Step 1
The reaction must be written and balanced:
2H2(g) + O2(g) → 2H2O(g)
(Numbers next to the chemical formulas in bold mean number of moles)
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Step 2
Procedure:
2 x 1 mole H2 -------- 1 mole O2
3.9 moles H2 -------- X
X = (3.9 moles H2 x 1 mole O2)/2 x 1 mole H2 = 1.95 moles O2
Answer: 1.95 moles O2
