Which tube will collect the smallest volume of gas, A or B, and why?

• In Tube A, magnesium (Mg) reacts with oxygen ($\text{O}_2$) to form magnesium oxide ($\text{MgO}$) and with nitrogen ($\text{N}_2$) to form magnesium nitride ($\text{Mg}_3\text{N}_2$) when heated: $$2\text{Mg(s)} + \text{O}_2\text{(g)} \xrightarrow{\Delta} 2\text{MgO(s)}$$ $$3\text{Mg(s)} + \text{N}_2\text{(g)} \xrightarrow{\Delta} \text{Mg}_3\text{N}_2\text{(s)}$$
• In Tube B, zinc (Zn) reacts with oxygen ($\text{O}_2$) to form zinc oxide ($\text{ZnO}$) when heated. Zinc is generally not reactive enough to react with nitrogen under these conditions: $$2\text{Zn(s)} + \text{O}_2\text{(g)} \xrightarrow{\Delta} 2\text{ZnO(s)}$$

Magnesium is more reactive than zinc. Therefore, magnesium reacts with both oxygen (approximately 21% of air) and nitrogen (approximately 78% of air). Zinc primarily reacts only with oxygen.

Since magnesium removes both oxygen and nitrogen from the air, a significantly larger volume of the gaseous components of air will be consumed in Tube A compared to Tube B, where only oxygen is removed. Thus, the volume of unreacted gas collected from Tube A will be smaller.

The smallest volume of gas will be collected from Tube A. This is because magnesium is more reactive than zinc and will react with both oxygen and nitrogen in the air, whereas zinc will primarily react only with oxygen.