Chemistry 9701/51 — October/November 2024

Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme

questions

marks

minutes

Topics Planning · Analysis, Conclusions and Evaluation

Q1Medium-EasyPlanningAnalysis, Conclusions and Evaluation

A student uses the following method to determine the percentage by mass of the painkiller aspirin, $\text{C}_9\text{H}_8\text{O}_4(\text{s})$ , in some tablets.

step 1 Grind five tablets into a powder.

step 2 Use a weighing boat to accurately weigh by difference approximately $0.4\text{ g}$ of powdered tablets into a pear-shaped flask containing anti-bumping granules.

step 3 Add $25\text{ cm}^3$ of aqueous $1\text{ mol dm}^{-3}$ sodium hydroxide, $\text{NaOH(aq)}$ , to the pear-shaped flask, forming mixture A.

step 4 Reflux mixture A for 20 minutes.

step 5 Allow mixture A to cool and then filter into a small beaker. Label the filtrate solution B.

step 6 Add $30\text{ cm}^3$ of alkaline aqueous iodine to solution B and leave to stand for 1 hour. A precipitate, C, $(\text{C}_6\text{H}_2\text{I}_2\text{O})_2(\text{s})$ , will form.

step 7 Filter the resulting mixture under reduced pressure. Wash the residue, C, with a small volume of cold distilled water.

step 8 Allow solid C to dry.

step 9 Weigh solid C and record its mass.

Alkaline aqueous iodine is irritating to the skin and eyes.

(a)

Identify an appropriate precaution, other than eye protection and a lab coat, that the student should take when using alkaline aqueous iodine.

1M Similar

DifficultyEasy

Worked solution

Answer

Wear chemically resistant gloves.

Final answer

Wear chemically resistant gloves.

Detailed explanation

Background Concept

When working with chemicals that are irritating to the skin or eyes, standard laboratory personal protective equipment (PPE) is required. While a lab coat protects clothing and general safety goggles protect the eyes, skin protection is specifically needed when handling irritating or corrosive substances that could cause dermatitis or chemical burns.

Understanding the Question

The question asks for a specific precaution when handling alkaline aqueous iodine, excluding eye protection and a lab coat. The mark scheme notes that the iodine is irritating, so the precaution must directly address skin contact.

Approach

Identify the hazard (irritation to skin and eyes) and select the appropriate PPE that is not already excluded (eye protection and lab coat). Gloves are the standard barrier for skin protection against chemical irritation.

Step-by-Step Reasoning

Identify the hazard: The question states alkaline aqueous iodine is irritating to the skin and eyes.
Exclude given PPE: Eye protection (safety goggles) and a lab coat are already mentioned.
Select appropriate PPE: Chemically resistant gloves (e.g., nitrile) are required to prevent the irritating solution from contacting the skin.

Key Takeaways

Always match PPE to the specific hazard. For skin-irritating chemicals, chemically resistant gloves are essential in addition to standard eye and clothing protection.

Common Mistakes

Stating 'wear gloves' without specifying 'chemically resistant' or 'nitrile' (standard cotton gloves offer little protection against chemicals).
Suggesting 'wear a face shield' (this is for eye/face protection, which is already excluded).

Things to Be Careful About

Be precise with terminology. 'Gloves' alone may not score if the examiner is looking for 'chemically resistant' or 'rubber' or 'nitrile' to distinguish from laboratory cotton gloves.

Techniques used

identify safety precautions for hazardous chemicals

(b)

Describe how the student should carry out step 2. Include a results table, with appropriate headings, for the student to fill in.

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DifficultyMedium-Easy

Worked solution

Answer

Description:

Measure the mass of the weighing boat with the powdered tablets on it.
Transfer the powder to the pear-shaped flask.
Measure the mass of the weighing boat with the remaining solid (residue).
Calculate the mass of solid transferred by subtracting the second mass from the first.

