C2 Calculations and analysis

Question 1

Calculate the relative formula mass (M_r) of Calcium hydroxide - Ca(OH)₂

Answer 1

40 + (16 + 1) x 2 = 74

Question 2

Calculate the percentage by mass of calcium in calcium hydroxide

Answer 2

(40 ÷ 74) x 100 = 54%

Question 3

Calculate the mass of calcium in 30g of calcium hydroxide

Answer 3

(54 ÷ 100) x 30 = 16g

Question 4

A sample of calcium compound contains 0.8g of calcium, 0.64g of oxygen and 0.04g of hydrogen, find its empirical formula.

Answer 4

Mass ÷ Mr

Ca – 0.8 O – 0.64 H – 0.04

40 = 0.02 16 = 0.04 1 = 0.04

Divide by the smallest

Ca – 0.02 O – 0.04 H – 0.04

0.02 = 1 0.02 = 2 0.02 = 2

= CaO₂H₂

Question 5

Calculate how much Iron could be made from 320g of Iron oxide

Fe₂O₃ + 3CO → 2Fe + 3CO₂

Answer 5

Moles of Fe₂O₃ = 320 ÷ 160 = Mass ÷ M_r = 2 Moles

Ratio of Fe₂O₃ 1 : Fe 2

Moles of Fe = Moles of Fe₂O₃ x 2 = 4 Moles

Mass of Fe = Moles of Fe x M_r of Fe = 4 x 56 = 224g

Question 6

What is the formula for percentage yield?

Answer 6

(Actual Yield ÷ Theoretical yield) x 100

Question 7

Give 3 reasons why the yield is not 100%.

Answer 7

Reversible reaction

Product can be lost when separated

Other reactions occur

Question 8

What is the percentage yield if a reaction only produces 20g when in theory it produces 40g.

Answer 8

% yield = (20 ÷ 40) x 100 = 50%

Question 9

Give 3 reasons why instrumental methods are useful.

Answer 9

Sensitive/ Accurate

Fast

Only needs samples that are small

Question 10

What method can be used to analysis food additives?

Answer 10

Paper Chromatography

Question 11

What is separated in gas chromatography?

Answer 11

Mixture of compounds

Question 12

In the column, what can be used to help identify the substance?

Answer 12

The retention time

Question 13

What can a mass spectrometer be used for?

Answer 13

Gives the relative formula mass

Question 14

What is shown by the molecular ion peak?

Answer 14

Relative formula mass of substance