$20 mL$ of a mixture of $CO$ and $H X_{2}$ was exploded with $25 mL$ of $O X_{2}$ . After cooling to room temperature the residual gas measured $27 mL$ . The volume of $CO$ in mL (nearest integer) in the original mixture was

Numerical answer type, answer in mL, tolerance 0.5.

Answer: 12 mL

Let

CO = a

H X_{2} = 20 - a

O X_{2}

used

= a /2 + (20 - a) /2 = 10 mL

for any

a

, leaving

15 mL

. Residual

= CO X_{2} + O X_{2} = a + 15 = 27

, so

a = 12 mL