Results Table:

Mass of weighing boat + solid before transfer / g	Mass of weighing boat + residue after transfer / g	Mass of solid transferred / g

Working

Mass transferred = (Mass of boat + solid before) - (Mass of boat + residue after)

Answer

See description and table above.

Final answer

Weigh by difference: measure mass of boat + solid, transfer to flask, measure mass of boat + residue. Table must have unambiguous headings and units (e.g., / g) for: mass of boat + solid before transfer, mass of boat + residue after transfer, and mass of solid transferred.

Detailed explanation

Background Concept

Weighing by difference is a standard quantitative technique used to transfer a specific mass of solid into a vessel without losing any material on the weighing container. It is more accurate than trying to weigh a solid directly into the vessel, as some powder may stick to the boat or spill.

Understanding the Question

The student needs to accurately weigh approximately 0.4 g of powdered tablets into a flask. The question asks for a description of step 2 and a results table to record the data.

Approach

Describe the weighing by difference method: weigh the container with the solid, transfer the solid, then weigh the container with the remainder. The difference is the mass transferred. Construct a table with clear headings and units to record these three values.

Step-by-Step Reasoning

Weighing by difference: Place the weighing boat with the powder on the balance and record the mass (boat + solid).
Transfer: Carefully pour the powder into the pear-shaped flask.
Weigh residue: Place the empty (or partially empty) weighing boat back on the balance and record the mass (boat + residue).
Calculate: Mass of solid transferred = Initial mass - Final mass.
Table format: The table must have three columns (or rows) with unambiguous headings. Each heading must include the quantity and its unit (e.g., 'Mass of weighing boat + solid before transfer / g').

Key Takeaways

Weighing by difference is essential for accurate transfer of solids. Results tables must always include units in the headings, not just in the data rows.

Common Mistakes

Describing weighing directly into the flask (loses accuracy due to spillage/sticking).
Forgetting units in the table headings (e.g., writing 'Mass of solid' instead of 'Mass of solid / g').
Not including the mass of the boat in the headings (the balance measures boat + solid, not just solid).

Things to Be Careful About

Ensure the table headings are unambiguous. 'Mass of solid' is not acceptable because the balance does not measure it directly; it measures 'Mass of weighing boat + solid'. The unit must be part of the heading, not a separate column.

Techniques used

describe weighing by differenceconstruct a results table with units

(c)

Complete Fig. 1.1 to show how step 4 is carried out in the laboratory.

Label your diagram fully.

Fig. 1.1

2M Similar

DifficultyMedium-Easy

Worked solution

Answer

Working

Step 4 is refluxing mixture A. This requires a vertical condenser attached to the flask to prevent loss of volatile components.

Answer

See diagram above.

Final answer

A vertical Liebig condenser attached to the top of the pear-shaped flask, with cold water entering at the bottom inlet and exiting at the top outlet.

Detailed explanation

Background Concept

Refluxing involves heating a reaction mixture to its boiling point while condensing the vapours and returning them to the flask. This allows prolonged heating without loss of solvent or volatile reactants/products. A Liebig condenser is typically used, consisting of an inner tube through which vapours pass and an outer jacket through which cooling water flows.

Understanding the Question

The student is refluxing mixture A (containing NaOH and aspirin) for 20 minutes. Figure 1.1 shows a pear-shaped flask with heat applied. The student must complete the diagram by adding the condenser and labeling the water flow.

Approach

Draw a vertical condenser on top of the flask. Label the water inlet at the bottom (to ensure the jacket fills completely) and the water outlet at the top.

Step-by-Step Reasoning

Condenser placement: Attach a Liebig condenser vertically to the top of the pear-shaped flask. It should have an outer jacket.
Water flow direction: Cold water must enter at the bottom (lower tap) and exit at the top (upper tap). This counter-current flow ensures the condenser jacket is always full of cold water, maximizing cooling efficiency.
Labels: Label the condenser, water inlet (bottom), and water outlet (top).

