To determine the number of moles of hydrogen gas evolved when an excess of sodium metal reacts with one mole of citric acid, we need to examine the balanced chemical equation for the reaction between sodium and citric acid.
The balanced chemical equation for the reaction can be written as follows:
3 Na + C6H8O7 → 3 NaC6H7O7 + 3 H2
From the balanced equation, we can see that 3 moles of sodium (Na) react with one mole of citric acid (C6H8O7) to produce 3 moles of sodium citrate (NaC6H7O7) and 3 moles of hydrogen gas (H2).
Since the excess of sodium metal is added, it implies that all of the sodium will react with citric acid. Therefore, the number of moles of hydrogen gas evolved will be equal to the stoichiometric coefficient of H2, which is 3.
Thus, when an excess of sodium metal is added to one mole of citric acid, three moles of hydrogen gas will be evolved.