CGB IGCSE Chemistry Practice problem answers - the mole
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The Mole

1. What is the mass, in g, of:
1. 1 mole of boron atoms

Relative atomic mass of boron = 11 Ar

1 mole of boron atoms= 11 g

1. 1 mole of nickel atoms

Relative atomic mass of nickel = 59 Ar

1 mole of nickel atoms = 59g

1. 3 moles of cobalt atoms

Relative atomic mass of cobalt = 59 Ar

1 mole of cobalt atoms = 59 g

3 moles of cobalt atoms = 59 g x 3 = 177g

1. 2.5 moles of scandium atoms

Relative atomic mass of scandium = 45 Ar

1 mole of scandium atoms = 45 g

2.5 moles of scandium atoms = 45 g x 2.5 = 112.5 g

1. 0.1 moles of fluorine atoms

Relative atomic mass of fluorine = 19 Ar

1 mole of fluorine atoms = 19 g

0.1 moles of fluorine atoms = 19 g x 0.1 moles = 1.9 g

1. 0.5 moles of argon atoms

Relative atomic mass of argon = 40 Ar

1 mole of argon atoms = 40 g

0.5 moles of argon atoms = 40 g x 0.5 moles = 20 g

1. 0.4 moles of silicon atoms

Relative atomic mass of silicon = 28 Ar

1 mole of silicon atoms = 28 g

0.4 moles of silicon atoms = 28g x 0.4 moles = 11.2 g

1. 0.3 moles of iodine atoms

Relative atomic mass of iodine = 127 Ar

1 mole of iodine atoms = 127 g

0.3 moles of iodine atoms = 127 g x 0.3 moles = 38.1 g

1. 1.7 moles of krypton atoms

Relative atomic mass of krypton = 84 Ar

1 mole of krypton atoms= 84 g

1.7 moles of krypton atoms = 84 g x 1.7 moles = 142.8 g

1. What is the mass, in g, of :
1. 1 mole of fluorine molecules, F2

Relative atomic mass of fluorine = 19 Ar

Relative formula mass of fluorine molecules = 19 Ar x 2 = 38 Mr

1 mole of fluorine molecules, F2 = 38 g

1. 4 moles of fluorine molecules, F2

Relative atomic mass of fluorine = 19 Ar

Relative formula mass of fluorine molecules = 19 Ar x 2 = 38 Mr

1 mole of fluorine molecules = 38 g

4 moles of fluorine molecules = 38 g x 4 = 152 g

1. 0.5 moles of bromine molecules, Br2

Relative atomic mass of bromine = 80 Ar

Relative formula mass of bromine molecules = 80 Ar x 2 = 160 Mr

1 mole of bromine molecules = 160 g

0.5  moles of bromine molecules = 160 g x 0.5 = 80 g

1. 1.2 moles of bromine molecules, Br2

Relative atomic mass of bromine = 80 Ar

Relative formula mass of bromine molecules= 80 Ar x 2 = 160 Mr

1 mole of bromine molecules = 160 g

1.2 moles of bromine molecules = 160 g x 1.2 moles = 192 g

