Molar MassMolar mass of formic acid (HCOOH)
The molar mass of formic acid, HCOOH, is 46.02 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 46.02 grams.
46.02 g/mol
2 × 1.0080 + 12.011 + 2 × 15.999 = 46.025 · 5 atoms in the formula · also called methanoic acid
About formic acid
Formic acid is the simplest carboxylic acid. It is the irritant in ant stings and nettle stings, and takes its name from the Latin word for ant. Only the hydrogen of the COOH group is acidic.
Other names: methanoic acid.
The working
The formula HCOOH has 2 hydrogen atoms, 1 carbon atom and 2 oxygen atoms, 5 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Hydrogen (H) | 2 | 1.0080 | 2.016 | 4.38% |
| Carbon (C) | 1 | 12.011 | 12.011 | 26.10% |
| Oxygen (O) | 2 | 15.999 | 31.998 | 69.52% |
2 × 1.0080 + 12.011 + 2 × 15.999 = 46.025 g/mol
With the rounded atomic masses used in many school books (H = 1, C = 12, O = 16) the answer is 46 g/mol. The difference from 46.02 is too small to matter in most school sums.
Percentage composition
By mass, HCOOH is 4.38% hydrogen, 26.10% carbon and 69.52% oxygen. The largest share comes from oxygen. In a 100 g sample there are 4.4 g of hydrogen, 26.1 g of carbon and 69.5 g of oxygen.
Worked examples
Grams to moles. How many moles are in 25 g of HCOOH? Divide the mass by the molar mass: 25 ÷ 46.02 = 0.5432 mol.
Moles to grams. What is the mass of 0.1 mol of HCOOH? Multiply: 0.1 × 46.02 = 4.603 g.
Counting particles. 25 g contains 0.5432 mol × 6.022 × 1023 = 3.271 × 1023 molecules, and 1.636 × 1024 atoms in all.
Making a solution. A 0.1 mol/L solution contains 0.1 × 46.02 = 4.603 g of HCOOH in each litre. For 250 mL, weigh 1.151 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 4.603 g |
| 0.5 mol | 23.01 g |
| 1 mol | 46.02 g |
| 2 mol | 92.05 g |
| 5 mol | 230.1 g |
| Mass | Moles |
|---|---|
| 1 g | 0.02173 mol |
| 10 g | 0.2173 mol |
| 100 g | 2.173 mol |
| 1 kg | 21.73 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of HCOOH?
The molar mass of formic acid (HCOOH) is 46.02 g/mol, or 46.025 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of HCOOH?
100 ÷ 46.02 = 2.173 mol.
What is the mass of one molecule of HCOOH?
Divide the molar mass by the Avogadro constant: 46.02 ÷ 6.022 × 1023 = 7.643 × 10-23 g.
What is the percentage of oxygen in HCOOH?
Oxygen makes up 69.52% of the mass of HCOOH: 2 × 15.999 ÷ 46.025 × 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.