Two formula mass calculations

Question 1

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

Answer 1

% mass = Ar x No. of atoms (of that element)
————————————————- x 100
Mr (of whole compound)

Question 2

What is the percentage mass of sodium in sodium carbonate (Na2CO3)?

Answer 2

Ar of sodium = 23
Ar of carbon = 12
Ar of oxygen = 16
Mr of Na2CO3 = 46 + 12 + 48 = 106
Percentage mass = Ar x n 23 x 2
——— x 100 = ———- x 100 = 43.4%
Mr 106
percentage mass of sodium in Na2CO3 = 43.3%

Question 3

How do we find the empirical formula (from masses or percentages)?

Answer 3

• List all the elements in the compound.
• Underneath them, write their experimental masses or percentages.
• Divide each mass or percentage by the Ar for that particular element.
• Turn the numbers you get into a simple ratio, by multiplying and/or dividing them by a number.
• Get the ratio in its simplest form, and that tells you the empirical formula of the compound.

Question 4

What is the empirical formula of the iron oxide produced when 44.8g of iron react with 19.2g of oxygen?

Answer 4

Fe                      O
44.8                  19.2
44.8                  19.2
------- = 0.8        ------ = 1.2
 56                      16
0.8 x 10 = 8       1.2 x 10 = 12
8 ÷ 4 = 2            12 ÷ 4 = 3
Empirical formula = Fe2O3