Question

$12\text{g}$ of $Mg$ will react completely with a monoprotic acid to give:

• A. 1 mole of $O_2$
• B. 1/2 mole of $H_2$
• C. 1 mole of acid
• D. 5 moles of $H_2$

Answer 1

The correct option is B 1/2 mole of $H_2$

$Mg$ on reaction with an acid always liberates $H_2$ gas.
So, let us take the reaction:
$Mg+2HCl\to MgC{l}_2+H_2$
1 mol of $Mg$ reacts to give 1 mol of $H_2$
No. of moles of $Mg$$=\frac{\text{given mass}}{\text{molar mass}}=\frac{12}{24}=0.5\text{mol}$
So, $0.5\text{mol}$ of $Mg$ on reaction with an acid will give $0.5\text{mol}$ of hydrogen gas.