How much H2 gas at STP can be produced by

the reaction
2 Na(s) + 2 H2O(ℓ) −→ H2(g) + 2 NaOH(aq)
of 2.10 g of Na and excess water?
Answer in units of L.

2Na(s) + 2 H2O(ℓ) −→ H2(g) + 2 NaOH(aq)

mols Na = grams/atomic mass = 2.10/about 23 = about 0.09
Using the equation coefficients, convert mols Na to mols H2. You get 1/2 as much so 0.09/2 = about 0.045
You want grams H2 ? That is mols x molar mass = ? g
You want volume H2 ? That is mols x 22.4 L = ?L
Note that the 0.09 is an estimate and you should recalculate all of these values.