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ASChemistryOCRExam Questions1. Development of Practical Skills in Chemistry1.1 Physical Chemistry Practicals

Physical Chemistry Practicals (OCR AS Chemistry)

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1.1 Physical Chemistry Practicals

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1.1 Physical Chemistry Practicals

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11 mark

A student used the apparatus below in an experiment to determine the enthalpy change of combustion of methanol.
The student measured 100 cm3 and poured it into the beaker.

ocr-a2-sq-h-61-3-10-calorimetry

The student measured a temperature rise of 10.5 oC.
The student calculated the amount of energy transferred to the water.
Which of the following uses the appropriate number of significant figures and correct standard form to represent the result of the calculation?

  • 4.389 × 103 J

  • 4.39 × 103 J

  • 43.9 × 102 J

  • 44.0 × 102 J

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2
Sme Calculator1 mark

15 g of hexane is burned and used to heat 50 cm3 of water by 21 oC.

What is the amount of energy transferred to the water?

Assume a water density of 1 g cm–3 and a specific heat capacity of 4.18 J g–1 K–1.

  • 1317 J

  • 4389 J

  • 1317 kJ

  • 4389 kJ

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31 mark

An experiment is set up to measure the volume of carbon dioxide released when copper carbonate reacts with sulfuric acid. The volume of carbon dioxide produced is measured using a gas syringe.

Which of the following are possible sources of error for this experiment?

  1. Carbon dioxide dissolving in water.
  2. Not all of the copper carbonate reacting.
  3. Not all of the copper carbonate being transferred to the reaction vessel.

  • 1, 2 and 3

  • Only 1 and 2

  • Only 2 and 3

  • Only 1 

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41 mark

An experiment is set up to determine the relative atomic mass of magnesium.

A known mass of magnesium is placed inside a conical flask with sulfuric acid. The conical flask is connected via a delivery tube to an inverted measuring cylinder in a water trough.

Which of the following statements is not correct?

  • Some carbon dioxide may escape before the delivery tube is connected.

  • Some magnesium may not react with the sulfuric acid.

  • The sulfuric acid should be in excess.

  • Using a balance to measure the mass lost is not appropriate for this experiment.

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51 mark

A known mass of magnesium powder is placed in a crucible and ignited to form magnesium oxide. The mass of magnesium oxide produced is recorded and calculations are performed to determine the chemical formula of magnesium oxide.

Which statement will result in a more accurate mass of magnesium oxide being recorded?

  • Keeping the lid on the crucible during the reaction.

  • Using magnesium ribbon instead of magnesium powder.

  • Measuring the mass of the weighing apparatus before and after the magnesium is added to the crucible.

  • Using sandpaper to remove any oxide coating from the magnesium used in the experiment.

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61 mark

Why is it important to fill the space below the tap in a burette with acid or alkali before beginning a titration?

Which reaction will distinguish between a primary and a secondary alcohol?

  • To give a more accurate result

  • The burette’s scale does not include this volume

  • To give concordant results

  • An incorrect volume would be recorded

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71 mark

Which piece of equipment is not needed to make a 250 cm3 standard solution of sodium hydroxide?

  • Burette

  • Volumetric flask

  • Beaker

  • Balance

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81 mark

A student was titrating a 25 cm3 sample of a solution of sodium hydrogen carbonate with 0.105 mol dm-3 hydrochloric acid. She wanted to reduce the percentage uncertainty in using the burette.

This could best be achieved by

  • Using more drops of the indicator

  • Repeating the titration several times and taking an average reading

  • Titrating a smaller volume of sodium hydrocarbonate solution

  • Titrating a larger volume of sodium hydrogen carbonate solution

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91 mark

A student planned to carry out an experiment to measure the enthalpy change when magnesium displaces copper from copper(II) sulfate solution.

The method they followed was

  • 50 cm3 of 0.20 mol dm-3 copper(II) sulfate solution was measured in a measuring cylinder and transferred to a polystyrene cup
  • A thermometer was placed in the cup and the initial temperature recorded
  • 1.86 g of magnesium powder was added to the cup and the mixture was stirred
  • The final temperature was recorded

In this experiment

  • The copper(II) sulfate solution should be the excess reactant

  • The magnesium powder is all used up

  • It is unnecessary to weigh the magnesium powder so precisely

  • The measuring cylinder is the largest source of error

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101 mark

The enthalpy of neutralisation of hydrochloric acid takes place as follows

                            HCl (aq)+ NaOH (aq) → NaCl (aq)+ H2O (l)

State the most suitable piece of laboratory equipment for carrying out the experiment described.

