OCR A LEVEL CHEMISTRY EXAM ANSWERS
A student carries out experiments using acids, bases and salts.
Calcium nitrate, Ca(NO3)2, is an example of a salt.
The student prepares a solution of calcium nitrate by reacting dilute nitric acid,
HNO3,
with the base calcium hydroxide, Ca(OH)2.
(i) Why is calcium nitrate an example of a salt? - CORRECT ANSWER ✅✅✅The
H+
ion in an (nitric) acid has been replaced by a metal ion
(ii) Write the equation for the reaction between dilute nitric acid and calcium
hydroxide. Include state symbols. - CORRECT ANSWER ✅✅✅2HNO3(aq) +
Ca(OH)2(aq) → Ca(NO3) 2 (aq)+ 2H2O(l)
(iii) Explain how the hydroxide ion in aqueous calcium hydroxide acts as a base
when it neutralises dilute nitric acid. - CORRECT ANSWER ✅✅✅Accepts a
proton OR accepts H+
(a) A student carries out a titration to find the concentration of some sulfuric acid.
The student finds that 25.00 cm3
of 0.0880 mol dm-3 aqueous sodium hydroxide,
NaOH, is neutralised by 17.60 cm3
of dilute sulfuric acid, H2SO4.
H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O(l)
(i) Calculate the amount, in moles, of NaOH used. - CORRECT ANSWER
✅✅✅0.0880 ×25.0/ 1000 = 2.20 × 10^-3 OR 0.00220 mol
(ii) Determine the amount, in moles, of H2SO4 used. - CORRECT ANSWER
✅✅✅00220.0/2 = 1.10 × 10^-3 OR 0.00110 mol
(iii) Calculate the concentration, in mol dm-3, of the sulfuric acid. - CORRECT
ANSWER ✅✅✅0.00110×1000/17.60 = 0.0625 mol dm^-3 OR 6.25 × 10^-2 mol
dm^-3
After carrying out the titration in (a), the student left the resulting solution to
crystallise. White crystals were formed, with a formula of Na2SO4•x H2O and a
molar mass of 322.1 g mol-1
(i) What term is given to the '•x H2O' part of the formula?
. - CORRECT ANSWER ✅✅✅(The number of) Water(s) of crystallisation
After carrying out the titration in (a), the student left the resulting solution to
crystallise. White crystals were formed, with a formula of Na2SO4•x H2O and a
molar mass of 322.1 g mol-1
(ii) Using the molar mass of the crystals, calculate the value of x. - CORRECT
ANSWER ✅✅✅142.1
(322.1-142.1) / 18.0 = 10
, Ammonium compounds such as ammonium sulfate, (NH4)2SO4, can be used as
fertilisers.
(i) Write a balanced equation to show how ammonium sulfate could be formed by the
reaction between aqueous ammonia and sulfuric acid. - CORRECT ANSWER
✅✅✅2NH3 + H2SO4 -> (NH4)2SO4
(ii) Ammonium sulfate is an example of a salt formed when an acid is neutralised by
a base.
Explain what is meant by the term salt. - CORRECT ANSWER ✅✅✅when the
H+ in an acid is replaced by a metal ion OR an
ammonium ion OR a + ion
(iii) Why is ammonia acting as a base in this neutralisation? - CORRECT ANSWER
✅✅✅accepts a proton OR accepts H+
(iv) What is the relative formula mass of (NH4)2SO4?
Give your answer to one decimal place. - CORRECT ANSWER ✅✅✅132.1
Epsom salts can be used as bath salts to help relieve aches and pains.
Epsom salts are crystals of hydrated magnesium sulfate, MgSO4•xH2O.
A sample of Epsom salts was heated to remove the water. 1.57 g of water was
removed leaving behind 1.51 g of anhydrous MgSO4.
(i) Calculate the amount, in mol, of anhydrous MgSO4 formed. - CORRECT
ANSWER ✅✅✅M(MgSO4) = 120.4 OR 120 (g mol-1)
mol MgSO4 = 1.51/120.4 = 0.0125 mol
(ii) Calculate the amount, in mol, of H2O removed. - CORRECT ANSWER
✅✅✅1.57/18.0 = 0.0872(2) (mol)
(iii) Calculate the value of x in MgSO4•xH2O. - CORRECT ANSWER ✅✅✅× = 7
Calcium oxide reacts with water and with nitric acid.
State the formula of the calcium compound formed when:
(i) calcium oxide reacts with water, .............................................................
[1]
(ii) calcium oxide reacts with nitric acid. ........................................................ -
CORRECT ANSWER ✅✅✅(i) Ca(OH)2
(ii) Ca(NO3)2
Calcium and its compounds, have properties typical of Group 2 in the Periodic Table.
Calcium carbonate, CaCO3, reacts with acids such as nitric acid.
A student neutralised 2.68 g of CaCO3 with 2.50 mol dm-3 nitric acid, HNO3.
The equation for this reaction is shown below.
CaCO3(s) + 2HNO3(aq) → Ca(NO3)2(aq) + CO2(g) + H2O(l)
(i) Determine the amount, in mol, of CaCO3 reacted. - CORRECT ANSWER
✅✅✅(i) Molar mass of CaCO3
= 100.1 g mol-1 (1)
2.68/100.1 = 0.0268/0.027 (1)
A student carries out experiments using acids, bases and salts.
