Formula Mass Calculations

Question 1

What is the equation for ‘percentage mass of an element in a compound’

Answer 1

= Aᵣ * no. of atoms (of that element) / Mᵣ (of whole compound) * 100

Question 2

What is Aᵣ?

Answer 2

Relative atomic mass (mass number)

Question 3

What is Mᵣ?

Answer 3

Relative Formula Mass (all the Aᵣ s in a compound added together)

Question 4

Work through example:

Find the percentage mass of sodium in sodium carbonate, Na₂CO₃

Answer 4

Aᵣ of sodium = 23
Aᵣ of carbon = 12
Aᵣ of oxygen = 16
Mᵣ of Na₂CO₃ = 2*23 + 12 + 3*16 = 106
so = 23 * 2 / 106 = 46/106 = 43.4%

Question 5

Find the percentage mass of oxygen in Fe₂O₃

Answer 5

30.0%

Question 6

Find the percentage mass of oxygen in H₂O

Answer 6

88.9%

Question 7

Find the percentage mass of oxygen in CaCO₃

Answer 7

48.0%

Question 8

Find the percentage mass of oxygen in H₂SO₄

Answer 8

65.3%

Question 9

How do you find the empirical formula from masses or percentages? (5)

Answer 9

1) List all the elements in the compound
2) Underneath them, write their experimental masses or percentages
3) Divide each mass or percentage by the Aᵣ of that particular element
4) Convert the numbers you get into a nice ratio
5) Get the ratio in its simplest form, and that is the empirical formula of the compound

Question 10

Find the empirical formula of the iron oxide produced when 44.8g of iron react with 19.2g of oxygen.
Aᵣ for iron = 56, Aᵣ for oxygen = 16

Answer 10

1) Fe O
2) 44.8 19.2
3) 44.8/56 19.2/16
= 0.8 1.2
4) 8 : 12
5) = 2 : 3
Answer = Fe₂O₃

Question 11

Find the empirical formula of the compound formed from 2.4g of carbon and 0.8g of hydrogen.

Answer 11

CH₄