Chemistry 9701/32 — May/June 2011

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

questions

marks

120

minutes

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

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

Many solid salts exist as hydrates. One example is washing soda – hydrated sodium carbonate, $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ .

You are to determine the value of $x$ in $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ by titration with hydrochloric acid.

FB 1 is hydrated sodium carbonate, $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ .
FB 2 is $0.200\text{ mol dm}^{-3}$ hydrochloric acid, $\text{HCl}$ .
methyl orange indicator

The equation for the reaction between hydrated sodium carbonate and hydrochloric acid is shown below.

\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O} + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{CO}_2 + (x + 1)\text{H}_2\text{O}

(a)

Method

Weigh the tube containing FB 1, the hydrated sodium carbonate. Record the mass in the space below.
Add all the FB 1 into a $250\text{ cm}^3$ glass beaker. Reweigh the tube containing any residual FB 1. Record the mass in the space below.
Calculate and record the mass of FB 1 used.

\text{mass of FB 1 used} = \dots\dots\dots\dots\dots\dots\dots\dots\dots\text{ g}

Use the $50\text{ cm}^3$ measuring cylinder to add, in total, about $100\text{ cm}^3$ of distilled water to the beaker.
Stir with a glass rod until all the solid has dissolved.
Pour the solution from the beaker into the $250\text{ cm}^3$ graduated (volumetric) flask.
Wash out the beaker thoroughly with distilled water and add the washings to the graduated flask.
Make up the contents of the graduated flask to the $250\text{ cm}^3$ mark with distilled water.
Shake the flask to mix the solution of FB 1.
Pipette $25.0\text{ cm}^3$ of your solution of FB 1 into a conical flask.
Add to the flask a few drops of methyl orange indicator and place the flask on a white tile.
Fill the burette with hydrochloric acid, FB 2.
Titrate the solution of FB 1 with the acid until the end-point is reached.

You should perform a rough titration.
In the space below record your burette readings for this rough titration.

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

Carry out as many accurate titrations as you think necessary to obtain consistent results.
Make certain any recorded results show the precision of your practical work.
Record, in an appropriate form below, all your burette readings and the volume of FB 2 added in each accurate titration.

(b)

From your accurate titration results, obtain a suitable value to be used in your calculations. Show clearly how you obtained this value.

25.0\text{ cm}^3\text{ of FB 1 required }\dots\dots\dots\dots\dots\dots\dots\dots\dots\text{ cm}^3\text{ of FB 2.}

(c)

Calculations

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

(i)

Calculate how many moles of $\text{HCl}$ were present in the volume of FB 2 calculated in (b).

\dots\dots\dots\dots\dots\dots\dots\dots\dots\text{ mol of HCl}

(ii)

Calculate how many moles of $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ were present in $25.0\text{ cm}^3$ of the solution of FB 1.

\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O} + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{CO}_2 + (x + 1)\text{H}_2\text{O}

\dots\dots\dots\dots\dots\dots\dots\dots\dots\text{ mol of Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}

(iii)

Calculate how many moles of $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ were present in $250\text{ cm}^3$ of the solution of FB 1.

\dots\dots\dots\dots\dots\dots\dots\dots\dots\text{ mol of Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}

(iv)

Use the mass of FB 1 that you weighed out to calculate the relative formula mass of $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ .

\text{relative formula mass} = \dots\dots\dots\dots\dots\dots\dots\dots\dots

(v)

Calculate the value of $x$ in $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ .
[ $A_r$ : H, 1.0; C, 12.0; O, 16.0; Na, 23.0]

x = \dots\dots\dots\dots\dots\dots\dots\dots\dots

(d)

The error in a single burette reading is $\pm 0.05\text{ cm}^3$ .

What is the percentage error in the titre volume calculated in (b)?

\text{percentage error} = \dots\dots\dots\dots\dots\dots\dots\dots\dots\%

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation10M
Q3Manipulation, Measurement and Observation · Qualitative Analysis · Analysis, Conclusions and Evaluation15M

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