Molar MassMolar mass of lactic acid (C3H6O3)
The molar mass of lactic acid, C3H6O3, is 90.08 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 90.08 grams.
90.08 g/mol
3 × 12.011 + 6 × 1.0080 + 3 × 15.999 = 90.078 · 12 atoms in the formula · also called 2-hydroxypropanoic acid
About lactic acid
Lactic acid forms when milk turns sour and in muscles during hard exercise. Its molecular formula is three times the empirical formula CH2O.
Other names: 2-hydroxypropanoic acid.
The working
The formula C3H6O3 has 3 carbon atoms, 6 hydrogen atoms and 3 oxygen atoms, 12 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Carbon (C) | 3 | 12.011 | 36.033 | 40.00% |
| Hydrogen (H) | 6 | 1.0080 | 6.048 | 6.71% |
| Oxygen (O) | 3 | 15.999 | 47.997 | 53.28% |
3 × 12.011 + 6 × 1.0080 + 3 × 15.999 = 90.078 g/mol
With the rounded atomic masses used in many school books (C = 12, H = 1, O = 16) the answer is 90 g/mol. The difference from 90.08 is too small to matter in most school sums.
Percentage composition
By mass, C3H6O3 is 40.00% carbon, 6.71% hydrogen and 53.28% oxygen. The largest share comes from oxygen. In a 100 g sample there are 40.0 g of carbon, 6.7 g of hydrogen and 53.3 g of oxygen.
Notice that hydrogen has more atoms in the formula (6) than oxygen (3), yet gives a smaller share of the mass. Percentage by mass depends on how heavy each atom is, not only on how many there are.
Worked examples
Grams to moles. How many moles are in 25 g of C3H6O3? Divide the mass by the molar mass: 25 ÷ 90.08 = 0.2775 mol.
Moles to grams. What is the mass of 0.1 mol of C3H6O3? Multiply: 0.1 × 90.08 = 9.008 g.
Counting particles. 25 g contains 0.2775 mol × 6.022 × 1023 = 1.671 × 1023 molecules, and 2.006 × 1024 atoms in all.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 9.008 g |
| 0.5 mol | 45.04 g |
| 1 mol | 90.08 g |
| 2 mol | 180.2 g |
| 5 mol | 450.4 g |
| Mass | Moles |
|---|---|
| 1 g | 0.0111 mol |
| 10 g | 0.111 mol |
| 100 g | 1.11 mol |
| 1 kg | 11.1 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of C3H6O3?
The molar mass of lactic acid (C3H6O3) is 90.08 g/mol, or 90.078 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of C3H6O3?
100 ÷ 90.08 = 1.11 mol.
What is the mass of one molecule of C3H6O3?
Divide the molar mass by the Avogadro constant: 90.08 ÷ 6.022 × 1023 = 1.496 × 10-22 g.
What is the percentage of oxygen in C3H6O3?
Oxygen makes up 53.28% of the mass of C3H6O3: 3 × 15.999 ÷ 90.078 × 100.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.