The molar mass of formic acid, CH2O2, is 46.025 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 46.025 g, and a gram of it is 1.308e+22 formula units. The calculator turns any weighed mass into that count, and reports the amount in moles, millimoles and the mass of a single formula unit alongside it.
The figure comes from the formula. Formic acid is 12.011 (C) + 2 × 1.008 (H) + 2 × 15.999 (O), which is 46.025 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Oxygen makes up the largest share of the mass: 2 of the 5 atoms in a formula unit are oxygen, and they account for 31.998 g of the 46.025 g, or 69.52% by mass. The derivation table below breaks the whole molecule down element by element.
The numbers a bench actually uses sit at the millimole scale. One millimole of formic acid is 46.025 mg and one micromole is 46.025 µg, so a balance reading to a milligram places you within 21.73 µmol. A single formula unit weighs 7.6426e-23 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
The simplest carboxylic acid, present in ant venom, used in leather tanning and as a preservative in silage. At 46.025 g/mol it is heavier than 1 of the 12 acids covered here, and that ranking matters more than it looks: hydrochloric acid has a molar mass of 36.458 g/mol, so a gram of it contains 26.2% more formula units than a gram of formic acid. Weigh by mass and you are not weighing equal amounts of substance.
One more step applies if you are starting from a concentrated bottle. Concentrated acids are labelled as a percentage by weight together with a density, not as a molarity, so getting to mol/L means multiplying density by that percentage and dividing by 46.025 g/mol. A page like this one gives you the last of those three numbers; the first two are on the bottle, and they vary between grades.
The formula
m- The mass of formic acid in grams
46.025- The molar mass of formic acid in g/mol, derived from its formula
N- The number of formula units that mass contains
How it works, step by step
- Enter a mass of formic acid in grams.
- It is divided by the molar mass, 46.025 g/mol, giving the amount in moles.
- That amount is multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole, to give the number of formula units.
- The panel also reports the amount in moles and millimoles and the mass of one formula unit.
Worked examples
0.5 g of formic acid as a molecule count
0.5 g divided by 46.025 g/mol is 0.0108637 mol, and multiplying by the Avogadro constant gives 6.542e+21 formula units of formic acid. The same mass is 10.864 mmol, which is the figure a reaction is actually planned in.
5 g of formic acid as a molecule count
5 g divided by 46.025 g/mol is 0.108637 mol, and multiplying by the Avogadro constant gives 6.542e+22 formula units of formic acid. The same mass is 108.64 mmol, which is the figure a reaction is actually planned in.
50 g of formic acid as a molecule count
50 g divided by 46.025 g/mol is 1.08637 mol, and multiplying by the Avogadro constant gives 6.542e+23 formula units of formic acid. The same mass is 1086.4 mmol, which is the figure a reaction is actually planned in.
How to read your score
Frequently asked questions
What is the molar mass of formic acid (CH2O2)?
It is 46.025 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 12.011 (C) + 2 × 1.008 (H) + 2 × 15.999 (O). One mole of formic acid therefore weighs 46.025 g, and a gram of it is 21.727 mmol.
What percentage of formic acid is oxygen?
69.52% by mass. Each formula unit contains 2 oxygen atoms contributing 31.998 g of the 46.025 g total, so 100 g of formic acid contains 69.52 g of oxygen and a kilogram contains 695.23 g of it.
How many molecules are in one gram of formic acid?
About 1.308e+22. One gram is 0.0217273 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 46.025.
How much does a single molecule of formic acid weigh?
7.6426e-23 g, which is 46.025 daltons. It is the molar mass divided by the Avogadro constant, and the number in daltons is numerically the same as the molar mass in g/mol — that equivalence is what makes the mole convenient.
Is molar mass the same as molecular weight?
They are the same number in ordinary use but not the same quantity. Molecular weight, properly relative molecular mass, is a ratio and has no unit: for formic acid it is 46.025. Molar mass carries grams per mole: 46.025 g/mol. Because the mole is defined so those agree, you can read one off the other.
Why is the molar mass not a whole number?
Because standard atomic weights are averages over the isotopes found in nature. Oxygen is quoted as 15.999 rather than a whole number for that reason, and the same applies to the other elements here, which is why formic acid comes out at 46.025 g/mol rather than 46.
Molar mass, composition and mole equivalents of formic acid
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 2 | 15.999 | 31.998 | 69.52% |
| Carbon (C) | 1 | 12.011 | 12.011 | 26.1% |
| Hydrogen (H) | 2 | 1.008 | 2.016 | 4.38% |
| Total — one mole of formic acid | 46.025 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 46.025 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000046 | 0.046025 | 6.022e+17 |
| 10 µmol | 0.00046025 | 0.46025 | 6.022e+18 |
| 100 µmol | 0.0046025 | 4.6025 | 6.022e+19 |
| 1 mmol | 0.046025 | 46.025 | 6.022e+20 |
| 10 mmol | 0.46025 | 460.25 | 6.022e+21 |
| 50 mmol | 2.30125 | 2301.25 | 3.011e+22 |
| 100 mmol | 4.6025 | 4602.5 | 6.022e+22 |
| 250 mmol | 11.5062 | 11,506.2 | 1.506e+23 |
| 500 mmol | 23.0125 | 23,012.5 | 3.011e+23 |
| 750 mmol | 34.5187 | 34,518.8 | 4.517e+23 |
| 1 mol | 46.025 | 46,025 | 6.022e+23 |
| 2 mol | 92.05 | 92,050 | 1.204e+24 |
| 5 mol | 230.125 | 230,125 | 3.011e+24 |
| 10 mol | 460.25 | 460,250 | 6.022e+24 |
Mass is the amount multiplied by 46.025 g/mol. The last column is that amount multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| Hydrochloric acid | HCl | 36.458 | 27.429 |
| Formic acid (this page) | CH2O2 | 46.025 | 21.727 |
| Acetic acid | CH3COOH | 60.052 | 16.652 |
| Boric acid | H3BO3 | 61.831 | 16.173 |
| Nitric acid | HNO3 | 63.012 | 15.87 |
| Oxalic acid | C2H2O4 | 90.034 | 11.107 |
| Lactic acid | C3H6O3 | 90.078 | 11.101 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.