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.
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
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