Worksheet for working out the number of moles of compounds. Pupils will need a data sheet or a list of relative atomic masses to be able to complete the questions.

1. 3. Moles of Compounds
We can work out the RMM of a compound by adding up the RAMs of all the atoms, and we can
work out the mass of a substance by using “mass = moles x RMM”, in other words, using the
moles triangle.
Example One
Find the amount (in moles) of 79.5g of sodium carbonate, Na2CO3.
RFM of Na2CO3 = 2 x RAM (Na) + RAM (C) + 3 x RAM (O)
= 46 + 12 + 48 mass
= 106
moles of Na2CO3 = mass / RFM
= 79. 5 / 106 moles RMM
= 0.75 mol
1) Calculate the amount (in moles) of:
a) 7g of magnesium oxide, MgO
b) 0.38g of carbon disulfide, CS2
c) 1 kg of sodium hydroxide, NaOH
d) 75g of zinc carbonate, ZnCO3
e) 4.88g of aluminium phosphate, AlPO4
f) 450g of glucose, C6H12O6
g) 34g of sodium nitrate, NaNO3
h) 10g of iron(III) sulfate, Fe2(SO4)3
i) 9.24g of ammonium sulfate, (NH4)2SO4
j) 21g of hydrated copper(II) sulfate, CuSO4.5H2O
Example Two
Find the mass 0.01 moles of 1,2-
dibromoethane, C2H4Br2.
RFM of C2H4Br2 = 2 x RAM (C) + 4 x
RAM (H) + 2 x RAM (Br)
= 24 + 4 + 160
= 188 2) Calculate the mass of:
mass of C2H4Br2 = moles x RFM a) 0.2 moles of sodium chloride, NaCl
= 0.01 x 188 b) 0.85 moles of silver bromide, AgBr
= 1.88 g c) 0.03 moles of sodium sulphate, Na2SO4
d) 0.055 moles of iron(III) oxide, Fe2O3
e) 0.5 moles of potassium carbonate, K2CO3
f) 4 moles of copper(II) nitrate, Cu(NO3)2
g) 1.25 moles of calcium hydroxide, Ca(OH)2
h) 0.01 moles of aluminium nitrate, Al(NO3)3
i) 0.2 moles of lead acetate, Pb(C2H3O2)2
j) 0.1 moles of hydrated nickel chloride,
NiCl2.6H2O

2. You might have used diagrams like
these to help you work out the OH- Ag+ Cl-
formulae of ionic compounds: Mg2+
Example Three
OH-
How many moles of hydroxide ions are OH-
there in 8 moles of magnesium
hydroxide?
From the diagram above, there are two
NH4+ Fe3+ OH-
moles of hydroxide ions for every mole SO42-
of magnesium hydroxide. Therefore NH4+ OH-
there should be 16 moles of hydroxide
ions in 8 moles of magnesium hydroxide.
-
Some people find it difficult to understand how you can get 16 moles of OH ions from only 8 moles of Mg(OH)2. We are not
saying you can get 16g of hydroxide ions from 8g of magnesium hydroxide. The hydroxide ions are only a smaller part of the
Mg(OH)2 unit – and there are two of them in each unit.
It might help to think of magnesium hydroxide as being like a chicken. The magnesium ion represents the chicken’s head and
body, and each hydroxide ion represents one of the chicken’s legs because the formula is Mg(OH) 2. Suppose you had a farm with
eight chickens in it. How many chicken legs would you have? Obviously you would have a total of 16 chicken legs.
3) This question is about working out the 4) Calculate the amount (in moles) of:
amount of ions in compounds. The diagrams a) chloride ions in 28.7g of silver
above might help you. Calculate the amount chloride, AgCl.
(in moles) of: b) hydroxide ions in 1.45g of magnesium
a) chloride ions in 0.65 moles of silver hydroxide, Mg(OH)2.
chloride, AgCl. c) hydroxide ions in 32.1g of iron(III)
b) hydroxide ions in 0.95 moles of hydroxide, Fe(OH)3.
magnesium hydroxide, Mg(OH)2. d) phosphate ions in 23.25g of calcium
c) ammonium ions in 2¼ moles of phosphate, Ca3(PO4)2.
ammonium sulfate, (NH4)2SO4.
d) hydrogen ions in 0.1 moles of sulphuric
acid, H2SO4.
Answers
1. Calculate the amount (in moles) of…(a) 7/40 = 0.175 mol, (b)0.38/76 = 0.005 mol , (c) 1000/40 = 25 mol, (d) 75/125 = 0.6 mol,
(e) 4.88/122 = 0.04 mol, (f) 450/180 = 2.5 mol, (g) 34/85 = 0.4 mol, (h) 10/400 = 0.025 mol, (i) 9.24/132 = 0.07 mol, (j) 21/250 =
0.084 mol
2. Calculate the mass of…(a) 0.2 x 58.5 = 11.7 g, (b) 0.85 x 188 = 159.8 g, (c)0.03 x 142 = 4.26 g, (d) 0.055 x 102 = 5.61 g, (e) 0.5 x
138 = 69 g, (f) 4 x 188 = 752 g, (g) 1.25 x 74 = 92.5 g, (h)0.01 x 213 = 2.13 g, (i) 0.2 x 325 = 65 g, (j) 0.1 x 238 = 23.8 g
3. Calculate the amount (in moles) of … (a) 0.65 mol, (b) 0.95 x 2 = 1.9 mol, (c)2¼ x 2 = 4½ mol, (d) 0.1 x 2 = 0.2 mol
4. Calculate the amount (in moles) of … (a) 0.2 mol, (b) 0.05 mol, (c)0.9 mol, (d)0.15 mol