To solve this problem, we need to use the balanced chemical equation and the molar mass of oxygen gas to determine the number of moles of oxygen present:
2H2 + O2 -> 2H2O
Molar mass of O2 = 32 g/mol
2.1 g O2 x (1 mol O2/32 g O2) = 0.0656 mol O2
From the balanced equation, we know that 1 mole of O2 reacts with 2 moles of H2. Therefore, we can use a mole ratio to determine the number of moles of hydrogen required:
0.0656 mol O2 x (2 mol H2/1 mol O2) = 0.1312 mol H2
Therefore, 0.1312 moles of hydrogen will be required to react with 2.1g of oxygen gas.