(a) Sodium metal tarnishes in air to form a layer of sodium oxide on its surface - Edexcel - GCSE Chemistry - Question 9 - 2018 - Paper 1

For oxygen, the mass is 0.32 g and the molar mass of oxygen (O) is 16 g/mol:

$\text{moles of O} = \frac{0.32 \text{ g}}{16 \text{ g/mol}} = 0.020 \text{ mol}$

Next, we divide each number of moles by the smallest number of moles, which is for oxygen:

• Moles of Na: $\frac{0.040}{0.020} = 2$
• Moles of O: $\frac{0.020}{0.020} = 1$

This gives us a ratio of 2:1.

Thus, the empirical formula of sodium oxide is $\text{Na}_2\text{O}$.

The reaction of sodium with water can be represented as:

$\text{Na (s)} + 2\text{H}_2\text{O (l)} \rightarrow \text{NaOH (aq)} + \text{H}_2 (g)$

In this equation:

• Na is a solid (s)
• H2O is a liquid (l)
• NaOH is in aqueous solution (aq)
• H2 is a gas (g)