(a) Of the three substances in the equation, let’s choose O2 as the reference because its coefficient is 1. For every molecule of O2 that disappears, two molecules of H2 disappear. Thus, the rate of [
b) Because [O2] is decreasing, the change in its concentration must be negative. We substitute the given rate as a negative value (−0.23 mol/L⋅s) into the expression and solve for Δ[H2O]/Δt.