Quantitative Chemistry: Calculating Percentage by Mass (DONE)

Question 1

What does “percentage by mass” tell us ?

Answer 1

The percentage by mass tells us what percentage of the relative formula mass is due to one of the elements in a compound.

Question 2

What s the formula used to calculate the percentage by mass?

Answer 2

The formula used to calculate the percentage by mass is:

percentage by mass of an element = total relative atomic mass of that element / relative formula mass of compound (Mr) X 100

Question 3

Calculate the percentage by mass of calcium in calcium chloride.

CaCl↓2

Mr CaCl↓2 = 111
Ar Ca = 40

Answer 3

Since we have one atom of calcium we do 40 x 1 which is a relative atomic mass of 40 so we then divide it by the relative formula mass of the compound which is 111. And then following the formula we multiply it by 100.

This is the percentage by mass for calcium in calcium chloride.

40 x 1 = 40

40 ÷ 111 x 100 = 36

36%

Question 4

Calculate the percentage by mass of hydrogen in methane.

CH↓4

Mr CH↓4 = 16
Ar H = 1

Answer 4

We have four atoms of hydrogen so we do 1 x 4 which is a relative atomic mass of 4 so we then divide it by the relative formula mass of the compound which is 16. We then X by 100.

This is the percentage of mass for hydrogen in methane. (how much hydrogen is in methane)

1 x 4 = 4

4 / 16 x 100 = 25

25%

Question 5

Calculate the percentage by mass of sodium in sodium sulphate. To the nearest decimal place.

Na↓2SO↓4

Mr Na↓2SO↓4 = 142
Ar Na = 23

Answer 5

This is the percentage of mass for sodium in sodium sulphate.

23 x 2 = 46

46 / 142 x 100 = 32.4

32.4%