Determining a Molecular Formula from an Empirical Formula
1. A compund has an experimental molar mass of 78g/mol. Its empirical formula is CH. What is its molecular formula?
To determine the molecular formula from the empirical formula, you need to know the molar mass of the compound.
1. Start by calculating the empirical formula mass. To do this, add up the atomic masses of the elements in the empirical formula. In this case, the empirical formula is CH, so the empirical formula mass is calculated as:
Empirical formula mass = (Atomic mass of C) + (Atomic mass of H)
Empirical formula mass = (12.01 g/mol) + (1.008 g/mol)
Empirical formula mass = 13.018 g/mol
2. Next, divide the experimental molar mass of the compound by the empirical formula mass to get the ratio between the experimental and empirical formulas:
Molar ratio = Experimental molar mass / Empirical formula mass
Molar ratio = 78 g/mol / 13.018 g/mol
Molar ratio ≈ 6
3. Finally, multiply the subscripts of the empirical formula by the molar ratio calculated in step 2 to obtain the molecular formula. In this case, the molecular formula is obtained as:
Molecular formula = (C subscript) × molar ratio + (H subscript) × molar ratio
Molecular formula = (C subscript) × 6 + (H subscript) × 6
Since the empirical formula is CH, the molecular formula will also be CH. Therefore, the molecular formula is CH.
In summary, the molecular formula is CH.