A mixture of hydrogen and oxygen has volume 2000 cm ^ 3 ,...

From the ideal gas equation, total number of moles in the mixture are,

$n = \dfrac{PV}{RT} = \dfrac{2 \times {10}^{- 3} \times 100 \times {10}^{3} \times 3}{25 \times 300}$$⇒ n = \dfrac{2}{25} = 0 . 08$

If number of moles of oxygen are ${n}_{1}$ and of hydrogen are ${n}_{2}$

${n}_{1} + {n}_{2} = n = 0 . 08 \ldots \left(1\right)$

And total mass of the mixture

${n}_{1}\left(32\right) + {n}_{2}\left(2\right) = 0 . 76 \ldots \left(2\right)$

Solving the above equations,

${n}_{1} = 0 . 02$ and ${n}_{2} = 0 . 06$

$\dfrac{{n}_{2}}{{n}_{1}} = 3$