4. (a) Air contains several gaseous elements - Edexcel - GCSE Chemistry Combined Science - Question 4 - 2020 - Paper 1

To calculate the density of argon, we first determine the mass of argon by subtracting the mass of the empty flask from the mass of the flask full of argon:

Mass of argon = 79.120 g - 78.639 g = 0.481 g.

Then, we use the volume of the flask, which is 250.0 cm³, to calculate the density using

density = \frac{mass}{volume}.

Thus, the density of argon = \frac{0.481 , \text{g}}{250.0 , \text{cm}^3} = 0.001924 , \text{g cm}^{-3}.

The complete ionic equation for the reaction of calcium carbonate with magnesium ions is:

CaCO₃ + Mg²⁺ \rightarrow CaMg(CO₃)₂ + Ca²⁺.

Based on the results shown in Figure 4, gas mixture P produced a white precipitate after reaching limewater after 4 minutes, suggesting little to no carbon dioxide, while gas mixture Q caused a white precipitate immediately. This indicates that gas mixture Q, which reacts faster with limewater, represents the early atmosphere, as it contained a higher level of carbon dioxide, a key component during Earth's early development.