Going from grams to moles for nitric acid means dividing by its molar mass, 63.012 g/mol. One gram is 0.01587 mol, or 15.87 mmol; 100 mg is 1.587 mmol; and a 63.012 g portion is exactly one mole. The division is the whole calculation, but the molar mass has to be the one for the exact formula you have, HNO3, which is where these figures come from.
The figure comes from the formula. Nitric acid is 1.008 (H) + 14.007 (N) + 3 × 15.999 (O), which is 63.012 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: 3 of the 5 atoms in a formula unit are oxygen, and they account for 47.997 g of the 63.012 g, or 76.17% by mass. The derivation table below breaks the whole molecule down element by element.
Balance resolution is the practical limit here. On a two-decimal balance the smallest step is 10 mg, which is 0.1587 mmol of nitric acid; on a three-decimal balance it is 1 mg, or 15.87 µmol. To hit a target within 1% you therefore need to weigh at least a gram on the first and at least 100 mg on the second, and below that a stock solution measured by pipette beats weighing.
The acid for nitration and metal digestion. Concentrated is 68% w/w, near 15.8 mol/L, and it stains skin yellow on contact. At 63.012 g/mol it is heavier than 4 of the 12 acids covered here, and that ranking matters more than it looks: boric acid has a molar mass of 61.831 g/mol, so a gram of it contains 1.91% more formula units than a gram of nitric 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 63.012 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 you weighed, in grams
63.012- The molar mass of nitric acid in g/mol, derived from its formula
n- The amount of substance in moles
How it works, step by step
- Enter the mass you weighed, in grams.
- It is divided by 63.012 g/mol, the molar mass of nitric acid.
- The result is the amount of substance in moles, with millimoles and micromoles beneath it.
- Check it against the table if you want the figure for a standard weighing rather than a typed mass.
Worked examples
0.25 g of nitric acid in moles
0.25 ÷ 63.012 = 0.0039675 mol, or 3.9675 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 15.87 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of nitric acid in moles
2 ÷ 63.012 = 0.03174 mol, or 31.74 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 15.87 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of nitric acid in moles
25 ÷ 63.012 = 0.39675 mol, or 396.75 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 15.87 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
How to read your score
Frequently asked questions
What is the molar mass of nitric acid (HNO3)?
It is 63.012 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 1.008 (H) + 14.007 (N) + 3 × 15.999 (O). One mole of nitric acid therefore weighs 63.012 g, and a gram of it is 15.87 mmol.
What percentage of nitric acid is oxygen?
76.17% by mass. Each formula unit contains 3 oxygen atoms contributing 47.997 g of the 63.012 g total, so 100 g of nitric acid contains 76.17 g of oxygen and a kilogram contains 761.71 g of it.
How do I convert grams of nitric acid to moles?
Divide the mass in grams by 63.012 g/mol. So 1 g is 0.01587 mol, 10 g is 0.1587 mol and 100 g is 1.587 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of nitric acid?
1.587 mol, which is 1587 mmol. Read the other way, one mole of nitric acid is 63.012 g, so 100 g is 1.587 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of nitric acid is 15.87 µmol, 10 mg is 158.7 µmol and 100 mg is 1.587 mmol. Working in µmol per mg avoids the string of leading zeros that mol per g produces at this scale.
Does purity change the number of moles?
Yes, in proportion. A 98% pure sample weighing 1 g contains 0.98 g of nitric acid, which is 0.0155526 mol rather than 0.01587 mol. The calculator assumes the mass you enter is the compound itself, so divide by the assay figure first if you need the corrected amount.
Grams to moles reference for nitric acid
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 3 | 15.999 | 47.997 | 76.17% |
| Nitrogen (N) | 1 | 14.007 | 14.007 | 22.23% |
| Hydrogen (H) | 1 | 1.008 | 1.008 | 1.6% |
| Total — one mole of nitric acid | 63.012 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 63.012 g/mol.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000016 | 0.01587 | 15.87 |
| 5 mg | 0.000079 | 0.07935 | 79.35 |
| 10 mg | 0.0001587 | 0.1587 | 158.7 |
| 25 mg | 0.00039675 | 0.39675 | 396.75 |
| 50 mg | 0.0007935 | 0.7935 | 793.5 |
| 100 mg | 0.001587 | 1.587 | 1587 |
| 250 mg | 0.0039675 | 3.9675 | 3967.5 |
| 500 mg | 0.007935 | 7.935 | 7935 |
| 1 g | 0.01587 | 15.87 | 15,870 |
| 2 g | 0.03174 | 31.74 | 31,740 |
| 5 g | 0.07935 | 79.35 | 79,350 |
| 10 g | 0.1587 | 158.7 | 158,700 |
| 25 g | 0.39675 | 396.75 | 396,750 |
| 50 g | 0.7935 | 793.5 | 793,500 |
| 100 g | 1.587 | 1587 | 1586999.301720 |
| 250 g | 3.9675 | 3967.5 | 3967498.254301 |
| 500 g | 7.935 | 7935 | 7934996.508602 |
Every row is the mass divided by 63.012 g/mol. A balance reading to 1 mg resolves 15.87 µmol of this compound, which is the smallest step the middle columns can really move in.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| Formic acid | CH2O2 | 46.025 | 21.727 |
| Acetic acid | CH3COOH | 60.052 | 16.652 |
| Boric acid | H3BO3 | 61.831 | 16.173 |
| Nitric acid (this page) | HNO3 | 63.012 | 15.87 |
| Oxalic acid | C2H2O4 | 90.034 | 11.107 |
| Lactic acid | C3H6O3 | 90.078 | 11.101 |
| Phosphoric acid | H3PO4 | 97.99376 | 10.205 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.