Explanation

(a)
$In\ 15\ L\ of\ H_{2}\ gas\ at\ STP,\ the\ number\ of\ molecules$
$= \frac{6.023 \times 10^{23}}{22.4} \times 15$
$= 4.033 \times 10^{23}$
$In\ 5\ L\ of\ N_{2}\ gas\ at\ STP,\ the\ number\ of\ molecules$
$= \frac{6.023 \times 10^{23} \times 5}{22.4} = 1.344 \times 10^{23}$
$In\ 0.5\ g\ of\ H_{2}\ gas,\ the\ number\ of\ molecules$
$= \frac{6.023 \times 10^{23} \times 0.5}{2}$
$= 1.505 \times 10^{23}$
$In\ 10\ g\ of\ O_{2}\ gas,\ the\ number\ of\ molecules$
$= \frac{6.023 \times 10^{23} \times 10}{32}$
$= 1.882 \times 10^{23}$
$Hence,\ maximum\ molecules\ are\ present\ in\ 15L\ of\ H_{2}\ at\ STP$.