Chemistry GCSE: C3 Quantitative Chemistry

Question 1

What is the relative formula mass (M_r) of Li₂O (lithium oxide)? Write down your workings.

A_r (relative atomic mass) of Li = 7

A_r (relative atomic mass) of O = 16

Answer 1

Li₂ = 7 x 2 = 14

O = 16

Li₂ + O = 14 + 16 = 30

Question 2

What is the relative formula mass (M_r) of C₃H₈ (propane)? Write down your workings.

A_r (relative atomic mass) of C = 12

A_r (relative atomic mass) of H = 1

Answer 2

C₃ = 12 x 3 = 36

H₈ = 1 x 8 = 8

C₃ + H₈ = 36 + 8 = 44

Question 3

What is the relative formula mass (M_r) of Ca(OH)₂ (calcium hydroxide)? Write down your workings.

A_r of Ca = 40

A_r of O = 16

A_r of H = 1

Answer 3

Ca = 40

O + H = 16 + 1 = 17

(OH)₂ = 17 x 2 = 34

Ca + (OH)₂ = 40 + 34 = 74

Question 4

What is the formula for calculating the percentage mass of an element in a compound?

Answer 4

A_r (relative atomic mass of element) x number of atoms x 100

M_r (relative formula mass of compound)

Question 5

What is the percentage mass of lithium in lithium oxide (Li₂O)? Write down your workings.

A_r of Li = 7

A_r of O = 16

Answer 5

M_r of Li₂O

• Li₂ = 7 x 2 = 14
• O = 16
• Li₂ + O = 14 + 16 = 30

Percentage mass of lithium in lithium oxide

• Li₂ ÷ Li₂O x 100 =
• (7x2) ÷ 30 x 100 =
• 14 ÷ 30 x 100 = 47%

Question 6

What is the percentage mass of hydrogen in propane (C₃H₈)? Write down your workings.

Ar of C = 12

Ar of H = 1

Answer 6

M_r of C₃H₈

• C₃ = 12 x 3 = 36
• H₈ = 1 x 8 = 8
• C₃ + H₈ = 36 + 8 = 44

Percentage mass of hydrogen in propane

• H₈ ÷ C₃H₈ x 100 =
• (1x8) ÷ 44 x 100 =
• 8 ÷ 44 x 100 = 18%

Question 7

What is the percentage mass of oxygen in calcium hydroxide (Ca(OH)₂)?Write down your workings.

Ar of Ca = 40

Ar of O = 16

Ar of H = 1

Answer 7

M_r of Ca(OH)₂

• Ca = 40
• O + H = 16 + 1 = 17
• (OH)₂ = 17 x 2 = 34
• Ca + (OH)₂ = 40 + 34 = 74

Percentage mass of hydrogen in calcium hydroxide

• O₂ ÷ Ca(OH)₂ x 100 =
• (16x2) ÷ 74 x 100 =
• 32 ÷ 74 x 100 = 43%

Question 8

If 6g of magnesium reacts completely with 4g of oxygen what mass of magnesium oxide is formed?

Answer 8

10g

Question 9

When calcium carbonate is hated it breaks down to give calcium oxide and carbon dioxide.

calcium carbonate →calcium oxide + carbon dioxide

6.00g of calcium carbonate is heated, this gives 3.36g of calcium oxide. What mass of carbon dioxide was made?

Extension: Explain how you work this out.

Answer 9

6.00g - 3.36g = 2.64g

Extension:

The total mass of products must equal the total mass of reactants. So to work out the mass of carbon dioxide yuo take away the mass of calcium oxide (product) from the mass of calcium carbonate (reactant).

Question 10

Liam adds two reactants to an open conical flask on a mass balance. The starting mass is 10g. At the end of the reaction the mass is 8.95g. What might have caused the decrease in mass during the reaction?

Answer 10

• One of the products was a gas
• The gas will have left the conical flask decreasing the mass of the flask.

Question 11

Sodium + Hydrochloric acid →Sodium chloride + Hydrogen

Jack reacts sodium with hydrochloric acid in an open (unsealed) conical flask. What will happen to the mass of the flask as the reaction takes place? Explain why this will happen.

