Chemistry 9701/52 — October/November 2017

Background Concept

Titration is a quantitative technique used to determine the concentration of a specific reactive species in a solution. It relies on a sharp, identifiable end-point (usually via an indicator) when the analyte has been completely neutralised or reacted. For titration to be useful for determining the composition of a mixture, the components must react differently with the titrant, or only one component must react.

Understanding the Question

The question asks why a titration cannot be used to find the percentage of copper(II) carbonate in verdigris, which is a mixture of CuCO3 and Cu(OH)2. Both compounds are basic and react with dilute HCl. The mark scheme notes that both form copper(II) chloride.

Approach

Look at the provided equations. Both CuCO3 and Cu(OH)2 consume HCl. If you titrate the mixture with HCl, the total volume of HCl used will reflect the total basicity of the mixture, not the specific amount of carbonate.

Step-by-Step Reasoning

1. The reaction of CuCO3 with HCl: $\text{CuCO}_3(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{CuCl}_2(\text{aq}) + \text{CO}_2(\text{g}) + \text{H}_2\text{O}(\text{l})$
2. The reaction of Cu(OH)2 with HCl: $\text{Cu(OH)}_2(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow \text{CuCl}_2(\text{aq}) + 2\text{H}_2\text{O}(\text{l})$
3. Both reactions consume 2 moles of HCl per mole of copper compound and both produce CuCl2(aq).
4. A titration with an indicator would only tell you the total moles of acid required to neutralise all the solid. You cannot tell how much acid reacted with the carbonate versus the hydroxide.

Key Takeaways

Titration is only suitable for mixtures where the components have different reactivities towards the titrant, or where only one component reacts. Here, both react identically in terms of acid consumption per mole of copper.

Common Mistakes

• Saying "both are bases" without explaining that this means both consume acid.
• Failing to mention that both produce the same product (CuCl2), which means no visual change indicates which reaction is happening.

Things to Be Careful About

Keep the answer concise. The mark scheme awards the mark for stating that both react with HCl or both form copper(II) chloride.

Background Concept

Preparing a dilute solution from a concentrated stock solution requires a dilution calculation using $C_1V_1 = C_2V_2$. To prepare an accurate volume of dilute solution, a volumetric flask is used. The concentrated acid must be added to a small amount of water first, then made up to the mark, to ensure safety and accuracy.

Understanding the Question

You have $10.0 \text{ mol dm}^{-3}$ HCl and need to prepare $250 \text{ cm}^3$ of $0.500 \text{ mol dm}^{-3}$ HCl. You must describe the procedure and include apparatus capacities.

Approach

1. Calculate the volume of concentrated HCl needed using the dilution formula.
2. Describe the correct laboratory procedure for making a dilution using a volumetric flask.

Step-by-Step Reasoning

1. Calculate $V_1$: $V_1 = \frac{C_2V_2}{C_1} = \frac{0.500 \times 250}{10.0} = 12.5 \text{ cm}^3$.
2. Apparatus needed: a pipette or burette (for accurate transfer of $12.5 \text{ cm}^3$), and a $250 \text{ cm}^3$ volumetric flask.
3. Procedure: Transfer the $12.5 \text{ cm}^3$ of concentrated HCl into the volumetric flask. Add some distilled water, swirl to mix, then add more distilled water until the bottom of the meniscus sits exactly on the $250 \text{ cm}^3$ mark.

Key Takeaways

Always specify the apparatus with its capacity (e.g., $250 \text{ cm}^3$ volumetric flask) and the action of 'making up to the mark' with distilled water.

Common Mistakes

• Forgetting to state 'make up to the mark' or 'add distilled water to the mark'.
• Using a measuring cylinder instead of a pipette/burette for the concentrated acid (not accurate enough).
• Not specifying the capacity of the volumetric flask.

Things to Be Careful About

The mark scheme requires both the transfer of the calculated volume AND the making up to the mark. Both are needed for the 2 marks.

Background Concept

To determine the percentage of one component in a mixture, you need a property that differs between the reactions of the components. Looking at the equations:

• CuCO3 + 2HCl -> CuCl2 + CO2(g) + H2O
• Cu(OH)2 + 2HCl -> CuCl2 + 2H2O
  The carbonate reaction produces a gas (CO2), while the hydroxide reaction does not. Also, the carbonate reaction produces a gas that leaves the system, causing a mass loss.

Understanding the Question

Identify a dependent variable (something you measure) that differs between the two reactions. The independent variable is the composition of the verdigris.

Approach

Look for products unique to one reaction. CO2 is only produced by the carbonate. Therefore, measuring CO2 volume or the mass loss due to CO2 evolution will allow you to calculate the amount of carbonate.

Step-by-Step Reasoning

1. CuCO3 produces CO2(g). Cu(OH)2 does not.
2. You can measure the volume of CO2 collected in a gas syringe.
3. Alternatively, the CO2 gas escapes, so the total mass of the reaction vessel decreases. You can measure this mass loss.