Key Takeaways

In reflux setups, water always enters at the bottom and exits at the top of the condenser to ensure the jacket is completely filled with cooling water.

Common Mistakes

Drawing the condenser at an angle (it must be vertical for reflux).
Reversing the water flow (in at top, out at bottom), which can lead to an empty jacket and poor cooling.
Forgetting to label the water inlet and outlet.

Things to Be Careful About

Ensure the condenser is drawn with an outer jacket. Simply drawing a tube is not sufficient. The labels 'cold water in' and 'cold water out' (or 'water inlet' and 'water outlet') must be clear.

Techniques used

draw a reflux condenser setuplabel water inlet and outlet

(d)

(i)

The student uses a measuring cylinder to measure the volume of alkaline aqueous iodine in step 6. Suggest why this is a suitable piece of apparatus to use.

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(ii)

Suggest why the student leaves the mixture to stand for 1 hour in step 6.

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(iii)

Explain why the residue is washed in step 7.

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(iv)

Explain why hot distilled water is not used in step 7.

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(e)

The equation for the reaction between aspirin, $\text{C}_9\text{H}_8\text{O}_4(\text{s})$ , and $\text{NaOH(aq)}$ , which takes place in step 4, is shown.

\text{C}_9\text{H}_8\text{O}_4(\text{s}) + 2\text{NaOH(aq)} \rightarrow \text{C}_7\text{H}_5\text{O}_3^-\text{Na}^+(\text{aq}) + \text{C}_2\text{H}_3\text{O}_2^-\text{Na}^+(\text{aq}) + \text{H}_2\text{O(l)}

The equation for the reaction in which solid C, $(\text{C}_6\text{H}_2\text{I}_2\text{O})_2(\text{s})$ , is formed in step 6 is shown.

2\text{C}_7\text{H}_5\text{O}_3^-\text{Na}^+(\text{aq}) + 6\text{I}_2(\text{aq}) + 8\text{OH}^-(\text{aq}) \rightarrow (\text{C}_6\text{H}_2\text{I}_2\text{O})_2(\text{s}) + 8\text{I}^-(\text{aq}) + 2\text{NaHCO}_3(\text{aq}) + 6\text{H}_2\text{O(l)}

The student's results are shown in Table 1.1.

Table 1.1


mass of powdered tablets added to the pear-shaped flask in step 2	$0.409\text{ g}$
mass of dry $(\text{C}_6\text{H}_2\text{I}_2\text{O})_2(\text{s})$ recorded in step 9	$0.764\text{ g}$

(i)

Calculate the amount, in mol, of $(\text{C}_6\text{H}_2\text{I}_2\text{O})_2(\text{s})$ collected in step 9.

[ $M_r$ : $(\text{C}_6\text{H}_2\text{I}_2\text{O})_2$ , 687.6]

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(ii)

Use your answer to (i) to calculate the mass, in g, of $\text{C}_9\text{H}_8\text{O}_4(\text{s})$ in the powdered tablets added to the flask in step 2.

1M Similar

DifficultyMedium-Easy

Worked solution

Working

From the equations:

1 mol of $(\text{C}_6\text{H}_2\text{I}_2\text{O})_2$ is produced from 2 mol of $\text{C}_7\text{H}_5\text{O}_3^-\text{Na}^+$ .
2 mol of $\text{C}_7\text{H}_5\text{O}_3^-\text{Na}^+$ is produced from 1 mol of $\text{C}_9\text{H}_8\text{O}_4$ .

Therefore, 1 mol of $(\text{C}_6\text{H}_2\text{I}_2\text{O})_2$ corresponds to 2 mol of $\text{C}_9\text{H}_8\text{O}_4$ .

