Edexcel A-Level Chemistry AS Practical Skills in Chemistry I Questions

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.

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

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.

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

The results of the experiment are shown.

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.

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

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

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

The results of the titration are shown.

Complete the table.

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.

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

The results of the experiment are shown.

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

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

A class of students carried out experiments to determine the enthalpy change for the reaction of magnesium metal with hydrochloric acid. The following method was used. Step1 A 1.00 m length of magnesium ribbon was cleaned using sandpaper, weighed and cut into 10 cm lengths. Step $250 \mathrm{~cm}^{3}$ of dilute hydrochloric acid (an excess) was placed into a polystyrene cup and the temperature measured. Step 3 A 10 cm length of magnesium ribbon was added to the hydrochloric acid. The solution was stirred gently and the maximum temperature recorded.

Results

Explain why the magnesium ribbon was cleaned with sandpaper before being weighed.

Sodium hydroxide solution reacts with carbon dioxide in the air and should be standardised before use. Ethanedioic acid may be used for this standardisation.

The titration results are shown.

Complete the table and calculate the mean titre.

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

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.

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

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

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

This question is about magnesium, magnesium oxide and magnesium sulfate.

Hydrated crystals of magnesium sulfate, $\mathrm{MgSO}_{4} \cdot 7 \mathrm{H}_{2} \mathrm{O}$, can be made by reacting magnesium with sulfuric acid.

In an experiment, magnesium was added to $30.0 \mathrm{~cm}^{3}$ of $0.500 \mathrm{~mol} \mathrm{dm}^{-3}$ sulfuric acid.
$\left[M_{\mathrm{r}}\right.$ value: $\mathrm{MgSO}_{4} \cdot 7 \mathrm{H}_{2} \mathrm{O}=246.4 \quad A_{\mathrm{r}}$ value: $\left.\mathrm{Mg}=24.3\right]$

State how the magnesium sulfate solution could be separated from the mixture produced in this experiment.

Magnesium ethanedioate ( $\mathrm{MgC}_{2} \mathrm{O}_{4}$ ) decomposes on gentle heating to form magnesium carbonate and carbon monoxide.

State why the thermal decomposition of magnesium ethanedioate should be carried out in a fume cupboard.

The use of ammonium carbonate in smelling salts is due to the formation of ammonia which counters the effects that cause fainting. When ammonium carbonate is heated gently, it decomposes to form ammonia, water and carbon dioxide.

Complete the table, giving a chemical test, not involving indicators, and its result for each of the products of the decomposition of ammonium carbonate.

Cyclohexene may be prepared from cyclohexanol using 85% phosphoric(V) acid.

A simplified procedure for this preparation is shown.
Step 1 Accurately weigh about 4 g of cyclohexanol into a pear-shaped flask.
Step 2 Add about $0.5 \mathrm{~cm}^{3}$ of 85% phosphoric(V) acid and a few anti-bumping granules to the flask.

Step 3 Set up the apparatus for fractional distillation.
Step 4 Heat the flask and collect the distillate that contains impure cyclohexene and water.

Step 5 Separate the impure cyclohexene and water, using a separating funnel.
Step 6 Add a few granules of anhydrous calcium chloride to the impure cyclohexene and allow the mixture to stand.

Step 7 Decant the impure cyclohexene into a clean pear-shaped flask. Distil the cyclohexene and weigh the distillate.

Data

Separate samples of cyclohexanol and the cyclohexene product were tested with phosphorus(V) chloride and with bromine water.

Complete the table to show the observations.

A student used a precipitation titration to determine the value of x in the formula of a sample of hydrated barium chloride, $\mathrm{BaCl}_{2} \cdot \times \mathrm{H}_{2} \mathrm{O}$.

Procedure

Step 1 Prepare a solution by dissolving 1.57 g of $\mathrm{BaCl}_{2} \cdot \times \mathrm{H}_{2} \mathrm{O}$ in deionised water, making the solution up to the mark in a $250.0 \mathrm{~cm}^{3}$ volumetric flask and then mixing thoroughly.

Step 2 Use a pipette to transfer $10.0 \mathrm{~cm}^{3}$ of the barium chloride solution into a conical flask.
Add excess sodium sulfate solution and swirl the mixture.
Step 3 Fill a burette with $0.0324 \mathrm{~mol} \mathrm{dm}^{-3}$ silver nitrate solution.
Step 4 Add three drops of potassium chromate(VI) solution to the conical flask and titrate the contents, while swirling, with the silver nitrate solution.
The end-point is shown by the appearance of a permanent pale red precipitate.

Step 5 Repeat Steps 2 to 4 until concordant results are obtained.
During the titration, two precipitation reactions occur.
Reaction 1 Silver ions react with chloride ions forming silver chloride.

Reaction 2 Once all chloride ions have reacted, silver ions react with chromate(VI) ions to form a red precipitate of silver chromate(VI).

Some data obtained in the experiment are shown.

Complete the table and use the concordant results to calculate the mean titre.

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

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.