Chemistry 9701/35 — October/November 2023

Cambridge AS Level · Advanced Practical Skills 1 · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation · Qualitative Analysis

Q123MManipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

Quantitative analysis

Read through the whole method before starting any practical work. Where appropriate, prepare a table for your results in the space provided.

Show the precision of the apparatus you used in the data you record.

Show your working and appropriate significant figures in the final answer to each step of your calculations.

1 You will determine the value of $x$ in hydrated sodium carbonate, $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ . $x$ is not an integer.

You will carry out two methods to determine the value of $x$ . Each method involves sodium carbonate reacting with excess hydrochloric acid to release carbon dioxide.

\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O(s)} + 2\text{HCl(aq)} \rightarrow 2\text{NaCl(aq)} + \text{CO}_2\text{(g)} + x\text{H}_2\text{O(l)}

(a)

Experiment 1

You will measure the volume of carbon dioxide released when hydrated sodium carbonate reacts with excess hydrochloric acid.

FA 1 is $0.500\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$ .
FA 2 is hydrated sodium carbonate, $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ .

Method

● Weigh the container with FA 2. Record the mass.
● Fill the tub with water to a depth of approximately $5\text{ cm}$ .
● Fill the $250\text{ cm}^3$ measuring cylinder completely with water. Holding a piece of paper towel firmly over the top, invert the measuring cylinder and place it in the water in the tub.
● Remove the paper towel and clamp the inverted measuring cylinder so the open end is in the water just above the base of the tub.
● Using the $50\text{ cm}^3$ measuring cylinder, transfer $50.0\text{ cm}^3$ of FA 1 into the flask labelled Z. Check that the bung fits tightly into the neck of flask Z, clamp flask Z and place the end of the delivery tube into the inverted $250\text{ cm}^3$ measuring cylinder.
● Remove the bung from the neck of the flask. Tip all the FA 2 from the container into the acid in the flask and replace the bung immediately. Remove the flask from the clamp and swirl it to mix the contents. You may need to shake the flask quite vigorously until the gas formed starts to collect in the measuring cylinder.
● Return the flask to the clamp. Leave for several minutes, shaking the flask occasionally.
● Weigh the container with any residual FA 2. Record the mass.
● Calculate the mass of FA 2 added to the flask. Record the mass.
● When no more gas is collected, measure the final volume of gas in the measuring cylinder. Record the volume.

Results

(b)

Calculations

(i)

Calculate the amount, in $\text{mol}$ , of carbon dioxide collected in the measuring cylinder at room conditions.

\text{amount of } \text{CO}_2 = \text{.............................. mol}

Hence deduce the amount, in $\text{mol}$ , of sodium carbonate present in the FA 2 you added in your experiment.

\text{amount of } \text{Na}_2\text{CO}_3 = \text{.............................. mol}

(ii)

Use your answer to (b)(i) and the mass of hydrated sodium carbonate, $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ , you used in Experiment 1 to calculate the relative formula mass, $M_r$ , of the $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ .

M_r \text{ of } \text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O} = \text{..............................}

(iii)

Use your answer to (b)(ii) to calculate the value of $x$ in the $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ .
Show your working.

x = \text{..............................}

(c)

A student suggests that it would be better to use hot water in the tub.

(i)

State whether using hot water would be an improvement. Explain your answer.

(ii)

State the effect, if any, that using hot water would have on the value of $x$ calculated.

(d)

Experiment 2

You will carry out a titration to measure the volume of hydrochloric acid that neutralises an aqueous solution of hydrated sodium carbonate, $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ .

\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O(s)} + 2\text{HCl(aq)} \rightarrow 2\text{NaCl(aq)} + \text{CO}_2\text{(g)} + x\text{H}_2\text{O(l)}

FA 3 is $0.100\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$ .
FA 4 is an aqueous solution containing $14.30\text{ g dm}^{-3}$ of hydrated sodium carbonate, $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ .
FA 5 is bromophenol blue indicator.

Method

● Fill the burette with FA 3.
● Pipette $25.0\text{ cm}^3$ of FA 4 into a conical flask.
● Add a few drops of FA 5.
● Carry out a rough titration and record your burette readings in the space below.

The rough titre is .............................. $\text{cm}^3$ .

● Carry out as many titrations as you think necessary to obtain consistent results.
● Make sure your recorded results show the precision of your practical work.
● Record, in a suitable form below, all of your burette readings and the volume of FA 3 added in each accurate titration.

(e)

From your accurate titration results, calculate a suitable mean value to use in your calculations. Show clearly how you obtain the mean value.

$25.0\text{ cm}^3$ of FA 4 required .............................. $\text{cm}^3$ of FA 3.

(f)

Calculations

(i)

Give your answers to (f)(ii), (f)(iii) and (f)(iv) to an appropriate number of significant figures.

(ii)

Calculate the amount, in $\text{mol}$ , of hydrochloric acid present in the volume of FA 3 you calculated in (e).

\text{amount of } \text{HCl} = \text{.............................. mol}

(iii)

Use the equation for the neutralisation to deduce the amount, in $\text{mol}$ , of sodium carbonate present in $25.0\text{ cm}^3$ of $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ .

\text{amount of } \text{Na}_2\text{CO}_3 = \text{.............................. mol}

Hence calculate the amount, in $\text{mol}$ , of sodium carbonate in $1.00\text{ dm}^3$ of $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ .

\text{amount of } \text{Na}_2\text{CO}_3 \text{ in } 1.00\text{ dm}^3 = \text{.............................. mol}

(iv)

Calculate the value of $x$ in the sample of $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ .
Show your working.

x = \text{..............................}

(g)

The aqueous solution of $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ , FA 4, was prepared by weighing and dissolving the solid to make $1.00\text{ dm}^3$ of solution.

\begin{aligned} \text{Mass of container } + \text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O} &= 32.509\text{ g} \\ \text{Mass of empty container} &= 18.209\text{ g} \\ \text{Mass of } \text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O} &= 14.300\text{ g} \end{aligned}

(i)

State the maximum uncertainty in a single balance reading for the balance used.

\text{maximum uncertainty} = \pm \text{.............................. g}

Calculate the maximum percentage uncertainty in this mass of $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ .
Show your working.

\text{maximum percentage uncertainty} = \pm \text{.............................. \%}

(ii)

Using the method in Experiment 2 a student calculated the relative formula mass, $M_r$ , of $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ to be 242.2. Assume that the uncertainty in the mass of FA 4 is the only source of error in the experiment.

Calculate the maximum value for the relative formula mass of FA 4.

\text{maximum value for the relative formula mass of } \textbf{FA 4} = \text{..............................}

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Q2Qualitative Analysis · Presentation of Data and Observations · Analysis, Conclusions and Evaluation · Manipulation, Measurement and Observation17M

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