\text{amount of } \text{C}_9\text{H}_8\text{O}_4 = 2 \times 1.111 \times 10^{-3} = 2.222 \times 10^{-3} \text{ mol}

M_r \text{ of } \text{C}_9\text{H}_8\text{O}_4 = (9 \times 12.0) + (8 \times 1.0) + (4 \times 16.0) = 180.0

\text{mass of } \text{C}_9\text{H}_8\text{O}_4 = 2.222 \times 10^{-3} \times 180.0 = 0.400 \text{ g}

Answer

$0.400 \text{ g}$

Final answer

0.400 g

Detailed explanation

Background Concept

Stoichiometry allows us to relate the amounts of reactants and products using balanced chemical equations. By following the mole ratios through a series of reactions, we can determine the amount of an initial reactant from the mass of a final product.

Understanding the Question

Using the answer from (e)(i), calculate the mass of aspirin ( $C_9H_8O_4$ ) in the tablets.

Approach

Use the mole ratio from the second equation to find moles of salicylate ion.
Use the mole ratio from the first equation to find moles of aspirin.
Calculate the mass of aspirin using its $M_r$ .

Step-by-Step Reasoning

Second equation: $2\text{C}_7\text{H}_5\text{O}_3^-\text{Na}^+ \rightarrow (\text{C}_6\text{H}_2\text{I}_2\text{O})_2$ . Ratio is 2:1.
Moles of $\text{C}_7\text{H}_5\text{O}_3^-\text{Na}^+ = 2 \times 1.111 \times 10^{-3} = 2.222 \times 10^{-3}$ mol.
First equation: $\text{C}_9\text{H}_8\text{O}_4 \rightarrow \text{C}_7\text{H}_5\text{O}_3^-\text{Na}^+$ . Ratio is 1:1.
Moles of $\text{C}_9\text{H}_8\text{O}_4 = 2.222 \times 10^{-3}$ mol.
Mass calculation: $M_r(\text{C}_9\text{H}_8\text{O}_4) = 180.0$ .
Mass $= 2.222 \times 10^{-3} \times 180.0 = 0.400$ g.

Key Takeaways

Chain stoichiometric calculations carefully, tracking the mole ratios through each equation.

Common Mistakes

Using the wrong mole ratio (e.g., 1:2 instead of 2:1).
Forgetting to calculate the $M_r$ of aspirin correctly.
Rounding intermediate values too early.

Things to Be Careful About

Ensure the mole ratios are taken from the correct equations and in the correct direction. Carry forward unrounded values.

Techniques used

use stoichiometric ratios to calculate moles of reactantcalculate mass from moles and molar mass

(iii)

Use your answer to (ii) to calculate the percentage by mass of aspirin, $\text{C}_9\text{H}_8\text{O}_4(\text{s})$ , in the tablets.

If you were unable to obtain an answer to (ii) you may use $0.374\text{ g}$ for the mass of $\text{C}_9\text{H}_8\text{O}_4(\text{s})$ . This is not the correct value.

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(f)

Another student follows the same method but does not allow solid C to dry completely in step 8.

State and explain the effect that this has on the calculated percentage by mass of aspirin, $\text{C}_9\text{H}_8\text{O}_4(\text{s})$ , in the tablets.

1M Similar

DifficultyMedium-Easy

Worked solution

Answer

If solid C is not completely dry, its measured mass will be greater than the true mass of the dry solid.

This will lead to a calculated amount of solid C that is too high, which in turn leads to a calculated mass of aspirin that is too high, and finally a percentage by mass of aspirin that is greater than the true value.

Final answer

The mass of solid C would be greater, leading to a calculated percentage by mass of aspirin that is greater than the true value.

Detailed explanation

Background Concept

In quantitative analysis, errors in intermediate measurements propagate through the calculation. If a mass is measured incorrectly (e.g., due to moisture), the final calculated value will be affected in the same direction.

Understanding the Question

The student does not allow solid C to dry completely. State and explain the effect on the calculated percentage by mass of aspirin.

Approach

Trace the error through the calculation: higher mass of C -> higher moles of C -> higher moles of aspirin -> higher mass of aspirin -> higher percentage.