Answer 11

• The mass of the flask will decrease
• Because, hydrogen is a gass and will be lost from the flask.

Question 12

Jamie reacts 6g of magnesium with oxygen (from the air) by heating it. At the end of the reaction he finds he has 9.3g of magnesium oxide.

How much oxygen was used in the reaction?

Answer 12

mass of magnesium oxide produced - mass of magnesium used = mass of oxygen used

9.3 - 6 = 3.3 g

Question 13

HIGHER:

How many atoms are in 1 mole of lithium (Li)?

Answer 13

6.02 x 10²³ (Avogadro’s constant)

Question 14

HIGHER:

How many molecules are in 1 mole of oxygen (O₂)?

Answer 14

6.02 x 10²³ (Avogadro’s constant)

Question 15

HIGHER:

How many atoms are in 1 mole of oxygen (O₂)?

Answer 15

2 x (6.02 x 10²³): Each oxygen molecule has two oxygen atoms

1.204 x 10²⁴

Question 16

HIGHER:

How many molecules are in 1 mole of carbon dioxide (CO₂)?

Answer 16

6.02 x 10²³

Question 17

HIGHER:

How many atoms are there in 1 mole of carbon dioxide (CO₂)?

Answer 17

3 x (6.02 x 10²³): Each carbon dioxide molecule is made from 3 atoms, 1 carbon atom and 2 oxygen atoms.

1.806 x 10²⁴

Question 18

HIGHER:

What is the mass of 5 moles of lithium oxide (Li₂O)? Write down your workings.

Ar of Li = 7

Ar of O = 16

Answer 18

Mass of 1 mole of lithium oxide

• Li₂ = 7 x 2 = 14
• O = 16
• Li₂ + O = 14 + 16 = 30g

Mass of 5 moles lithium oxide

• 5 x 30g = 150g

Question 19

HIGHER:​

What is the mass (in grams) of 3C₃H₈ (propane)? Write down your workings.

Ar (relative atomic mass) of C = 12

Ar (relative atomic mass) of H = 1

Answer 19

Mass of 1 mole of propane (C₃H₈)

• C₃ = 12 x 3 = 36
• H₈ = 1 x 8 = 8
• C₃ + H₈ = 36 + 8 = 44g

Mass of 3 moles propane

• 3 x 44g = 132g

Question 20

HIGHER:

How many moles of water (H₂O) are there in 36g?

Ar (relative atomic mass) of H = 1

Ar (relative atomic mass) of O = 16

Answer 20

Mass of 1 mole of water (H₂O)

• H₂ = 1 x 2 = 2
• O = 16
• H₂ + O = 2 + 16 = 18g

Moles of water in 36g

• number of moles = total mass ÷ mass of 1 mole
• 36 ÷ 18 = 2 moles

Question 21

HIGHER:

How many moles of ammonia (NH₃) are there is 8.5g.

Ar (relative atomic mass) of N = 14

Ar (relative atomic mass) of H = 1

Answer 21

Mass of 1 mole of water (NH₃)

• N = 14
• H₃ = 1 x 3 = 3
• N + H₃ = 14 + 3 = 17

Moles of ammonia in 8.5g

• number of moles = total mass ÷ mass of 1 mole
• 8.5 ÷ 17 = 0.5 moles

Question 22

HIGHER:

3.5 g of Li reacts completely with 4g of O₂ to produce 7.5 g of Li₂O. Use this to work out a balanced equation for the reaction.

Ar (relative atomic mass) of Li = 7

Mr (relative formula mass) of O₂ = 32 (16 x2)

Mr (relative formula mass of Li₂O = 30 ( 14 x 2 + 16)

Write you answer out.

Answer 22

• Li + O₂ → Li₂O
• Moles of Li in 3.5 g = 3.5 ÷ 7 = 0.5
• Moles of O₂ in 4g = 4 ÷ 32 = 0.125
• Moles of Li₂O in 7.5 g = 7.5 ÷ 30 = 0.25
• ratio of atoms/molecules = 0.5: 0.125: 0.25
• To get whole number divide each by the smallest (0.125)
• ratio of atoms/molecules = 4:1:2
• Balanced equation = 4Li + 1O₂ → 2Li₂O

Question 23

4Al + 3O₂ → 2Al₂O₃

Calculate the number of moles of aluminium oxide formed when 135g of aluminum is burned in air.

