How many hydrogen atoms are there in 2.25 grams of methane?

Molar mass of CH₄ (methane) = (12.0 + 1.0×4) g/mol = 16.0 g/mol
No. of moles of CH₄ = (2.25 g) / (16.0 g/mol) = 2.25/16.0 mol

1 mole of CH₄ contains 4 moles of H atoms.
No. of moles of H atoms = (2.25/16.0 mol) × 4 = 2.25/4.00 mol

1 mole of H atoms contains 6.02 × 10²³ H atoms.
No. of H atoms = (2.25/4.00 mol) × (6.02 × 10²³ H atoms/mol) = 3.39 × 10²³ H atoms