The amount of each reactant required to produce 750 ml of carbon dioxide, when two volumes of carbon monoxide combine with one volume of oxygen to produce two volumes of carbon dioxide. 2CO + O₂ ⟶ 2CO₂

All three are gases under the same conditions, so the coefficients give the volume ratio directly.

CO : O₂ : CO₂ = 2 : 1 : 2

To produce 750 ml of CO₂: volume of CO = (2 ÷ 2) × 750 = 750 ml volume of O₂ = (1 ÷ 2) × 750 = 375 ml

750 ml of carbon monoxide and 375 ml of oxygen are required.

Read the ratio off the balanced equation and scale it to the product you are asked for. Carbon monoxide and carbon dioxide are both 2, so they are equal — 750 ml each. Oxygen is half of that.

The mistake is scaling from the wrong end: multiplying 750 by 2 for the oxygen instead of halving it. Check the direction each time — the equation needs LESS oxygen than carbon monoxide, so the oxygen figure must be the smaller one.