Question

If 35.0 mL of 0.210 M HCl reacts with excess Mg, how many mL of hydrogen gas are produced at STP?

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

Is the answer 0.8176 L

Convert moles of HCl:

2.10 x 0.035= 0.00735 mol of HCl

Convert to moles of H2:

0.0735 x 1 mol H2/ 2 mols HCl= 0.03675 mol of H2

Convert to STP (L):

0.0365mol H2 x 22.4 L/1mol H2=0.8176

Answer 1

I think your last step is a little off; I have 82.3 mL.
I agree with your numbers up to multiplying by 22.4.