Step-by-Step Reasoning

Error in mass: Wet solid C has a higher mass than dry solid C (due to water).
Moles of C: $n = \frac{m}{M_r}$ . Higher mass $m$ leads to higher calculated moles of C.
Moles of aspirin: Moles of aspirin are proportional to moles of C (ratio 2:1). Higher moles of C leads to higher calculated moles of aspirin.
Mass of aspirin: Mass = moles $\times M_r$ . Higher moles leads to higher calculated mass of aspirin.
Percentage: Percentage = $\frac{\text{mass of aspirin}}{\text{mass of tablets}} \times 100$ . Higher mass of aspirin leads to a higher percentage.

Key Takeaways

Errors in mass measurements propagate through stoichiometric calculations. Always ensure precipitates are completely dry before weighing.

Common Mistakes

Stating 'the percentage would be lower' (wrong direction of error).
Not explaining the chain of reasoning (mass -> moles -> percentage).

Things to Be Careful About

Be clear about the direction of the error: 'greater than' or 'higher than', not just 'wrong'. Explain the causal chain.

Techniques used

evaluate the effect of an experimental error on the final calculation

Q2MediumAnalysis, Conclusions and EvaluationPlanning

Crystal violet, $\text{C}_{25}\text{H}_{30}\text{N}_3\text{Cl}(\text{s})$ , is a purple dye.

Some light is absorbed when it passes through $\text{C}_{25}\text{H}_{30}\text{N}_3\text{Cl}(\text{aq})$ .

Absorbance is the proportion of light absorbed at a particular wavelength. This is measured using a colorimeter.

A graph of absorbance against wavelength for $\text{C}_{25}\text{H}_{30}\text{N}_3\text{Cl}(\text{aq})$ is shown in Fig. 2.1.

Fig. 2.1

A student investigates how to determine the concentration of aqueous crystal violet, $\text{C}_{25}\text{H}_{30}\text{N}_3\text{Cl}(\text{aq})$ , using colorimetry.

(a)

Suggest the best wavelength of light to use in the colorimeter when measuring the concentration of $\text{C}_{25}\text{H}_{30}\text{N}_3\text{Cl}(\text{aq})$ .

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(b)

Solution D is $500.0\text{ cm}^3$ of $2.50 \times 10^{-2}\text{ mol dm}^{-3}$ $\text{C}_{25}\text{H}_{30}\text{N}_3\text{Cl}(\text{aq})$ .

(i)

Calculate the mass of $\text{C}_{25}\text{H}_{30}\text{N}_3\text{Cl}(\text{s})$ needed to prepare solution D.

Give your answer to three significant figures.

[ $M_r$ : $\text{C}_{25}\text{H}_{30}\text{N}_3\text{Cl}(\text{s})$ , 407.5]

1M Similar

DifficultyMedium-Easy

Worked solution

Working

n = c \times V = 2.50 \times 10^{-2} \text{ mol dm}^{-3} \times 0.5000 \text{ dm}^3 = 1.25 \times 10^{-2} \text{ mol}

\text{mass} = n \times M_r = 1.25 \times 10^{-2} \text{ mol} \times 407.5 \text{ g mol}^{-1} = 5.09 \text{ g}

Answer

5.09 g

Final answer

5.09 g

Detailed explanation

Background Concept

To prepare a solution of a known concentration, you need to calculate the mass of solute required. This involves using the relationship $n = c \times V$ to find the number of moles, and then $m = n \times M_r$ to find the mass. Remember that volume must be in dm³ when using concentration in mol dm⁻³.

Understanding the Question

Calculate the mass of crystal violet needed to make 500.0 cm³ of a 2.50 × 10⁻² mol dm⁻³ solution.

Approach

Convert volume from cm³ to dm³.
Calculate moles using $n = c \times V$ .
Calculate mass using $m = n \times M_r$ .
Round to three significant figures.

