Question

A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.

Answer 1

Mass of the compound = 0.24g
Mass of boron = 0.096 g 
Mass of oxygen = 0.144 g

Percentage of boron = \(\frac{\text{Mass of boron}}{\text{ Mass of compound}}\) x 100 = \(\frac{ 0.096g}{0.240 g}\) x 100 = 40%

Percentage of oxygen =\(\frac{\text{Mass of oxygen}}{\text{ Mass of compound }}\)x 100 = \(\frac{0.144g}{0.240g }\)x 100 = 60%

Alternative method 
Percentage of oxygen =100 - percentage of boron 
=100 - 40 
= 60%