When a 19.34 gram piece of lithium is placed in a sample of water, how many grams of hydrogen gas are?

Molar mass of Li = 6.941 g/mol
Molar mass of H₂ = 1.008 × 2 g/mol = 2.016 g/mol

2 Li (s) + 2 H₂O(l) → 2 LiOH(aq) + H₂(g)
OR: Mole ratio Li : H₂ = 2 : 1

No. of moles of Li = (19.34 g) / (6.941 g/mol) = 2.786 mol
No. of moles of H₂ = (2.786 mol) × (1/2) = 1.393 mol
Mass of H₂ = (1.393 mol) × (2.016 g/mol) = 2.808 g

Calculate how many moles of Li you have

From the balanced equation, moles of H2 = moles of Li / 2

mass of H2 = moles of H2 * 2.016 g/mole. About 2.5 g