Chemistry 9701/35 — May/June 2012

Cambridge AS Level · Advanced Practical Skills · 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

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

You are advised to begin work on question 2, and return to question 1 later.

In this experiment you are to investigate the reaction between phosphoric acid and sodium hydroxide in order to determine the chemical equation.

FA 1 is an aqueous solution of phosphoric acid, $\text{H}_3\text{PO}_4$ .
FA 2 is $0.115\text{ mol dm}^{-3}$ sodium hydroxide, $\text{NaOH}$ .
phenolphthalein indicator

(a)

Method

Dilution

Weigh the $100\text{ cm}^3$ beaker provided. Record the mass in the space below.
Use the measuring cylinder to add about $10\text{ cm}^3$ of FA 1 to the beaker. Weigh the beaker with FA 1 and record the mass.
Calculate the mass of FA 1 used and record this in the space below.
Pour the FA 1 from the beaker into the $250\text{ cm}^3$ graduated (volumetric) flask provided, labelled FA 3.
Wash the beaker twice with small quantities of water and add these washings to the volumetric flask.
Make the solution up to the mark using distilled water. This diluted solution of phosphoric acid is FA 3.
Ensure that the contents of the flask are thoroughly mixed before using FA 3 for your titrations.

Titration

Fill the burette with FA 2.
Pipette $25.0\text{ cm}^3$ of FA 3 into a conical flask.
Add 5 drops of phenolphthalein indicator to the flask. The indicator should remain colourless.
Titrate FA 3 with FA 2 until the indicator changes to a permanent pale pink colour.
Perform a rough titration and record your burette readings in the space below.

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

Carry out as many accurate titrations as you think necessary to obtain consistent results.
Make sure 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 FA 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 have obtained this value.

$25.0\text{ cm}^3$ of FA 3 required ................ $\text{cm}^3$ of FA 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 sodium hydroxide were present in the volume of FA 2 calculated in (b).

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

(ii)

The phosphoric acid, FA 1, that you weighed out contained $8.40\%$ by mass of $\text{H}_3\text{PO}_4$ . Calculate the mass of $\text{H}_3\text{PO}_4$ that you weighed out.

mass of $\text{H}_3\text{PO}_4$ = ..................... g

(iii)

Calculate how many moles of $\text{H}_3\text{PO}_4$ were present in $25.0\text{ cm}^3$ of the diluted solution, FA 3.
( $A_r$ : $\text{H}, 1.0$ ; $\text{O}, 16.0$ ; $\text{P}, 31.0$ )

moles of $\text{H}_3\text{PO}_4$ = .................... mol

(iv)

Use your answers to (i) and (iii) to calculate how many moles of $\text{NaOH}$ react with $1$ mole of $\text{H}_3\text{PO}_4$ .
Give your answer to the nearest whole number.

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

(v)

When $\text{NaOH}$ reacts with $\text{H}_3\text{PO}_4$ , the salt formed could be $\text{NaH}_2\text{PO}_4$ , $\text{Na}_2\text{HPO}_4$ or $\text{Na}_3\text{PO}_4$ .
Use your answer to (iv) to deduce which one of these three salts was the major product formed during the titration.
Write the equation for the reaction of $\text{NaOH}$ with $\text{H}_3\text{PO}_4$ to produce this salt.

(d)

(i)

A $25\text{ cm}^3$ pipette is accurate to $\pm 0.06\text{ cm}^3$ .
Calculate the maximum percentage error when the pipette was used to measure solution FA 3.

percentage error in measuring FA 3 = ...................... %

(ii)

State the maximum error in the mass of the $100\text{ cm}^3$ beaker used in the dilution of FA 1.

maximum error = ............. g

(iii)

Calculate the maximum percentage error in the mass of FA 1 used.

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

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2 more questions

Q2Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation12M
Q3Qualitative Analysis · Presentation of Data and Observations · Analysis, Conclusions and Evaluation · Manipulation, Measurement and Observation13M

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