Which of the following is the correctly balanced chemical equation for the reaction between hydrogen gas and oxygen gas to form water?

Balance the oxygen atoms first by adjusting the coefficient in front of water. Since oxygen appears as \(\mathrm{O_2}\) on the left and as a single oxygen atom in \(\mathrm{H_2O}\) on the right, place a coefficient of 2 in front of \(\mathrm{H_2O}\) to have 2 oxygen atoms on the right: \(\mathrm{H_2 + O_2 \rightarrow 2H_2O}\).

Now balance the hydrogen atoms. With 2 molecules of water, there are 4 hydrogen atoms on the right, so place a coefficient of 2 in front of \(\mathrm{H_2}\) on the left to have 4 hydrogen atoms: \(\mathrm{2H_2 + O_2 \rightarrow 2H_2O}\).