The Earth’s atmosphere today has changed, when compared with the Earth’s earliest atmosphere - Edexcel - GCSE Chemistry - Question 3 - 2015 - Paper 1

Photosynthesis contributed to the composition of the Earth's atmosphere by reducing the levels of carbon dioxide. During this process, carbon dioxide is converted into glucose and oxygen. As a result, the concentration of oxygen in the atmosphere increased while the levels of carbon dioxide decreased.

The balanced equation for the reaction is:

$2Cu + O_2 \rightarrow 2CuO$

To calculate the volume of oxygen reacting with copper, we first consider the volumes involved in the experiment. Based on the values given,

egin{align*} 21 \text{ cm}^3 \times 50 \text{ (number of reactions)} \div 100 = 10.5 \text{ cm}^3 ext{(for 1 reaction)} ext{After 50 minutes, total volume is:} ext{50 reactions} = 50 \times 10.5 \text{ cm}^3 = 525 \text{ cm}^3 \end{align*}

The final calculation gives us:

(79 \text{ cm}^3 - 21 \text{ cm}^3 = 58 \text{ cm}^3)

So, the correct volume of oxygen is:

(79 \text{ cm}^3 - 58 \text{ cm}^3 = 39.5 \text{ cm}^3)