1. 3 moles of oxygen molecules, O2

Relative atomic mass of oxygen = 16 Ar

Relative formula mass of oxygen molecules= 16 Ar x 2 = 32 Mr

1 mole of oxygen molecules = 32 g

3 moles of oxygen molecules = 32 g x 3 = 96 g

1. Find how many moles of atoms there are, in:
1. 14 g of Lithium

Relative atomic mass of lithium = 7 Ar

1 mole of lithium atoms = 7 g

14 g  x 1 mole/ 7g = 2 moles

14 g of lithium = 2 moles of lithium

1. 3 g of hydrogen

Relative atomic mass of hydrogen = 1 Ar

1 mole of hydrogen atoms = 1 g

3 g x 1 mole/1 g = 3 moles

3 g of hydrogen = 3 moles of hydrogen

1. 24 g of iron

Relative atomic mass of iron = 56 Ar

1 mole of iron atoms = 56 g

24 g x 1 mole/56 g = 0.43 moles

24 g of iron = 0.43 moles of iron

1. 400 g of cobalt

Relative atomic mass of cobalt = 59 Ar

1 mole of cobalt atoms = 59 g

400 g x 1 mole/59 g = 6.8 moles

400 g of cobalt = 6.8 moles of cobalt

1. 256 g of nickel

Relative atomic mass of nickel = 59 Ar

1 mole of nickel atoms = 59 g

256 g x 1 mole/59 g = 4.3 moles

256 g of nickel = 4.3 moles of nickel

1. 2.5 g of helium

Relative atomic mass of helium = 4 Ar

1 mole of helium atoms = 4 g

2.5 g x 1 mole/4 g = 0.63 moles

2.5 g of helium = 0.63 moles of helium

1. 74 g of phosphorus

Relative atomic mass of phosphorus = 31 Ar

1 mole of phosphorus atoms = 31 g

74 g x 1 mole/ 31 g = 2.4 moles

74 g of phosphorus = 2.4 moles

1. 12.7 g of boron

Relative atomic mass of boron = 11 Ar

1 mole of boron atoms = 11 g

12.7 g x 1 mole/11g = 1.2 moles

12.7 g of boron = 1.2 moles of boron

1. 16 g of carbon

Relative atomic mass of carbon = 12 Ar

1 mole of carbon atoms = 12 g

16 g x 1 mole/12 g = 1.3 moles

16 g of carbon = 1.3 moles of carbon

1. 902 g of gold

Relative atomic mass of gold = 197 Ar

1 mole of gold atoms = 197 g

902 g x 1 mole/ 197 g = 4.6 moles

902 g of gold = 4.6 moles of gold

1. 34 g of silver

Relative atomic mass of silver = 108 Ar

1 mole of silver atoms = 108 g

34 g x 1 mole/ 108 g = 0.3 moles

34 g of silver = 0.3 moles of silver

1. 5 g of arsenic

Relative atomic mass of arsenic = 75 Ar

1 mole of arsenic atoms = 75 g

5 g x 1 mole/75 g = 0.07 moles

5 g of arsenic = 0.07 moles of arsenic

1. For each pair, decide which of the two substances contains the greater number of atoms.  Remember, there are 6.02 x 1023 atoms per mole.
1. 12 g of sodium or 12 g of magnesium

