How many hydrogen atoms are there in 3.50 grams of methane?

Answer is: there are 5,275·10²³ hydrogen atoms in methane.
m(CH₄) = 3,50 g.
n(CH₄) = m(CH₄) ÷ M(CH₄).
n(CH₄) = 3,5 g ÷ 16 g/mol.
n(CH₄) = 0,219 mol.
n(CH₄) : n(H) = 1 : 4.
n(H) = 4 · 0,219 mol = 0,876 mol.
N(H) = n(H) · Na.
N(H) = 0,876 mol · 6,022·10²³ 1/mol.
N(H) = 5,275·10²³.
Na - Avogadro number.