Precise Relative Molecular Mass

The precise relative molecular mass of a molecule is the sum of the precise relative isotopic masses of the atoms in its molecular formula, using the most abundant isotope of each element.Values usually given in questions:
¹H = 1.0078, ¹²C = 12.0000, ¹⁴N = 14.0031, ¹⁶O = 15.9949Method
1. Write each possible molecular formula with the same whole-number M_r.
2. Multiply the number of each type of atom by its precise mass and add them up.
3. Choose the formula whose total matches the measured m/z of the molecular ion.Worked example: a gas gives a molecular ion peak at m/z = 28.0312. Is it N₂, CO or C₂H₄?
N₂: 2 × 14.0031 = 28.0062
CO: 12.0000 + 15.9949 = 27.9949
C₂H₄: (2 × 12.0000) + (4 × 1.0078) = 24.0000 + 4.0312 = 28.0312
The gas is ethene, C₂H₄.Second example: a compound with a molecular ion at m/z = 60.0210 could be C₃H₈O or C₂H₄O₂.
C₃H₈O: 36.0000 + 8.0624 + 15.9949 = 60.0573
C₂H₄O₂: 24.0000 + 4.0312 + 31.9898 = 60.0210
The measured value matches C₂H₄O₂.Tips
Keep all four decimal places throughout and compare only at the end.
The differences are small, often in the second or third decimal place, so a rounding error can give the wrong answer.
Check the whole-number M_r first: every candidate must add up to the same integer.