Question

The 64.8-g sample of the compound X2O5 contains 48.0 g of oxygen atoms. What is the molar mass of element X?

Answer 1

The answer is 14 g/mol.

The number of moles (n) is the ratio of mass (m) and molar mass (M):
n = m : M

Step 1. Calculate number of moles of oxygen in the compound.
n = ?
m = 48 g
M = 16 g/mol

n = m/M = 48 g : 16 g/mol = 3 mol


Step 2. Calculate the number of moles of the element X:
5 atoms of oxygen is 3 mol.
2 atoms of element X is x
5 : 3  = 2 :x
x = 2 * 3 / 5 = 6/5 = 1.2 mol

Step 3. Calculate the molar mass of element X
n = 1.2 mol
m = 64.8 g - 48 g = 16.8 g
M = ?

n = m : M
M = m : n
M = 16.8 g : 1.2 mol = 14 g/mol