2a.4 - Relative Formula Mass

Question 1

What is relative atomic mass?

Answer 1

How heavy different atoms are compared with the mass of carbon-12

Question 2

How do you calculate relative formula mass?

Answer 2

Add up all the relative atomic masses

Question 3

What is a mole?

Answer 3

The relative formula mass of a substance in grams is known as one mole of that substance

Question 4

How do you calculate the number of moles?

Answer 4

Mass in g (of element or compound) % relative formula mass (of element or compound)

Question 5

How do you calculate the percentage mass of an element in a compound?

Answer 5

Ar x no. of atoms (of that element
—————————-x100
Mr (of whole compound)

Question 6

Find the percentage mass of sodium in sodium carbonate, Na2CO3
Ar Na - 23
Ar C - 12
Ar O - 16

Answer 6

Calculate Mr of Na2CO3:
(2x23) + 12 + (3x16) = 106

Now use the formula:
Ar x no. of atoms (of that element
———————————————–x100
Mr (of whole compound)

23 x 2
———– x 100 = 43.4%
106

Question 7

Find the percentage mass of oxygen in Fe2O3
Ar Fe - 56
Ar O - 16

Answer 7

Calculate Mr of Fe2O3:
(56 x 2) + (16 x 3) = 160

Now use the formula:
Ar x no. of atoms (of that element
———————————————–x100
Mr (of whole compound)

16 x 3
——— x100 = 30.00%
160

Question 8

Work out the percentage mass of oxygen in H2O
Ar H - 1
Ar O - 16

Answer 8

Calculate the Mr:
(1 x 2) + 16 = 18

Use the formula:
Ar x no. of atoms (of that element
———————————————–x100
Mr (of whole compound)

16 x 1
———x100 = 88.9%
18

Question 9

Calculate the percentage mass of oxygen in CaCO3
Ar Ca - 40
Ar C - 12
Ar O - 16

Answer 9

Calculate Mr:
40 + 12 + (16 x 3) = 100

Use the formula:
Ar x no. of atoms (of that element
———————————————–x100
Mr (of whole compound)

16 x 3
———x100 = 48%
100

Question 10

Find the percentage mass of oxygen in H2SO4
Ar H - 1
Ar S - 32
Ar O - 16

Answer 10

Calculate Mr:
(1 x 2) + 32 + (16 x 4) = 98

Use the formula:
Ar x no. of atoms (of that element
———————————————–x100
Mr (of whole compound)

16 x 4
———- x100 = 65.3%
98

Question 11

How do you calculate empirical formula (5)

Answer 11

• List all the elements in the compound
• Write the experimental mass percentage (Ar underneath)
• Calculate the percentage
• Find a ratio (times by 10 is necessary)
• Make the ratio is in it’s lowest form

Question 12

Find the empirical formula of the iron oxide produced when 44.8 g of iron react with 19.2g of oxygen
Ar Fe - 56
Ar O - 16

Answer 12

List elements: Fe O
Write percentages:
Fe = 44.8 % 16 = 0.8 O = 19.2%16 = 1.2
Multiply by 10: 8 12
Divide by 4 (lowest): 2 3
Empirical formula: Fe2 O3

Question 13

Find the empirical formula of the compound formed from 2.4 g of carbon and 0.8 g of hydrogen
Ar C - 12
Ar H - 1

Answer 13

List elements: C H
Write percentages:
C = 2.4 % 12 = 0.2 H = 0.8 % 1 = 0.8
Multiply by 10: 2 8
Divide by 2: 1 4
Empirical formula: CH4

Question 14

What are the steps to calculating masses in reactions?

Answer 14

• Write out the balanced equation
• Write out the RFM of each
• Apply the rule: Divide to get one, then multiply to get all

Question 15

What mass of magnesium oxide is produced when 60 g of magnesium is burned in air?
Mg - 24
O - 16

Answer 15

1)Write a balanced symbol equation:
2Mg + O2 -> 2MgO
2)Work out the RFM:
2Mg = 2 x 24 = 48
2MgO = 2 x (24 + 16) = 80
3)Apply the rule:
48g of Mg……..80g of MgO
(%48) (%48)
1 g of Mg…….1.67 g of MgO
(x60) (x60)
60g of Mg…..100g of MgO
60 g of magnesium will produce 100 g of MgO