Solid FB 1 is hydrated sodium carbonate, $\text{Na}_2\text{CO}_3 \cdot x\text{H}_2\text{O}$. You will determine the value of $x$ in a sample of FB 1 . The experiment involves three steps: Step 1 React a known mass of sodium carbonate, FB 1 , with an excess of acid. Step 2 Dilute the products of Step 1 to a known volume. Step 3 Carry out a titration to find out how much acid remained after the reaction in Step 1 . You will use the results of these three steps to find $x$. FB…

Chemistry 9701/36 — October/November 2021

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

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

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

Solid FB 1 is hydrated sodium carbonate, $\text{Na}_2\text{CO}_3 \cdot x\text{H}_2\text{O}$ . You will determine the value of $x$ in a sample of FB 1.

The experiment involves three steps:

Step 1 React a known mass of sodium carbonate, FB 1, with an excess of acid.
Step 2 Dilute the products of Step 1 to a known volume.
Step 3 Carry out a titration to find out how much acid remained after the reaction in Step 1.

You will use the results of these three steps to find $x$ .

FB 1 is hydrated sodium carbonate, $\text{Na}_2\text{CO}_3 \cdot x\text{H}_2\text{O}$ .
FB 2 is $0.800\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$ .
FB 4 is $0.100\text{ mol dm}^{-3}$ sodium hydroxide, $\text{NaOH}$ .
bromophenol blue indicator

(a)

Method

Step 1

Label a burette FB 2 and fill this burette with FB 2.
Run $50.00\text{ cm}^3$ of FB 2 into the $250\text{ cm}^3$ beaker.
Weigh the container with FB 1. Record the mass.
Slowly, and in small portions, add FB 1 to the acid.
Stir the mixture until the fizzing has stopped. Leave the stirring rod in the beaker.
Reweigh the container with any residue. Record the mass.
Calculate and record the mass of FB 1 added to the acid.

Step 2

Stir the mixture from Step 1 and ensure that all the solid has dissolved. Transfer this solution to the graduated flask.
Rinse the beaker and stirring rod twice with distilled water, then add the washings into the graduated flask.
Make the solution up to $250\text{ cm}^3$ with distilled water. Thoroughly mix the contents of the flask. This solution is FB 3.

Step 3

Label the other burette FB 4. Fill this burette with FB 4.
Pipette $25.0\text{ cm}^3$ of FB 3 into a conical flask.
Add several drops of bromophenol blue indicator.
Perform 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 are necessary to obtain consistent results.
Make certain any recorded results show the precision of your practical work.
Record, in a suitable form below, all of your burette readings and the volume of FB 4 added in each accurate titration.

(b)

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

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

(c)

Calculations

(i)

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

(ii)

Calculate the number of moles of hydrochloric acid in the FB 2 used in Step 1.

moles of $\text{HCl}$ in FB 2 used in Step 1 = .............................. mol

(iii)

Use your answer to (b) to calculate the number of moles of sodium hydroxide, FB 4, required to react with $25.0\text{ cm}^3$ of FB 3 in Step 3.

moles of $\text{NaOH}$ required = .............................. mol

Use this answer to deduce the number of moles of hydrochloric acid in $250\text{ cm}^3$ of FB 3. This is the number of moles remaining after the reaction in Step 1.

moles of $\text{HCl}$ in $250\text{ cm}^3$ of FB 3 = .............................. mol

(iv)

Use your answers to (c)(ii) and (c)(iii) to calculate the number of moles of hydrochloric acid that reacted with sodium carbonate in FB 1.

moles of $\text{HCl}$ that reacted with FB 1 = .............................. mol

(v)

Write an equation for the reaction of sodium carbonate with hydrochloric acid in Step 1. Include state symbols.

(vi)

Use the equation and your answer to (c)(iv) to determine the moles of sodium carbonate present in FB 1.

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

(vii)

Use your answer to (c)(vi) and your mass of FB 1 to calculate the formula mass of hydrated sodium carbonate.
Hence find the value for $x$ .

(If you were unable to calculate the number of moles of $\text{Na}_2\text{CO}_3$ in (c)(vi) assume that it is $5.55 \times 10^{-3}\text{ mol}$ . This is not the correct value.)

$x$ = ..............................

(d)

State the maximum error in a single balance reading.

maximum error in a balance reading = .............................. g

Calculate the maximum percentage error in the mass of FB 1 you used. Show your working.

maximum percentage error = .............................. %

(e)

A student decided to use a larger mass of FB 1 when carrying out the same method.

What effect would this have on the titre volume in Step 3?
Explain your answer.

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