Determine the molecular formula of a compound that contains 26.7% , 12.1% , and 61.2% , and has a molar mass of 812 g/mol.

Answer is: the molecular formula of a compound is P₇N₇Cl₁₄.

If we use 100 grams of substance:

ω(P) = 26.7% ÷ 100% = 0.267.

m(P) = 0.267 · 100 g = 26.7 g.

n(P) = 26.7 g ÷ 31 g/mol = 0.86 mol.

n(N) = 12.1 g ÷ 14 g/mol = 0.86 mol.

n(Cl) = 61.2 ÷ 35.45 g/mol = 1.72 mol.

n(P) : n(N) : n(Cl) = 0.86 mol : 0.86 mol : 1.72 mol/ 0.86 mol.

n(P) : n(N) : n(Cl) =  1 : 1 : 2.

M(PₓNₓCl₂ₓ) = 812 g/mol.

M(PₓNₓCl₂ₓ) = 31x + 14x + 70.9x · g/mol.

812 g/mol = 115.9x · g/mol.

x = 7.

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