Step-by-Step Reasoning

Volume $V = 500.0 \text{ cm}^3 = 0.5000 \text{ dm}^3$ .
Moles $n = 2.50 \times 10^{-2} \times 0.5000 = 1.25 \times 10^{-2} \text{ mol}$ .
Mass $= 1.25 \times 10^{-2} \times 407.5 = 5.09375 \text{ g}$ .
To three significant figures, mass = 5.09 g.

Key Takeaways

Always convert volume to dm³ when using concentration in mol dm⁻³. Pay attention to significant figures in the final answer.

Common Mistakes

Forgetting to convert cm³ to dm³ (using 500 instead of 0.5).
Rounding too early in the calculation.

Things to Be Careful About

The question asks for three significant figures. 5.09375 rounds to 5.09, not 5.10.

Techniques used

calculate moles from concentration and volumecalculate mass from moles and molar mass

(ii)

The student is given a small beaker containing the mass of $\text{C}_{25}\text{H}_{30}\text{N}_3\text{Cl}(\text{s})$ calculated in (i).

Describe how the student should prepare $500.0\text{ cm}^3$ of solution D.

Include the name and capacity of the key apparatus which should be used and describe how the student should ensure the volume is exactly $500.0\text{ cm}^3$ .

3M Similar

DifficultyMedium-Easy

Worked solution

Answer

Add a small volume of distilled water to the beaker and dissolve the solid. Transfer the solution to a 500.0 cm³ volumetric flask, including washings. Make up to the calibration mark with distilled water and mix by inverting the flask.

Final answer

See answer above

Detailed explanation

Background Concept

Preparing a standard solution of a precise concentration requires a volumetric flask. The solid is first dissolved in a small amount of solvent, then transferred quantitatively to the flask, and finally the volume is made up to the mark. Washings ensure all solute is transferred.

Understanding the Question

Describe the method to prepare 500.0 cm³ of solution D from the solid calculated in (b)(i). Must include apparatus name/capacity and how to ensure exact volume.

Approach

Describe the standard procedure step-by-step, ensuring all mark scheme points are covered: dissolving, transferring with washings, making up to the mark, and mixing.

Step-by-Step Reasoning

M1: Add a small volume of distilled water to the beaker containing the solid and stir to dissolve it completely.
M2: Pour the solution into a 500.0 cm³ volumetric flask. Rinse the beaker and stirring rod with distilled water and add the washings to the flask to ensure all solute is transferred.
M3: Add distilled water until the bottom of the meniscus sits exactly on the calibration mark. Stopper the flask and invert it several times to mix the solution thoroughly.

Key Takeaways

A volumetric flask is used for precise volume measurements. Washings are essential to ensure quantitative transfer of solute. Making up to the mark and mixing are critical final steps.

Common Mistakes

Forgetting to mention washings.
Saying "fill to 500 cm³" instead of "make up to the calibration mark".
Not mentioning mixing/inverting at the end.

Things to Be Careful About

Use the exact apparatus name: "volumetric flask" (not just "flask").
Specify the capacity: "500.0 cm³ volumetric flask".

Techniques used

describe volumetric flask preparation methodensure accurate volume measurement

(c)

A small sample of solution D was diluted to form solution E, $2.50 \times 10^{-4}\text{ mol dm}^{-3}$ $\text{C}_{25}\text{H}_{30}\text{N}_3\text{Cl}(\text{aq})$ .

The student prepares solutions 2 to 6 as shown in Table 2.1.

The total volume needed for each of solutions 2 to 6 is $20.00\text{ cm}^3$ .

Each solution is placed into a colorimeter and the absorbance is measured.

(i)

Complete Table 2.1 to show the volumes of solution E and distilled water needed to prepare each of the solutions from 2 to 6. Give all volumes to two decimal places.