Answer 23

• 4Al + 3O₂ → 2Al₂O₃
• Number of moles of aluminium = mass of aluminium ÷ relative formula mass
• 135 ÷ 27 = 5
• Ratio of aluminium to aluminium oxide 4:2
• There are half the number of moles of aluminium oxide as there are of aluminium
• There will therefore be 5 x 0.5 = 2.5 moles of aluminium oxide

Question 24

• What volume does a mole of gas occupy at toom temperature (20^oC) and pressure (1 atm)

Answer 24

• 24 dm³

Question 25

If you were given the mass of a gas, how would you calculate the volume it takes up?

Answer 25

Volume of gas (in dm³) = number of moles x 24, therefore..

Volume of gas (in dm³) = (mass ÷ relative formula mass) x 24

Question 26

What is the volume of 80 g of oxygen at room temp and pressure?

Answer 26

Volume of gas (in dm³) = number of moles x 24

Volume of gas (in dm³) = (mass ÷ relative formula mass) x 24

= (80 ÷ 16) x 24 = 5 x 24

= 120 dm³

Question 27

What volume of ammonia is formed when 100 dm³ of nitrogen reacts with hydrogen.

N₂ + 3H₂ → 2NH₃

Answer 27

N₂ + 3H₂ → 2NH₃

The ratio of nitrogen to ammonia is 1:2

Therefore if you use 100 dm³ of nitrogen you will get 200 dm³ of ammonia.

Question 28

What is the formula for working out the concentration of a solution in g/dm³?

Answer 28

Concentration (g/dm³) = mass of solute (g) ÷ volume of solvent (dm³)

Question 29

What is the formula for working out the concentration of a solution in mol/dm³?

Answer 29

Concentration (mol/dm³) = moles of solute ÷ volume of solvent (dm³)

Question 30

If you were given the concentration of a solution (mol/dm³) and the volume (dm³), how would you calculate the number of moles of solute present?

Answer 30

moles of solute = Concentration (mol/dm³) x volume of solvent (dm³)

Question 31

How do you convert cm³ into dm³?

e.g how many dm³ is 100 cm³

Answer 31

divide by 1000

e.g 100 cm³ = (100 ÷ 1000) dm³ = 0.1 dm³

Question 32

What is the concentration in g/dm³ of a 2 mol/dm³ hydrochloric acid (HCl) solution?

Ar (relative atomic mass) of H = 1

Ar (relative atomic mass) of Cl = 35.5

Answer 32

Calculate the mass of 1 mol of HCl: 1 + 35.5 = 36.5 g

36.5 x 2 = 73 g

Therefore the solution has a concentration of 73 g/dm³

Question 33

What is the concentration in g/dm³ of a 0.2 mol/dm³ hydrochloric acid (HCl) solution?

Ar (relative atomic mass) of H = 1

Ar (relative atomic mass) of Cl = 35.5

Answer 33

Calculate the mass of 1 mol of HCl: 1 + 35.5 = 36.5 g

36.5 x 0.2 or 36.5 ÷ 5 = 7.3 g

Therefore the solution has a concentration of 7.3 g/dm³

Question 35

What is the formula for calculating atom economy?

Answer 35

(relative formula mass of desired product ÷ relative formula mass of reactants) x 100

Question 36

CH₄ + H₂O → CO + 3H₂

If the reaction shown above is used to make hydrogen, what is the atom economy of this reaction.

Ar (relative atomic mass) of C = 12

Ar (relative atomic mass) of H = 1

Ar (relative atomic mass) of H = 16

Answer 36

• (relative formula mass of desired product ÷ relative formula mass of reactants) x 100
• Relative formula mass of 3H₂ = 3 (1 x 2) = 3 x 2 = 6
• Relative formula mass of CH₄ = 12 + (1 x 4) = 12 + 4 = 16
• Relative formula mass of H₂O = (2x1) + 16 = 2 + 16 = 18
• Relative formula mass of reactants = 18 + 16 = 34
• Atom economy = (6 ÷ 34) x 100 = 17.6%