2. Amount of Substance Exam Questions With Complete
Solutions
When aluminium is added to an aqueous solution of copper(II)
chloride, CuCl2, copper metal and aluminium chloride, AlCl3,
are formed. Write an equation to represent this reaction. (1)
2A1+ 3CuCl2 → 2A1C13 + 3Cu
Lead(II) nitrate may be produced by the reaction between nitric
acid and lead(II) oxide as shown by the equation:
PbO + 2HNO3 → Pb(NO3)2 + H2O
An excess of lead(II) oxide was allowed to react with 175cm3 of
1.50moldm-3 nitric acid.
Calculate the maximum mass of lead(II) nitrate which could be
obtained from this reaction. (4)
n HNO3: 1.5 x 175/1000 = 0.2625mol
n Pb(NO3)2 = 0.2625/2 = 0.131mol
m Pb(NO3)2 = 331.2 x 0.131 = 43.5g
An equation representing the thermal decomposition of lead(II)
nitrate: 2Pb(NO3)2(s) → 2PbO(s) + 4NO2(g) + O2(g)
A sample of lead(II) nitrate was heated until the decomposition
was complete. At a temperature of 500K and a pressure of
100kPa, the total volume of the gaseous mixture produced was
found to be 1.50×10^-4m3.
State the ideal gas equation and use it to calculate the total
number of moles of gas
produced in this decomposition. (3)
,Ideal gas equation: pV=nRT
Total number of moles of gas: 100kPa = 100,000Pa
100,000 x 1.5x10^-4 = n x 8.31 x 500
n = 3.61 x 10^-3
Deduce the number of moles, and the mass, of NO2 present in
this gaseous mixture. (4)
Number of moles of NO2: 4/5 × 3.61x10^-3 = 2.89x10^-3
Mass of NO2: 46 × 2.89×10^-3 = 0.133g
Ammonia, NH3, reacts with sodium to form sodium amide,
NaNH2, and hydrogen. Write an equation for the reaction
between ammonia and sodium. (1)
2Na + 2NH3 → 2NaNH2 + H2
State what is meant by the term empirical formula. (1)
Simplest ratio of atoms of each element in a compound
A salt, X, contains 16.2% by mass of magnesium, 18.9% by
mass of nitrogen and 64.9% by mass of oxygen. Determine the
empirical formula of X. (2)
Mg: 16.2/24 = 0.675
N: 18.9/14 = 1.37
O: 64.9/16 = 4.06
1:2:6 therefore EF = MgN2O6
Calculate the original number of moles of NaOH in 100cm3 of
0.500moldm-3 aqueous sodium hydroxide. (1)
0.500 x 100/1000 = 0.05mol
, Calculate the number of moles of HCl in 27.3 cm3 of 0.600 mol
dm-3 hydrochloric acid. (1)
0.600 x 27.3/1000 = 0.01638mol
Deduce the number of moles of the unreacted NaOH neutralised
by the hydrochloric acid. (1)
0.01638mol
Calculate the number of moles of NaOH which reacted with the
ammonium sulphate. (1)
0.05 - 0.01638 = 0.03362mol
(NH4)2SO4 + 2NaOH → 2NH3 + Na2SO4 + 2H2O. Calculate
the number of moles and the mass of ammonium sulphate in the
sample. (3)
Moles of ammonium sulphate: 0.03362/2 = 0.01681mol
Mass of ammonium sulphate: 0.01681 x 132.1 = 2.22g
A 0.143g gaseous sample of ammonia occupied a volume of
2.86 × 10-4 m3 at a temperature T and a pressure of 100 kPa.
State the ideal gas equation, calculate the number of moles of
ammonia present and deduce the value of the temperature T. (4)
Ideal Gas Equation: pV=nRT
Moles of Ammonia: n = 0.143/17 = 8.41x10^-3
T: 100,000 × 2.86×10^−4 = 8.4×10^−3 x 8.31 x T
T = 409.23K
Compound A is an oxide of sulphur. At 415K, a gaseous sample
of A, of mass 0.304g, occupied a volume of 127cm3 at a
pressure of 103kPa.
