Questions on Atoms, Moles & Equations
1.
The element sulphur, S, has been known since ancient times and was originally
named ‘brimstone’.
(a) Sulphur exists as a mixture of four isotopes,
32
S,
33
S,
34
S and 36S.
(i) What is meant by the term isotopes?
..........................................................................................................................................
..................................................................................................................................... [1]
(ii) Complete the table below to show the composition of the
isotope
protons
neutrons
33
S and 34S isotopes.
electrons
33
S
34
S
[2]
(b) A sample of sulphur was collected from a volcano. The relative atomic mass of the
sulphur was determined.
(i) Define the term relative atomic mass.
..........................................................................................................................................
..........................................................................................................................................
..........................................................................................................................................
..................................................................................................................................... [3]
(ii) The sample of sulphur had the following percentage composition by mass:
32
S, 94.93%;
33
S, 0.76%;
34
S, 4.29%;
36
S, 0.02%.
Calculate the relative atomic mass of the sample of sulphur.
Give your answer to four significant figures.
Answer = ……………………………………… [2]
c) Sulphur also occurs naturally in sulphates. Gypsum is a common sulphate ore, containing
mainly calcium sulphate as CaSO4.2H2O.
(i) What is meant by the ‘2H2O’ in the formula CaSO4.2H2O?
..........................................................................................................................................
..................................................................................................................................... [1]
(ii) What is the molar mass of CaSO4.2H2O?
answer = .............................................g mol –1 [1]
[Total: 10]
2.
Write equations for each of the following reactions:
a. potassium hydroxide and nitric acid  potassium nitrate and water
………………………………………………………………………………………………
b. calcium hydroxide and hydrochloric acid  calcium chloride and water
………………………………………………………………………………………………
c. magnesium and aluminium sulfate  magnesium sulfate and aluminium
………………………………………………………………………………………………
[3]
3.
Compound B is a calcium compound used in making paper. Compound B is
manufactured by passing SO2 gas through a solution of calcium hydroxide.
Compound B has the following percentage composition by mass:
Ca, 19.82%; H, 0.99%; S, 31.74%; O, 47.45%.
(i)
Determine the empirical formula for compound B.
[2]
[Total: 2]
4. Aluminium reacts with acids, releasing hydrogen gas.
A student reacted 2.025 g of aluminium metal with an excess of 1.80 mol dm–3 hydrochloric
acid, HCl.
The equation for this reaction is shown below.
2Al(s) + 6HCl(aq)
(i)
→
2AlCl3(aq) + 3H2(g)
Calculate how many moles of Al reacted.
answer = ....................................................mol [1]
(ii) Calculate the volume, in dm3, of H2(g) that formed from 2.025 g Al at room
temperature and pressure, r.t.p.
1.00 mol of H2(g) has a volume of 24.0 dm3 at r.t.p.
volume = ...................................................dm3 [2]
(iii) Calculate the volume, in cm3, of 1.80 mol dm–3 HCl that reacts exactly with 2.025g
of Al.
volume = ................................................... cm3 [2]
[Total: 5]
5. A student prepared some chlorine gas on a small scale by reacting hydrochloric acid with
household bleach.
The reaction is shown below.
2HCl (aq) + NaClO(aq)
→
Cl2(g) + NaCl (aq) + H2O(l)
The student reacted 1.0 cm3 of 6.0 mol dm–3 HCl with 3.0 cm3 household bleach. 55 cm3 of
chlorine gas were produced. The hydrochloric acid was in excess and this ensured that all
the NaCl O in the bleach was reacted.
Under these conditions, 1.0 mol of Cl2(g) has a volume of 24 dm3.
(i)
Calculate how many moles of Cl2(g) were produced.
answer = ...................................... mol [1]
(ii)
Calculate the concentration, in mol dm–3, of NaCl O in the bleach.
concentration = ...................................... mol dm–3 [1]
(iii)
Calculate the number of moles of HCl that remained after the reaction.
answer = ...................................... mol [3]
[Total: 5]
6.(a) Compound X is a volatile liquid and is known to contain 52.17% carbon, 13.0 %
hydrogen and 34.78% oxygen by mass. Calculate the empirical formula of
compound X. Show your working.
[2]
(b) 0.25g of X is vapourised at 90⁰C and 101kPa pressure and is found to have a
volume of 162cm3. Calculate the molecular formula of X.
[3]