Calcium nitrate, Ca(NO3)2, is an example of a salt.
The student prepares a solution of calcium nitrate by reacting dilute nitric acid,
HNO3,
with the base calcium hydroxide, Ca(OH)2.
(i) Why is calcium nitrate an example of a salt? - CORRECT ANSWER ✅✅✅The
H+
ion in an (nitric) acid has been replaced by a metal ion
(ii) Write the equation for the reaction between dilute nitric acid and calcium
hydroxide. Include state symbols. - CORRECT ANSWER ✅✅✅2HNO3(aq) +
Ca(OH)2(aq) → Ca(NO3) 2 (aq)+ 2H2O(l)
(iii) Explain how the hydroxide ion in aqueous calcium hydroxide acts as a base
when it neutralises dilute nitric acid. - CORRECT ANSWER ✅✅✅Accepts a
proton OR accepts H+
(a) A student carries out a titration to find the concentration of some sulfuric acid.
The student finds that 25.00 cm3
of 0.0880 mol dm-3 aqueous sodium hydroxide,
NaOH, is neutralised by 17.60 cm3
of dilute sulfuric acid, H2SO4.
H2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O(l)
(i) Calculate the amount, in moles, of NaOH used. - CORRECT ANSWER
✅✅✅0.0880 ×25.0/ 1000 = 2.20 × 10^-3 OR 0.00220 mol
(ii) Determine the amount, in moles, of H2SO4 used. - CORRECT ANSWER
✅✅✅00220.0/2 = 1.10 × 10^-3 OR 0.00110 mol
(iii) Calculate the concentration, in mol dm-3, of the sulfuric acid. - CORRECT
ANSWER ✅✅✅0.00110×1000/17.60 = 0.0625 mol dm^-3 OR 6.25 × 10^-2 mol
dm^-3
After carrying out the titration in (a), the student left the resulting solution to
crystallise. White crystals were formed, with a formula of Na2SO4•x H2O and a
molar mass of 322.1 g mol-1
(i) What term is given to the '•x H2O' part of the formula?
. - CORRECT ANSWER ✅✅✅(The number of) Water(s) of crystallisation
After carrying out the titration in (a), the student left the resulting solution to
crystallise. White crystals were formed, with a formula of Na2SO4•x H2O and a
molar mass of 322.1 g mol-1
(ii) Using the molar mass of the crystals, calculate the value of x. - CORRECT
ANSWER ✅✅✅142.1
(322.1-142.1) / 18.0 = 10
, Ammonium compounds such as ammonium sulfate, (NH4)2SO4, can be used as
fertilisers.
(i) Write a balanced equation to show how ammonium sulfate could be formed by the
reaction between aqueous ammonia and sulfuric acid. - CORRECT ANSWER
✅✅✅2NH3 + H2SO4 -> (NH4)2SO4
(ii) Ammonium sulfate is an example of a salt formed when an acid is neutralised by
a base.
Explain what is meant by the term salt. - CORRECT ANSWER ✅✅✅when the
H+ in an acid is replaced by a metal ion OR an
ammonium ion OR a + ion
(iii) Why is ammonia acting as a base in this neutralisation? - CORRECT ANSWER
✅✅✅accepts a proton OR accepts H+
(iv) What is the relative formula mass of (NH4)2SO4?
Give your answer to one decimal place. - CORRECT ANSWER ✅✅✅132.1
Epsom salts can be used as bath salts to help relieve aches and pains.
Epsom salts are crystals of hydrated magnesium sulfate, MgSO4•xH2O.
A sample of Epsom salts was heated to remove the water. 1.57 g of water was
removed leaving behind 1.51 g of anhydrous MgSO4.
(i) Calculate the amount, in mol, of anhydrous MgSO4 formed. - CORRECT
ANSWER ✅✅✅M(MgSO4) = 120.4 OR 120 (g mol-1)
mol MgSO4 = 1.51/120.4 = 0.0125 mol
(ii) Calculate the amount, in mol, of H2O removed. - CORRECT ANSWER
✅✅✅1.57/18.0 = 0.0872(2) (mol)
(iii) Calculate the value of x in MgSO4•xH2O. - CORRECT ANSWER ✅✅✅× = 7
Calcium oxide reacts with water and with nitric acid.
State the formula of the calcium compound formed when:
(i) calcium oxide reacts with water, .............................................................
[1]
(ii) calcium oxide reacts with nitric acid. ........................................................ -
CORRECT ANSWER ✅✅✅(i) Ca(OH)2
(ii) Ca(NO3)2
Calcium and its compounds, have properties typical of Group 2 in the Periodic Table.
Calcium carbonate, CaCO3, reacts with acids such as nitric acid.
A student neutralised 2.68 g of CaCO3 with 2.50 mol dm-3 nitric acid, HNO3.
The equation for this reaction is shown below.
CaCO3(s) + 2HNO3(aq) → Ca(NO3)2(aq) + CO2(g) + H2O(l)
(i) Determine the amount, in mol, of CaCO3 reacted. - CORRECT ANSWER
✅✅✅(i) Molar mass of CaCO3
= 100.1 g mol-1 (1)
2.68/100.1 = 0.0268/0.027 (1)
