The diagram shows a model of a water molecule - Edexcel - GCSE Chemistry - Question 4 - 2012 - Paper 1

The relative formula mass of water can be calculated by adding the relative atomic masses of its components:

• For 2 hydrogen atoms: $2 \times 1.0 = 2.0$
• For 1 oxygen atom: $1 \times 16 = 16.0$

Thus, the total relative formula mass of water is:

$2.0 + 16.0 = 18.0$

The balanced equation for the combustion of hydrogen in oxygen to form water is: $2H_2 + O_2 \rightarrow 2H_2O$

The percentage yield can be calculated using the formula:

$\text{Percentage Yield} = \left( \frac{\text{actual yield}}{\text{theoretical yield}} \right) \times 100$

Substituting in the values:

$\text{Percentage Yield} = \left( \frac{2.0g}{4.0g} \right) \times 100 = 50\%$

One reason for obtaining less than 4.0 g of water could be due to losses during the experiment, such as evaporation or incomplete reaction.