What is the molecular formula for Lactic acid if an elemental analysis of this compound shows that it contains 40.0 mass % C, 6.71 mass % H and 53.3 mass % O and a mass spectrometry experiment indicates its molar mass is 90.08 g/mol.

Question

Quizplus · Accepted Answer

To determine the molecular formula, we need to find the empirical formula first. 
- Assume we have 100 g of the compound, which means we have 40.0 g C, 6.71 g H, and 53.3 g O.
- Convert each of the masses to moles by dividing by their molar masses: 
    - 40.0 g C / 12.01 g/mol = 3.33 mol C
    - 6.71 g H / 1.01 g/mol = 6.64 mol H
    - 53.3 g O / 16.00 g/mol = 3.33 mol O
- Divide each of the mole values by the smallest number of moles to get a ratio of whole numbers:
    - C: 3.33 mol / 3.33 mol = 1
    - H: 6.64 mol / 3.33 mol = 1.99, ~2
    - O: 3.33 mol / 3.33 mol = 1
- The empirical formula is thus CH2O.
- The molar mass of this empirical formula is:
    - 12.01 g/mol (C) + 2.02 g/mol (H) + 16.00 g/mol (O) = 30.03 g/mol
- The molecular formula has a molar mass of 90.08 g/mol, which is exactly 3 times the empirical formula mass. Therefore, the molecular formula is simply 3 times the empirical formula: C3H6O3, which matches option C.

What Is the Molecular Formula for Lactic Acid If an Elemental