Relative atomic mass of sodium = 23 Ar

1 mole of sodium atoms = 23g

12 g x 1 mole/23 g = 0.52 mole

1 mole = 6.02 x 1023 atoms

0.5 mole = (6.02 x 1023 atoms/mole) x 0.52 mole = 3.13 x 1023 atoms

12 g of sodium = 3.13 x 1023 atoms of sodium

Relative atomic mass of magnesium = 24 Ar

1 mole of magnesium atoms = 24 g

12 g x 1 mole/24 g = 0.5 moles

1 mole = 6.02 x 1023 atoms

0.5 mole = (6.02 x 1023 atoms/mole) x 0.5 moles = 3.01 x 1023 atoms

12 g of magnesium = 3.01 x 1023 atoms of magnesium

12 g of sodium has more atoms than 12 g of magnesium

1. 14 g of calcium or 14 g of potassium

Relative atomic mass of calcium = 40 Ar

1 mole of calcium atoms = 40 g

14 g x 1 mole/40 g = 0.35 moles

1 mole = 6.02 x 1023 atoms

0.35 moles = (6.02 x 1023 atoms/mole) x 0.35 moles = 2.11 x 1023 atoms

14 g of calcium = 2.11 x 1023 atoms

Relative atomic mass of potassium = 39 Ar

1 mole of potassium atoms = 39 g

14 g x 1 mole/39 g = 0.36 moles of potassium

1 mole = 6.02 x 1023 atoms

0.36 moles = (6.02 x 1023 atoms/mole) x 0.36 moles = 2.17 x 1023 atoms

14 g of potassium = 2.17 x 1023 atoms

14 g of potassium has more atoms than 14 g of calcium

1. 26 g of aluminum or 58 g of bromine

Relative atomic mass of aluminum = 27 Ar

1 mole of aluminum atoms = 27 g

26 g x 1 mole/27 g = 0.96 moles

1 mole = 6.02 x 1023 atoms

0.96 moles = (6.02 x 1023 atoms/mole) x 0.96 moles = 5.78 x 1023 atoms

26 g of aluminum = 5.78 x 1023 atoms of aluminum

Relative atomic mass of bromine = 80 Ar

1 mole of bromine atoms = 80 g

58 g x 1 mole/80g = 0.73 moles

1 mole = 6.02 x 1023 atoms

0.73 moles = (6.02 x 1023 atoms/mole) x 0.73 moles = 4.4 x 1023 atoms

58 g of bromine = 4.4 x 1023 atoms of bromine

26 g of aluminum has more atoms than 58 g of bromine

1. 67 g of antimony or 15 g of selenium

Relative atomic mass of antimony = 122 Ar

1 mole of antimony atoms = 122 g

67 g x 1 mole/122g = 0.55 moles

1 mole = 6.02 x 1023 atoms

0.55 moles = (6.02 x 1023 atoms/mole) x 0.55 moles = 3.31 x 1023 atoms

67 g of antimony = 3.31 x 1023 atoms of antimony

Relative atomic mass of selenium = 79 Ar

1 mole of selenium atoms = 79 g

15 g x 1 mole/79 g = 0.19 moles

1 mole = 6.02 x 1023 atoms

0.19 moles = (6.02 x 1023 atoms/mole) x 0.19 moles = 1.14 x 1023 atoms

15 g of selenium = 1.14 x 1023 atoms of selenium

67 g of antimony has more atoms than 15 g of selenium

1. 1 mole of chromium or 1 mole of cobalt

1 mole of chromium = 6.02 x 1023 atoms

1 mole of cobalt  = 6.02 x 1023 atoms

1 mole of chromium and 1 mole of cobalt have the same number of atoms

1. 2 moles of oxygen atoms or ½ mole of oxygen molecules (O2)

2 moles of oxygen atoms = (6.02 x 1023 atoms/mole) x 2 moles = 12.04 x 1023 atoms

½  mole of oxygen molecules = 1 mole of oxygen atoms = 6.02 x 1023 atoms

There are more atoms in 2 moles of oxygen atoms than in ½ mole of oxygen molecules

1. What is the mass, in g, of:
1. 1 mole of sulphuric acid, H2SO4

Relative formula mass of H2SO4

2H = 2 x 1 Ar = 2

S = 1 x 32 Ar = 32

4O = 4 x 16 Ar = 64

2 + 32 + 64 = 98 Mr

1 mole of H2SO4 = 98 g

1. 2 moles of magnesium oxide, MgO

Relative formula mass of MgO

Mg = 24 Ar

O = 16 Ar

24 + 16 = 40 Mr

1 mole of MgO = 40 g

2 moles of MgO = 40 g/mole x 2 moles = 80 g

1. 1.5 moles of sodium chloride, NaCl

Relative formula mass of NaCl

Na = 23 Ar

Cl = 35.3 Ar

23 + 35.5 = 58.5 Mr

1 mole of NaCl = 58.5 g

1.5 moles of NaCl = 58.5 g /mole x 1.5 moles = 87.8 g

1. 10 moles of lithium fluoride, LiF

Relative formula mass of LiF

Li = 7 Ar

F = 19 Ar

7 + 19 = 26 Mr

1 mole of LiF = 26 g

10 moles of LiF = 26 g/mole x 10 moles = 260 g

1. 0.2 moles of ethane, C2H4

Relative formula mass C2H4

2 C = 2 x 12 Ar = 24

4 H = 4 x 1 Ar = 4

4 + 24 = 28 Mr

1 mole of C2H4 = 28 g

0.2 moles of C2H4 = 28 g/mole x 0.2 moles = 5.6 g