Question

An unknown substance has the composition of 77.87% C, 2.63% H and 55.62% O. The compound has a molar mass of 345.2 g/mole. What is the molecular formula?

Answer 1

If we use 100 grams of compound:
m(C) = 0,7787 · 100 g = 77,87 g.
n(C) = 77,87 g ÷ 12 g/mol = 6,49 mol.
m(H) = 0,0263 · 100 g = 2,63 g.
n(H) = 2,63 g ÷ 1 g/mol = 2,63 mol.
m(O) = 0,5562 · 100 g = 55,62 g.
n(O) = 55,62 g ÷ 16 g/mol = 3,48 mol.
n(C) : n(H) : n(O) = 6,49 mol : 2,63 mol : 3,48 mol.
n(C) : n(H) : n(O) = 2,5 : 1 : 1,33.
From molecular mass only possible is C₁₂H₉O₁₂.