Table 2.1

solution	volume of $2.50 \times 10^{-4}\text{ mol dm}^{-3}$ $\text{C}_{25}\text{H}_{30}\text{N}_3\text{Cl}(\text{aq})$ (solution E) / $\text{cm}^3$	volume of distilled water / $\text{cm}^3$	$[\text{C}_{25}\text{H}_{30}\text{N}_3\text{Cl}(\text{aq})]$ / $\text{mol dm}^{-3}$	absorbance
1	0.00	20.00	0.00	0.000
2			$0.50 \times 10^{-4}$	0.191
3			$1.00 \times 10^{-4}$	0.270
4			$1.50 \times 10^{-4}$	0.545
5			$2.00 \times 10^{-4}$	0.711
6			$2.50 \times 10^{-4}$	0.860

1M Similar

DifficultyMedium-Easy

Worked solution

Answer

solution	volume of solution E / cm³	volume of distilled water / cm³
2	4.00	16.00
3	8.00	12.00
4	12.00	8.00
5	16.00	4.00
6	20.00	0.00

Final answer

See table above

Detailed explanation

Background Concept

When diluting a solution, the number of moles of solute remains constant. This is expressed by $C_1V_1 = C_2V_2$ , where $C_1$ and $V_1$ are the concentration and volume of the stock solution, and $C_2$ and $V_2$ are the concentration and volume of the diluted solution. The total volume $V_2$ is fixed at 20.00 cm³.

Understanding the Question

Complete the table showing volumes of solution E (2.50 × 10⁻⁴ mol dm⁻³) and distilled water needed to make 20.00 cm³ of solutions 2–6 with specified concentrations.

Approach

Use $C_1V_1 = C_2V_2$ to find the volume of solution E needed for each target concentration. Subtract this from 20.00 cm³ to find the volume of water.

Step-by-Step Reasoning

For solution 2: $C_2 = 0.50 \times 10^{-4} \text{ mol dm}^{-3}$ , $V_2 = 20.00 \text{ cm}^3$ .

V_1 = \frac{C_2 V_2}{C_1} = \frac{0.50 \times 10^{-4} \times 20.00}{2.50 \times 10^{-4}} = 4.00 \text{ cm}^3

Volume of water $= 20.00 - 4.00 = 16.00 \text{ cm}^3$ .

Similarly:

Solution 3: $V_1 = 8.00 \text{ cm}^3$ , water $= 12.00 \text{ cm}^3$ .
Solution 4: $V_1 = 12.00 \text{ cm}^3$ , water $= 8.00 \text{ cm}^3$ .
Solution 5: $V_1 = 16.00 \text{ cm}^3$ , water $= 4.00 \text{ cm}^3$ .
Solution 6: $V_1 = 20.00 \text{ cm}^3$ , water $= 0.00 \text{ cm}^3$ .

Key Takeaways

Dilution calculations rely on $C_1V_1 = C_2V_2$ . The volume of solvent added is the total volume minus the volume of stock solution.

Common Mistakes

Forgetting that the total volume is 20.00 cm³, not just the volume of solution E.
Not giving volumes to two decimal places as required.

Things to Be Careful About

Ensure all volumes are given to two decimal places (e.g., 4.00, not 4).

Techniques used

calculate dilution volumes using C1V1 = C2V2proportional reasoning for dilutions

(ii)

Identify the dependent variable.

1M Similar

(d)

(i)

Plot a graph of absorbance against $[\text{C}_{25}\text{H}_{30}\text{N}_3\text{Cl}(\text{aq})]$ on the grid in Fig. 2.2.

Use a cross ( $\times$ ) to plot each data point.

Draw a straight line of best fit.

Fig. 2.2

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(ii)

Circle the point on the graph you consider to be most anomalous.

Suggest one reason why this anomaly may have occurred during this experimental procedure.

Assume no error was made in the measurement of absorbance.

