Edexcel IAL Chemistry Unit 3 Practical Skills

[Maximum number: 9]

A group of students carried out a thermochemistry experiment to determine the relative atomic mass of a metal, M.

Procedure Step 1 Transfer $50.0 \mathrm{~cm}^{3}$ of a $1.35 \mathrm{~mol} \mathrm{dm}^{-3}$ solution of copper(II) sulfate to an expanded polystyrene cup placed in a glass beaker. Step 2 Weigh out, as accurately as possible, a known mass of the finely powdered metal M. Step 3 Measure the temperature of the copper(II) sulfate solution. Step 4 Quickly add all of the powdered metal, stir the mixture continuously and note the highest temperature reached.

Figure for Question Unit 3: Practical Skills in Chemistry I question 1 — Edexcel A-Level Chemistry AS

(a)

Each student carried out the experiment using a different mass of the metal.

[ 3 ]

(i)

Give a reason, other than preventing heat loss, for placing the polystyrene cup in a glass beaker.

[ 1 ]

(ii)

Name the piece of apparatus suitable for measuring $50.0 \mathrm{~cm}^{3}$ of copper(II) sulfate solution.

[ 1 ]

pipette

Accept

$50 \mathrm{~cm}^{3}$ pipette

$25 \mathrm{~cm}^{3}$ pipette (twice) graduated pipette

Allow

burette / measuring cylinder

/ volumetric flask

Do not award beaker / flask

(1)

(iii)

Powdered metal reacts much faster than filings or granules. Suggest why this is important in this experiment.

[ 1 ]

(b)

The students' results were collected in a table.

Table for Question (b) — Edexcel A-Level Chemistry AS

[ 6 ]

(i)

Plot a labelled graph of mass of metal on the horizontal axis against temperature change on the vertical axis.

Table for Question (i) — Edexcel A-Level Chemistry AS

[ 3 ]

(ii)

Determine the mass of metal M that reacts exactly with $50.0 \mathrm{~cm}^{3}$ of $1.35 \mathrm{~mol} \mathrm{dm}^{-3}$ copper(II) sulfate by drawing appropriate best-fit straight lines. You must show your working on the graph. Mass of metal M g

[ 2 ]

two best fit lines

(1)

- correct reading of the mass at the intersection of the best fit lines (1)

Allow reasonable best fit line close to points 1, 2, 4 and 5

If the line is continued to the axis it must be within one small square of the origin

Do not award a BFL that includes point 3

Second line should be horizontal at about $55^{\circ} \mathrm{C}$

Do not award if horizontal line deviates 2 small squares from extreme left to extreme right

4.1-4.5 (g)

TE on lines drawn but do not award unless value is obtained from the intersection of a straight, angled best fit line and a straight best fit line through the last four points

(2)

(iii)

One mass of M in the experiment gave an anomalous data point. Suggest a reason, other than measurement error, for this anomaly.

[ 1 ]

[Maximum number: 5]

A student carried out an experiment to identify the metal M in the hydrated carbonate $\mathrm{M}_{2} \mathrm{CO}_{3} \cdot 10 \mathrm{H}_{2} \mathrm{O}$ . A solution was made by dissolving 3.56 g of the hydrated metal carbonate in distilled water and making the volume up to $250.0 \mathrm{~cm}^{3}$ in a volumetric flask. $25.0 \mathrm{~cm}^{3}$ of this solution was placed in a conical flask and titrated with $0.100 \mathrm{~mol} \mathrm{dm}^{-3}$ of hydrochloric acid. The equation for the reaction is

\mathrm{M}_{2} \mathrm{CO}_{3}(\mathrm{aq})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow 2 \mathrm{MCl}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})+\mathrm{CO}_{2}(\mathrm{~g})

(a)

Name a suitable piece of apparatus to measure the $25.0 \mathrm{~cm}^{3}$ of solution.

[ 1 ]

(b)

The results of the titration are shown.

