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Changes in concentration are proportional to the molar quantities in the balanced equation, so by knowing just one of the reactant's initial and equilibrium concentrations, we can determine those of the rest of the reactants and products. We are given the initial and equilibrium concentration of NO, which we can use to find its change in concentration. Since the volume of the container is 1.0 L , the molar value of a given substance is equivalent to the concentration of the substance.
Change in concentration \( =[ \) Final \( ]-[ \) Initial \( ] \)
Change in \( [\mathrm{NO}]=0.062-0.100=-3.8 \times 10^{-2} M \)
Since NO decreases in concentration by \( 3.8 \times 10^{-2} M \), you can expect a proportional change in \( \mathrm{H}_{2} \). Both NO and \( \mathrm{H}_{2} \) have a coefficient of 2 in the balanced equation, so they are \( 1: 1 \) in the reaction and will undergo the same change in concentration. Since \( \mathrm{H}_{2} \) is a reactant, it will be consumed as the reaction proceedes, and therefore its change is a decrease in concentration, just as it was for NO.
Equilibrium Concentration \( =[ \) Initial \( ]+[ \) Change \( ] \) Equilibrium \( \left[\mathrm{H}_{2}\right]=0.050+\left(-3.8 \times 10^{-2}\right)=1.2 \times 10^{-2} M \)
