Question

A substance contains 43.4% sodium, 11.3% carbon and 43.3% oxygen. Calculate its empirical formula.

Answer 1

Step 1: Defining the empirical formula

The simplest ratio of atoms of different elements present in a molecule of the compound.

Step 2: Recalling the Atomic mass

• The atomic mass of sodium = 23u
• The atomic mass of carbon = 12u
• The atomic mass of oxygen = 16u

Step 3: Identifying the relative number of moles

The relative number of moles = $\frac{\%\mathrm{of}\mathrm{composition}}{\mathrm{Atomic}\mathrm{mass}}$

The relative number of moles for sodium $=\frac{43.04}{23}=1.88$ mol

The relative number of moles for carbon $=\frac{11.3}{12}=0.94$ mol

The relative number of moles for oxygen $=\frac{43.3}{16}=2.71$ mol

Step 4: Identifying the simple ratio

To identify the simple ratio, divide each of the relative numbers of moles by the least value amongst them.

Simple ratio of sodium $=\frac{1.88}{0.94}=2$

Simple ratio of carbon $=\frac{0.94}{0.94}=1$

Simple ratio of oxygen $=\frac{2.71}{0.94}=3$

Step 5: Finding the empirical formula

From the simple ratio identified above, the empirical formula is ${\mathrm{Na}}_2{\mathrm{CO}}_3$