Chemistry · Ch 1 — Basic Concepts of Chemistry and Chemical Calculations
Calculation of Molecular Formula from Empirical Formula
Calculation of Molecular Formula from Empirical Formula
The empirical formula only tells you the ratio of atoms -- to get the actual molecular formula, you additionally need the compound's molar mass. Since the molecular formula is always a whole-number multiple of the empirical formula, that multiple n is found from:
n = (molar mass of the compound) ÷ (calculated empirical formula mass)
and the molecular formula is then (empirical formula) × n. Table 1.6.2 works this out for five compounds (acetic acid, hydrogen peroxide, lactic acid, tartaric acid and benzene) -- notice that acetic acid and lactic acid share the same empirical formula, CH₂O, but differ in their multiplier n (2 versus 3) and hence in their actual molecular formula. …
| Compound | Empirical formula | Molar mass | Empirical formula mass | n | Molecular formula |
|---|---|---|---|---|---|
| Acetic acid | CH₂O | 60 | 30 | 60/30 = 2 | C₂H₄O₂ |
| Hydrogen peroxide | HO | 34 | 17 | 34/17 = 2 | H₂O₂ |
| Lactic acid | CH₂O | 90 | 30 | 90/30 = 3 | C₃H₆O₃ |
Worked out. Both compounds share the same mass-percentage composition (C-40%, H-6.6%, O-53.4%), so both share the empirical formula CH₂O (empirical-formula mass 30 g mol⁻¹). For the vinegar acid (molar mass 60), n = 60/30 = 2, so the molecular formula is (CH₂O)₂ = C₂H₄O₂ (acetic acid). For the sour-milk acid (molar mass 90), n = 90/30 = 3, so the molecular formula is (CH₂O)₃ = C₃H₆O₃ (lactic acid). …
Worked out. An in-text practice box: a compound of elements x, y, z has composition x = 32%, y = 24%, z = 44%, with relative numbers of atoms of x, y, z equal to 2, 1 and 0.5 respectively, and molecular mass 400 g. Find (i) the atomic masses of x, y, z, (ii) the empirical formula, and (iii) the molecular formula. …