All the substances are gases, so we can use Gay-Lussac's Law of Combining Volumes:
The ratio of the volumes in which gases react and form products is the same as the ratio of their moles.
The equation for the reaction is
"Theor. Vol./cm"^3: color(white)(ml)2color(white)(mmll)1color(white)(mmm)2Theor. Vol./cm3:ml2mmll1mmm2
color(white)(mmmmmmmmm)"2CO" + "O"_2 → "2CO"_2mmmmmmmmm2CO+O2→2CO2
"Actual Vol./cm"^3: color(white)(m)20color(white)(mm)30Actual Vol./cm3:m20mm30
"Divide by:" color(white)(mmmmml)2color(white)(mmll)1Divide by:mmmmml2mmll1
"Moles of reaction:" color(white)(ml)10 color(white)(mm) 30Moles of reaction:ml10mm30
The coefficients of "CO"CO and "O"_2O2 in the equation tell us that the molecules react in a 2:1 ratio.
A quick way to identify a limiting reactant is to calculate its "moles of reaction".
We divide the number of moles (here they are "cm"^3cm3) by their coefficients in the balanced equation.
I did that for you in the lines above.
We see that "CO"CO is the limiting reactant, because it gives the fewest moles of reaction.
Calculate volume of "CO"_2CO2 formed
"Vol. of CO"_2 = 20 color(red)(cancel(color(black)("cm"^3color(white)(l) "CO"))) × ("2 cm"^3 color(white)(l)"CO"_2)/(2 color(red)(cancel(color(black)("cm"^3 color(white)(l)"CO")))) = "20 cm"^3color(white)(l) "CO"
Calculate volume of "O"_2 reacted
"Vol of O"_2 color(white)(l)"reacted" = 20 color(red)(cancel(color(black)("cm"^3color(white)(l) "CO"))) × ("1 cm"^3color(white)(l) "O"_2)/(2 color(red)(cancel(color(black)("cm"^3 color(white)(l)"CO")))) = "10 cm"^3color(white)(l) "O"_2
Calculate the volume of "O"_2 remaining
"O"_2color(white)(l) "remaining" = "30 cm"^3 - "10 cm"^3 = "20 cm"^3