Solutions
When aluminium is added to an aqueous solution of copper(II)
chloride, CuCl2, copper metal and aluminium chloride, AlCl3,
are formed. Write an equation to represent this reaction. (1)
2A1+ 3CuCl2 → 2A1C13 + 3Cu
Lead(II) nitrate may be produced by the reaction between nitric
acid and lead(II) oxide as shown by the equation:
PbO + 2HNO3 → Pb(NO3)2 + H2O
An excess of lead(II) oxide was allowed to react with 175cm3 of
1.50moldm-3 nitric acid.
Calculate the maximum mass of lead(II) nitrate which could be
obtained from this reaction. (4)
n HNO3: 1.5 x 175/1000 = 0.2625mol
n Pb(NO3)2 = 0.2625/2 = 0.131mol
m Pb(NO3)2 = 331.2 x 0.131 = 43.5g
An equation representing the thermal decomposition of lead(II)
nitrate: 2Pb(NO3)2(s) → 2PbO(s) + 4NO2(g) + O2(g)
A sample of lead(II) nitrate was heated until the decomposition
was complete. At a temperature of 500K and a pressure of
100kPa, the total volume of the gaseous mixture produced was
found to be 1.50×10^-4m3.
State the ideal gas equation and use it to calculate the total
number of moles of gas
produced in this decomposition. (3)
,Ideal gas equation: pV=nRT
Total number of moles of gas: 100kPa = 100,000Pa
100,000 x 1.5x10^-4 = n x 8.31 x 500
n = 3.61 x 10^-3
Deduce the number of moles, and the mass, of NO2 present in
this gaseous mixture. (4)
Number of moles of NO2: 4/5 × 3.61x10^-3 = 2.89x10^-3
Mass of NO2: 46 × 2.89×10^-3 = 0.133g
Ammonia, NH3, reacts with sodium to form sodium amide,
NaNH2, and hydrogen. Write an equation for the reaction
between ammonia and sodium. (1)
2Na + 2NH3 → 2NaNH2 + H2
State what is meant by the term empirical formula. (1)
Simplest ratio of atoms of each element in a compound
A salt, X, contains 16.2% by mass of magnesium, 18.9% by
mass of nitrogen and 64.9% by mass of oxygen. Determine the
empirical formula of X. (2)
Mg: 16.2/24 = 0.675
N: 18.9/14 = 1.37
O: 64.9/16 = 4.06
1:2:6 therefore EF = MgN2O6
Calculate the original number of moles of NaOH in 100cm3 of
0.500moldm-3 aqueous sodium hydroxide. (1)
0.500 x 100/1000 = 0.05mol
, Calculate the number of moles of HCl in 27.3 cm3 of 0.600 mol
dm-3 hydrochloric acid. (1)
0.600 x 27.3/1000 = 0.01638mol
Deduce the number of moles of the unreacted NaOH neutralised
by the hydrochloric acid. (1)
0.01638mol
Calculate the number of moles of NaOH which reacted with the
ammonium sulphate. (1)
0.05 - 0.01638 = 0.03362mol
(NH4)2SO4 + 2NaOH → 2NH3 + Na2SO4 + 2H2O. Calculate
the number of moles and the mass of ammonium sulphate in the
sample. (3)
Moles of ammonium sulphate: 0.03362/2 = 0.01681mol
Mass of ammonium sulphate: 0.01681 x 132.1 = 2.22g
A 0.143g gaseous sample of ammonia occupied a volume of
2.86 × 10-4 m3 at a temperature T and a pressure of 100 kPa.
State the ideal gas equation, calculate the number of moles of
ammonia present and deduce the value of the temperature T. (4)
Ideal Gas Equation: pV=nRT
Moles of Ammonia: n = 0.143/17 = 8.41x10^-3
T: 100,000 × 2.86×10^−4 = 8.4×10^−3 x 8.31 x T
T = 409.23K
Compound A is an oxide of sulphur. At 415K, a gaseous sample
of A, of mass 0.304g, occupied a volume of 127cm3 at a
pressure of 103kPa.