  • Conical flask

  • Evaporating basin

  • Copper calorimeter

  • Polystyrene cup

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11 mark

A student completes a practical, at a temperature of 17 oC and pressure of 100 kPa, to determine the composition of copper(II) carbonate, CuCO3•Cu(OH)2. The volume of carbon dioxide that the student records is 108 cm3.

Which calculation should the student perform to determine the number of moles of copper carbonate?

  • begin mathsize 14px style n equals fraction numerator 108 over denominator 24.0 end fraction end style

  • n equals fraction numerator 8.314 cross times 290 over denominator 100000 cross times left parenthesis 108 cross times 10 to the power of negative 6 end exponent right parenthesis end fraction

  • n equals fraction numerator 100000 cross times 108 over denominator 8.314 cross times 290 end fraction

  • n equals fraction numerator 100000 cross times open parentheses 108 cross times 10 to the power of negative 6 end exponent close parentheses over denominator 8.314 cross times 290 end fraction

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21 mark

The composition of copper(II) carbonate, CuCO3•Cu(OH)2, can be determined by reacting magnesium with sulfuric acid.

Which of the following statements about this practical is not correct?

  • The loss of gas while replacing the bung results in a smaller mass of copper(II) carbonate

  • The percentage uncertainty in the volume of sulfuric acid used should be calculated

  • Carbon dioxide dissolving in the water results in a smaller mass of copper(II) carbonate

  • The reaction can be performed with nitric acid instead of sulfuric acid

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31 mark

Which statement(s) is/are correct when heating magnesium in a crucible to determine the formula of magnesium oxide?

  1. Leaving the crucible open will give a lower mass of magnesium oxide
  2. Covering the crucible with a lid will give a lower mass of magnesium oxide
  3. Using magnesium powder will give a lower mass of magnesium oxide

  • 1, 2 and 3

  • Only 1 and 2

  • Only 2 and 3

  • Only 1

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41 mark

The following mass measurements were taken when preparing a 250 cm3 standard solution.

  Mass / g
Weighing boat  5.63
Weighing boat + solute 10.28

The uncertainty of the balance is ±0.005 g.

What is the percentage uncertainty in the mass of the solute used?

  • 0.22%

  • 0.11%

  • 0.18%

  • 0.10%

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51 mark

A group of students titrate 25.0 cm3 of the same solution of hydrochloric acid, HCl(aq), with sodium hydroxide, NaOH (aq).

Compared to the other students, one student obtains a smaller titre.

Which statement explains the smaller titre?

  • The jet space of the burette is still filled with air before starting the titration

  • The pipette is rinsed with water before filling with HCl (aq)

  • The burette readings are taken from the top of the meniscus instead of the bottom of the meniscus

  • The conical flask is rinsed with distilled water before completing the titration

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61 mark

A student made up a standard solution of sodium hydrogen carbonate using a two decimal place balance and a 250 cm3 volumetric flask.

Which of the following steps would make the final concentration of the solution more accurate?

  • Using a dropping pipette to add water up to the graduated mark

  • Using a three decimal place balance

  • Using a 100 cm3 flask instead of a 250 cm3 flask

  • Adding a stopper and inverting the flask 10 times

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71 mark

The enthalpy change of a chemical reaction can be found using the expression below.

q = mcΔT

Which statement about this expression is correct?

  • m represents the amount of substance in moles

  • The temperature is measured in Centigrade

  • c is the specific heat capacity of the substance

  • The unit of q is kJ

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81 mark

The enthalpy of neutralisation can be determined by measuring the temperature changes when a known amount of hydrochloric acid is added to a known amount of sodium hydroxide in a calorimeter.

What assumption is not made when calculating the enthalpy change?

  • There are negligible heat losses in the calorimeter

  • The specific heat capacity of the solution is the same as water

  • The density of solution is the same as water

  • The hydrochloric acid and sodium hydroxide react completely

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91 mark

A student carried out an experiment to measure the enthalpy change of combustion of methanol by using the methanol to heat water.