[ 1 ]

(i)

Complete the table.

[ 1 ]

(c)

The titration was repeated without using an indicator. Describe how you would obtain large, dry crystals of the metal chloride, MCl , from this titration solution. Diagrams are not required.

[ 3 ]

[Maximum number: 5]

Precipitation reactions can be used to determine the formulae of compounds.
A bottle has a torn label which shows that it contains a nitrate with the formula X(NO_3)_2.

Figure for Question Unit 3: Practical Skills in Chemistry I question 3 — Edexcel A-Level Chemistry AS

Procedure
- a technician dissolved a sample of 12.41 g of the nitrate in deionised water to make 100 cm^3 of solution
- the technician pipetted 5.0 cm^3 of 1.50 mol dm^-3 potassium iodide into each of a series of test tubes
- each test tube then had a volume of the nitrate solution added to it as shown in the table
- a cloudy yellow solution formed, and the precipitate was allowed to settle
- the height of the precipitate was then measured.

The equation for the reaction is

\mathrm{X}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+2 \mathrm{KI}(\mathrm{aq}) \rightarrow \mathrm{XI}_{2}(\mathrm{~s})+2 \mathrm{KNO}_{3}(\mathrm{aq})

The results of the experiment are shown.

Table for Question Unit 3: Practical Skills in Chemistry I question 3 — Edexcel A-Level Chemistry AS

(a)

Plot the data on the grid.

Figure for Question (a) — Edexcel A-Level Chemistry AS

[ 2 ]

(b)

Determine the minimum volume of nitrate solution needed to react completely with 5.0 cm^3 of potassium iodide solution. You must show your working on the graph.

[ 2 ]

(c)

Suggest why some of the precipitate heights may be above the maximum height expected. Assume there were no measurement errors.

[ 1 ]

[Maximum number: 4]

In an experiment to measure the enthalpy change of combustion of ethanol, a student uses the apparatus shown.

Figure for Question Unit 3: Practical Skills in Chemistry I question 4 — Edexcel A-Level Chemistry AS

(a)

A student devised the procedure shown to determine the enthalpy change of combustion for ethanol.
- $150 \mathrm{~cm}^{3}$ of water is placed in the beaker and its temperature recorded
- the burner containing ethanol is weighed, placed under the beaker of water and then lit
- after 5 minutes the burner is removed
- the temperature of the water in the beaker is recorded again
- the flame is extinguished, and the burner and its contents are reweighed.

[ 1 ]

(i)

The uncertainty in each reading of the thermometer was $\pm 0.1^{\circ} \mathrm{C}$ .

Calculate the percentage uncertainty in the temperature change in the student's experiment.

[ 1 ]

calculation of percentage uncertainty

Example of calculation:
$\begin{aligned} (2 \times 0.1 \div 7.3) \times 100=2.7397 / 2.740 / 2.74 / 2.7 / 3 \% \text { Ignore SF } \end{aligned}$

(b)

The student repeated the experiment using the same apparatus, in an attempt to improve its accuracy. The water was heated for 15 minutes before measuring the final temperature and reweighing the burner.

The data book value for the standard enthalpy change of combustion of ethanol is $-1367 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

The teacher said that the changes would improve the percentage uncertainty in the measurement of mass and temperature but have little effect on the accuracy of the value obtained.

[ 3 ]

(i)

State how the changes in the experiment improve the percentage uncertainties in the measurement of mass and temperature. No calculation is required.

[ 1 ]

(ii)

Explain how the difference between uncertainty and accuracy led the teacher to make this statement.

[ 2 ]

Edexcel IAL Chemistry Unit 3 Practical Skills in Chemistry I

Covered syllabus points

Covered syllabus points

Unit 3: Practical Skills in Chemistry I question 1

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Question (iii)

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Unit 3: Practical Skills in Chemistry I question 2

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Unit 3: Practical Skills in Chemistry I question 3

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Unit 3: Practical Skills in Chemistry I question 4

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