Key Takeaways

When designing an experiment for a mixture, look for unique products (gases, precipitates) that can be measured to isolate the contribution of one component.

Common Mistakes

• Suggesting measuring the volume of water produced (hard to measure in aqueous solution).
• Suggesting measuring the pH (both consume acid, so pH change is not specific to carbonate).

Things to Be Careful About

The question asks for a difference based on the reactions. Gas volume or mass loss are the standard answers here.

Background Concept

To measure the volume of gas evolved from a reaction between a solid and a liquid, a common setup is a reaction vessel (conical flask) sealed with a bung, connected via a delivery tube to a gas syringe. The gas syringe allows direct measurement of gas volume.

Understanding the Question

Draw a diagram for the apparatus to measure the dependent variable (volume of CO2 or mass loss). Labels are required.

Approach

If measuring gas volume: conical flask + bung + delivery tube + gas syringe. Label all parts and the chemicals inside.
If measuring mass loss: conical flask on a balance, possibly with a cotton wool plug to prevent spray.

Step-by-Step Reasoning

1. Draw a conical flask.
2. Add solid (verdigris) and liquid (dilute HCl) inside.
3. Add a rubber bung with a hole.
4. Insert a delivery tube through the bung.
5. Connect the delivery tube to a gas syringe.
6. Label: 'Conical flask', 'Verdigris', 'Dilute HCl', 'Bung', 'Delivery tube', 'Gas syringe'.

Key Takeaways

A valid gas collection apparatus must be a closed system (sealed bung) to prevent gas escape. Labels must identify both apparatus and chemicals.

Common Mistakes

• Forgetting the bung (gas will escape).
• Not labelling the chemicals inside the flask.
• Drawing an open system like a beaker without a balance (if measuring mass loss).

Things to Be Careful About

The diagram must be clear and labelled. The gas syringe should be shown connected to the delivery tube. If using a measuring cylinder over water, the delivery tube must go under the water.

Background Concept

A graph of product formed (or reactant consumed) against time for a reaction that slows down as reactants are used up will show a curve starting at the origin, with a decreasing gradient, eventually reaching a horizontal asymptote (plateau) when the reaction is complete.

Understanding the Question

Sketch a graph of the dependent variable (volume of CO2 or mass loss) against time. Extend beyond the completion point. Label axes.

Approach

Y-axis: dependent variable (volume or mass). X-axis: time. Shape: curve from origin to plateau.

Step-by-Step Reasoning

1. Label Y-axis: 'Volume of CO$_2$ (cm$^3$)' or 'Loss in mass (g)'.
2. Label X-axis: 'Time (s)' or 't'.
3. Draw a curve starting at (0,0).
4. The curve should have a steep initial gradient (fast reaction) that decreases over time.
5. The curve must flatten to a horizontal line (plateau) indicating the reaction is complete and no more gas is produced/mass is lost.
6. Extend the horizontal line beyond the plateau point.

Key Takeaways

Reaction progress graphs always start at zero, rise with decreasing gradient, and plateau when complete.

Common Mistakes

• Drawing a straight line (implies constant rate, which is incorrect for most reactions).
• Forgetting to label the axes with units.
• Not extending the line beyond the plateau.

Things to Be Careful About

The curve must be smooth, not jagged. The plateau must be clearly horizontal.

Background Concept

Accuracy in quantitative experiments depends on minimising systematic and random errors. For gas collection, leaks are a major source of error. For mass loss, spray or splashing of liquid can cause false mass loss readings.

Understanding the Question

Describe how to make the single experiment as accurate as possible.

Approach

Think about potential errors in the apparatus from part (iv) and suggest a fix.

Step-by-Step Reasoning

• If using gas syringe: Check that the bung is tight and the delivery tube connections are secure to prevent gas leaking out.
• If measuring mass loss: The reaction may produce spray/fumes. Insert a cotton wool plug in the flask neck to allow gas to escape but prevent liquid spray.
• General: Use equipment with higher precision (e.g., 0.01g balance instead of 0.1g).

Key Takeaways

Always consider how the measured quantity could be lost or falsely increased, and suggest a physical barrier or better equipment.

Common Mistakes

• Vague answers like 'be more careful' or 'repeat the experiment' (the question asks about this one experiment).
• Suggesting 'use a better balance' without specifying why or how it improves accuracy in context.

Things to Be Careful About

The answer must be specific to the method chosen (gas collection vs mass loss). 'Check apparatus is sealed' is a good general answer.

Background Concept

Stoichiometry calculations require converting mass to moles, using the balanced equation to find moles of the second substance, and then converting back to mass or volume using concentration or molar volume.

Understanding the Question

Calculate the minimum volume of 0.500 mol dm$^{-3}$ HCl needed to react completely with 0.5 g of pure CuCO$_3$.

Approach

1. Calculate moles of CuCO$_3$.
2. Use the 1:2 ratio from the equation to find moles of HCl.
3. Calculate volume of HCl using $V = n/C$.