Which statement(s) is / are correct?

  1. The value of this enthalpy change is always negative
  2. The mass of the methanol burned is used in the q = mcΔT calculation
  3. The number of moles of water is required to calculate the enthalpy change

  • 1, 2 and 3

  • Only 1 and 2

  • Only 2 and 3

  • Only 1

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101 mark

When a sample of sodium hydroxide, NaOH, is dissolved in 250 cm3 of distilled water. 25.0 cm3 of this solution is titrated against sulfuric acid with a concentration of 2.00 mol dm-3. Complete neutralisation takes place with 15.0 cm3 of sulfuric acid.

2NaOH + H2SO4 → Na2SO4 + 2H2O

What is the mass of the original sample of sodium hydroxide dissolved in 250 cm3 of distilled water?

  • 1.20 g

  • 12.0 g

  • 2.40 g

  • 24.0 g

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11 mark

The student’s calculated enthalpy change was less exothermic than the value in data books.
Which of the following errors could have contributed to this result?

Error 1: After the final temperature was recorded, the student removed the burner from under the beaker. The flame burnt for a further 5 minutes before weighing the spirit burner.
Error 2: The student recorded the final temperature 5 minutes after removing the burner.
Error 3: The student spilt some water on the bench when pouring the water from the measuring cylinder into the beaker.

  • 1, 2 and 3

  • Only 1 and 2

  • Only 2 and 3

  • Only 1

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21 mark

A student prepares a standard solution and carries out a titration.
The standard solution is placed in the burette.
Which of the following would result in a titre that is larger than it should be?

1. Water is added to completely fill the volumetric flask, rather than to the graduation line
2. The conical flask is washed out with water before carrying out each titration
3. The pipette is washed out with water before carrying out each titration

  • 1, 2 and 3

  • Only 1 and 2

  • Only 2 and 3

  • Only 1

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31 mark

2.27 g of an unknown Group 1 carbonate was dissolved in 250 cm3 of water. 25 cm3 of this solution was titrated against 0.3 mol dm-3 hydrochloric acid. The titration was repeated and the average titre was found to be 10.95 cm3.

What is the identity of the metal in the Group 1 carbonate?

  • Lithium

  • Sodium

  • Potassium

  • Rubidium

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41 mark

25.0 cm3 of 0.300 mol dm-3 ethanoic acid was reacted with 50.0 cm3 of 0.150 mol dm-3 sodium hydroxide. The temperature over the course of the reaction was recorded below.

Time (s)

0.00

30.0

60.0

90.0

120

150

180

210

240

270

300

Temperature (oC)

21.0

22.0

25.0

27.0

31.0

33.0

35.0

39.0

38.0

38.0

37.0

What is the enthalpy change of this reaction?

  • 752 kJ mol-1

  • -752 kJ mol-1

  • 5.64 kJ mol-1

  • -5.64 kJ mol-1

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51 mark

The enthalpy change of combustion can be calculated using a simple calorimeter and a spirit burner.

Which equation would be used to calculate the enthalpy change of combustion using the data from this experiment?

  • begin mathsize 16px style increment straight H space equals space fraction numerator 1000 cross times straight m cross times straight c cross times left parenthesis initial space temperature space minus space final space temperature right parenthesis over denominator Mr space cross times left parenthesis initial space mass space of space burner space minus space final space mass space of space burner right parenthesis end fraction end style

  • increment straight H space equals space fraction numerator Mr cross times straight m cross times straight c cross times left parenthesis final space temperature space minus space initial space temperature right parenthesis over denominator 1000 space cross times left parenthesis initial space mass space of space burner space minus space final space mass space of space burner right parenthesis end fraction

  • increment straight H space equals space fraction numerator 1000 cross times straight m cross times straight c cross times left parenthesis final space temperature space minus space initial space temperature right parenthesis over denominator Mr space cross times left parenthesis initial space mass space of space burner space minus space final space mass space of space burner right parenthesis end fraction

  • increment straight H space equals space fraction numerator Mr cross times straight m cross times straight c cross times left parenthesis initial space temperature space minus space final space temperature right parenthesis over denominator 1000 space cross times left parenthesis initial space mass space of space burner space minus space final space mass space of space burner right parenthesis end fraction

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