9 (a) The rate of reaction between magnesium ribbon and dilute hydrochloric acid at room temperature is investigated - Edexcel - GCSE Chemistry - Question 9 - 2018 - Paper 1

To calculate the rate of reaction, you will need to find the tangent to the curve at the chosen point. From the graph in Figure 12, measure the difference in volume and time. Assuming a volume change of 15 cm³ over a time interval of 60 s, the rate of reaction is calculated as:

$ext{Rate of reaction} = \frac{\text{change in volume}}{\text{time taken}} = \frac{15 \text{ cm}³}{60 s} = 0.25 \text{ cm}³/s$

The expected line on the graph would be steeper, indicating a faster rate of reaction, due to the increased surface area of the powdered magnesium compared to the ribbon. It should start at the same initial volume and continue to rise before leveling off at a similar final volume.

To find the number of moles of magnesium in the sample:

$\text{Number of moles} = \frac{\text{mass (g)}}{\text{relative atomic mass}} = \frac{0.1}{24} = 0.00417 \text{ moles}$

The balanced equation shows that 1 mole of Mg reacts with 2 moles of HCl. In this case, if 0.5 moles of HCl are present,

$0.5 \text{ moles of HCl} \times \frac{1 \text{ mole of Mg}}{2 \text{ moles of HCl}} = 0.25 \text{ moles of Mg needed}$

Since we have 0.5 moles of Mg available, it is clear that Mg is in excess.