Step-by-Step Reasoning

1. $n(\text{CuCO}_3) = \frac{m}{M_r} = \frac{0.5}{123.5} = 4.0486 \times 10^{-3} \text{ mol}$
2. From $\text{CuCO}_3 + 2\text{HCl} \rightarrow \dots$, ratio is 1:2.
   $n(\text{HCl}) = 2 \times 4.0486 \times 10^{-3} = 8.097 \times 10^{-3} \text{ mol}$
3. $V(\text{HCl}) = \frac{n}{C} = \frac{8.097 \times 10^{-3}}{0.500} = 0.01619 \text{ dm}^3$
4. Convert to cm$^3$: $0.01619 \times 1000 = 16.19 \approx 16.2 \text{ cm}^3$

Key Takeaways

Always carry out calculations in the order: mass -> moles -> ratio -> moles -> volume/mass. Keep extra significant figures during intermediate steps.

Common Mistakes

• Forgetting the 1:2 stoichiometric ratio (using 1:1 gives half the volume).
• Forgetting to convert dm$^3$ to cm$^3$ (giving 0.0162 instead of 16.2).
• Using the wrong $M_r$ (e.g., including the hydroxide part).

Things to Be Careful About

The question asks for the volume in cm$^3$. Ensure the final unit is correct. Use 3 significant figures as appropriate (16.2).

Background Concept

Titration calculations involve finding moles of titrant from volume and concentration, using the stoichiometric ratio to find moles of analyte, converting to mass, and then calculating percentage purity.

Understanding the Question

Calculate the percentage by mass of $\text{Cu}_3(\text{CO}_3)_2(\text{OH})_2$ in a 1.50 g sample, given that 24.15 cm$^3$ of 0.400 mol dm$^{-3}$ H$_2$SO$_4$ was used.

Approach

1. Calculate moles of H$_2$SO$_4$.
2. Use the 1:3 ratio to find moles of azurite compound.
3. Calculate mass of azurite compound.
4. Calculate percentage in the 1.50 g sample.

Step-by-Step Reasoning

1. $n(\text{H}_2\text{SO}_4) = C \times V = 0.400 \times \frac{24.15}{1000} = 0.00966 \text{ mol}$
2. Equation: $\text{Cu}_3(\text{CO}_3)_2(\text{OH})_2 + 3\text{H}_2\text{SO}_4 \rightarrow \dots$
   Ratio is 1:3.
   $n(\text{compound}) = \frac{0.00966}{3} = 0.00322 \text{ mol}$
3. $m(\text{compound}) = n \times M_r = 0.00322 \times 344.5 = 1.1093 \text{ g} \approx 1.11 \text{ g}$
4. $\% = \frac{1.1093}{1.50} \times 100 = 73.95\% \approx 74.0\%$ (to 3 s.f.)

Key Takeaways

Always check the stoichiometric ratio carefully. Here it is 1:3, not 1:2. Round only at the final step to 3 significant figures.

Common Mistakes

• Using a 1:2 ratio instead of 1:3 (forgetting there are 3 H$_2$SO$_4$ molecules).
• Forgetting to divide by 1000 when converting cm$^3$ to dm$^3$.
• Rounding intermediate values too early (e.g., using 1.11 g exactly is fine, but using 0.0032 mol gives 1.102 g -> 73.5%).

Things to Be Careful About

The question asks for 3 significant figures. 74.0% is correct (not 74%).

Background Concept

Evaluating experimental data involves checking for concordance (repeatability) and considering systematic errors like indicator choice in coloured solutions.

Understanding the Question

The student used 24.15 cm$^3$ (average of 23.90 and 24.40). Identify two problems and suggest improvements.

Approach

1. Look at the titre values: 23.90 and 24.40. Difference is 0.50 cm$^3$. Concordant titres should be within 0.10 cm$^3$.
2. Look at the products: CuSO$_4$(aq) is blue/green. The indicator is bromophenol blue (yellow to blue/purple). The colour change might be hard to see.

Step-by-Step Reasoning

Problem 1: Titres 23.90 and 24.40 are not concordant (difference > 0.10 cm$^3$). The average is unreliable.
Improvement 1: Carry out further titrations until two results are within 0.10 cm$^3$ of each other, then average those.

Problem 2: The solution contains Cu$^{2+}$ ions, which are blue/green. Bromophenol blue changes from yellow to blue. The blue colour of copper sulfate may mask the endpoint.
Improvement 2: Use an indicator with a different colour change (e.g., methyl orange, though pH range must be suitable), or use a pH meter to detect the endpoint precisely.

Key Takeaways

Always check titre concordance in titration data. Also consider the colour of the solution when choosing an indicator.

Common Mistakes

• Saying 'the indicator is wrong' without explaining why (masking by Cu$^{2+}$ colour).
• Suggesting 'repeat more times' without specifying the concordance criterion (within 0.10 cm$^3$).

Things to Be Careful About

The improvement must directly address the problem. If the problem is non-concordant titres, the improvement is 'repeat until concordant'. If the problem is colour masking, the improvement is 'use a different indicator' or 'pH meter'.