A hydrogen chloride gas reacted with oxygen gas to yield water and chlorine gas. The mole ratio of the hydrogen chloride gas to water is

Analyze the question: We need the balanced equation for HCl reacting with O₂ to give H₂O and Cl₂, then compare the coefficients of HCl and H₂O.

Concept: The mole ratio of reactants and products comes from the coefficients of the balanced equation.

Solution:

Unbalanced equation: HCl + O₂ → H₂O + Cl₂

Balance step by step:

• Oxygen: 2 O atoms on the left (in O₂), so we need 2 H₂O on the right.
• Hydrogen: 2 H₂O contains 4 H, so we need 4 HCl on the left.
• Chlorine: 4 HCl contains 4 Cl, so we need 2 Cl₂ on the right.

Balanced equation: 4HCl + O₂ → 2H₂O + 2Cl₂

Mole ratio of HCl : H₂O = 4 : 2 = 2 : 1

Checking the options: The ratio 4:2 simplifies to 2:1, which is option B. Option D (4:1) is a common trap. It comes from reading the coefficient of HCl (4) and forgetting that H₂O has a coefficient of 2, not 1.

Answer: B.

Tip: Always balance the equation first, then simplify the ratio of the coefficients. Don't just take the first numbers you see. Balance O first, then H, then Cl, since oxygen appears in only one place on each side.