If 8.2 L of hydrogen gas at STP are produced in this
reaction, how many atoms of sodium react?

The reaction is;

2Na + 2H2O → 2NaOH + H2

4.4 × 10^(23) atoms

Explanation:

The reaction is;

2Na + 2H2O → 2NaOH + H2

2 moles of Na produces 1 mole of H2 from the equation reaction.

Thus, since 8.2 L of hydrogen gas at STP are produced. And we know that at STP, 1 mole of a gas will have a volume of 22.4 L, and also, We know according to avogadro's number that, 1 mol = 6.02 × 10^(23) atoms, Thus;

8.2 L of H2 × (1 mole of H2/22.4 L) × (2 mole of Na/1 mole of H2) × (6.02 × 10^(23) atoms/1 mole of Na) = 4.4 × 10^(23) atoms

your answer would be a. 12.5 kg

12 is it do you think so