2M Similar

DifficultyMedium-Easy

Worked solution

Answer

Most anomalous point: solution 3 (concentration $1.00 \times 10^{-4} \text{ mol dm}^{-3}$ , absorbance 0.270).

Reason: Volume of distilled water added to the mixture was too large (or volume of solution E added was too small).

Final answer

Solution 3 anomalous; water volume too large or solution E volume too small

Detailed explanation

Background Concept

An anomalous result is a data point that does not fit the general trend of the data. In a calibration curve, this usually indicates a procedural error during preparation or measurement. Since the question states no error was made in measuring absorbance, the error must be in preparing the solution.

Understanding the Question

Circle the anomalous point on the graph and suggest one reason for it, assuming absorbance measurement was correct.

Approach

Look at the graph: the point at (1.00, 0.270) is significantly below the line of best fit.
Since absorbance is lower than expected for this concentration, the actual concentration in the colorimeter must have been lower than intended.
This could happen if too much water was added or too little solution E was added during preparation.

Step-by-Step Reasoning

The point for solution 3 (1.00 × 10⁻⁴ mol dm⁻³, 0.270) is clearly below the line.
A lower absorbance means lower concentration in the cuvette.
During dilution, if the volume of solution E was measured too small (e.g., using a pipette incorrectly) or the volume of water was too large (e.g., using a measuring cylinder instead of a pipette), the final concentration would be lower than 1.00 × 10⁻⁴ mol dm⁻³.

Key Takeaways

Anomalous points can arise from errors in preparing solutions (volumetric errors) or measuring instruments. Always consider both possibilities.

Common Mistakes

Suggesting "human error" or "not accurate" without being specific.
Suggesting an error in absorbance measurement when the question says to assume there was none.

Things to Be Careful About

Be specific about the error: "volume of water too large" or "volume of solution E too small".

Techniques used

identify anomalous data pointpropose source of error in dilution

(iii)

State the relationship between $[\text{C}_{25}\text{H}_{30}\text{N}_3\text{Cl}(\text{aq})]$ and absorbance.

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(iv)

Suggest how the student could improve the reliability of the data obtained in the experiment in (c).

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(e)

The student carries out a further experiment to examine the kinetics of the reaction between crystal violet, $\text{C}_{25}\text{H}_{30}\text{N}_3\text{Cl}(\text{aq})$ , and aqueous sodium hydroxide, $\text{NaOH(aq)}$ .

\text{C}_{25}\text{H}_{30}\text{N}_3\text{Cl}(\text{aq}) + \text{OH}^-(\text{aq}) \rightarrow \text{C}_{25}\text{H}_{30}\text{N}_3\text{OH}(\text{aq}) + \text{Cl}^-(\text{aq})

The disappearance of the purple colour as the reaction proceeds can be monitored by measuring how the absorbance of light by the mixture changes using a colorimeter.

The student mixes $5\text{ cm}^3$ of solution 6 with $5\text{ cm}^3$ of $\text{NaOH(aq)}$ , a large excess, and immediately starts the stopwatch.

The resulting mixture is then placed in a colorimeter. The absorbance of this mixture is measured every 100 seconds after starting the stop-watch.

Fig. 2.3 shows a graph of the student's results.

Fig. 2.3

(i)

Suggest why it is not possible for the student to measure the absorbance of the mixture at $t = 0\text{ s}$ .

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(ii)

Use the graph in Fig. 2.3 to find the half-life, $t_{\frac{1}{2}}$ , starting at $100\text{ s}$ .

State the coordinates of both points on the line of best fit used in your calculation.

coordinates 1 .............................. coordinates 2 ..............................

half-life .............................. s

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(iii)

Another student repeats the experiment at a different temperature and measures two half-life values. The values obtained are $420\text{ s}$ and $425\text{ s}$ .

Use these values to deduce the order of the reaction with respect to $\text{C}_{25}\text{H}_{30}\text{N}_3\text{Cl}(\text{aq})$ . Explain your answer.

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