Going from grams to moles for paracetamol means dividing by its molar mass, 151.165 g/mol. One gram is 0.00661529 mol, or 6.61529 mmol; 100 mg is 0.66153 mmol; and a 151.165 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, C8H9NO2, which is where these figures come from.
The figure comes from the formula. Paracetamol is 8 × 12.011 (C) + 9 × 1.008 (H) + 14.007 (N) + 2 × 15.999 (O), which is 151.165 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Carbon makes up the largest share of the mass: 8 of the 20 atoms in a formula unit are carbon, and they account for 96.088 g of the 151.165 g, or 63.56% 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 0.06615 mmol of paracetamol; on a three-decimal balance it is 1 mg, or 6.615 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.
Acetaminophen: 500 mg per standard tablet. The therapeutic and the hepatotoxic doses are closer together than for most drugs, hence the warnings. At 151.165 g/mol it is heavier than 13 of the 21 organic compounds covered here, and that ranking matters more than it looks: Tris base has a molar mass of 121.136 g/mol, so a gram of it contains 24.8% more formula units than a gram of paracetamol. Weigh by mass and you are not weighing equal amounts of substance.
Two practical things move the result. Purity below 100% means the weighed mass overstates how much compound you have — at 98% assay, 3.0233 g of every 151.165 g is not the compound — and any water picked up from the air does the same. Both are systematic, so they shift every solution made from that jar in the same direction.
The formula
m- The mass you weighed, in grams
151.165- The molar mass of paracetamol 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 151.165 g/mol, the molar mass of paracetamol.
- 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 paracetamol in moles
0.25 ÷ 151.165 = 0.00165382 mol, or 1.6538 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 6.615 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of paracetamol in moles
2 ÷ 151.165 = 0.0132306 mol, or 13.231 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 6.615 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of paracetamol in moles
25 ÷ 151.165 = 0.165382 mol, or 165.38 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 6.615 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 paracetamol (C8H9NO2)?
It is 151.165 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 8 × 12.011 (C) + 9 × 1.008 (H) + 14.007 (N) + 2 × 15.999 (O). One mole of paracetamol therefore weighs 151.165 g, and a gram of it is 6.6153 mmol.
What percentage of paracetamol is carbon?
63.56% by mass. Each formula unit contains 8 carbon atoms contributing 96.088 g of the 151.165 g total, so 100 g of paracetamol contains 63.56 g of carbon and a kilogram contains 635.65 g of it.
How do I convert grams of paracetamol to moles?
Divide the mass in grams by 151.165 g/mol. So 1 g is 0.00661529 mol, 10 g is 0.0661529 mol and 100 g is 0.661529 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of paracetamol?
0.661529 mol, which is 661.529 mmol. Read the other way, one mole of paracetamol is 151.165 g, so 100 g is 0.6615 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of paracetamol is 6.6153 µmol, 10 mg is 66.153 µmol and 100 mg is 0.66153 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 paracetamol, which is 0.00648298 mol rather than 0.00661529 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 paracetamol
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Carbon (C) | 8 | 12.011 | 96.088 | 63.56% |
| Oxygen (O) | 2 | 15.999 | 31.998 | 21.17% |
| Nitrogen (N) | 1 | 14.007 | 14.007 | 9.266% |
| Hydrogen (H) | 9 | 1.008 | 9.072 | 6.001% |
| Total — one mole of paracetamol | 151.165 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 151.165 g/mol.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000007 | 0.00661529 | 6.61529 |
| 5 mg | 0.000033 | 0.0330764 | 33.0764 |
| 10 mg | 0.000066 | 0.0661529 | 66.1529 |
| 25 mg | 0.000165382 | 0.165382 | 165.382 |
| 50 mg | 0.000330764 | 0.330764 | 330.764 |
| 100 mg | 0.000661529 | 0.661529 | 661.529 |
| 250 mg | 0.00165382 | 1.65382 | 1653.82 |
| 500 mg | 0.00330764 | 3.30764 | 3307.64 |
| 1 g | 0.00661529 | 6.61529 | 6615.29 |
| 2 g | 0.0132306 | 13.2306 | 13,230.6 |
| 5 g | 0.0330764 | 33.0764 | 33,076.4 |
| 10 g | 0.0661529 | 66.1529 | 66,152.9 |
| 25 g | 0.165382 | 165.382 | 165,382 |
| 50 g | 0.330764 | 330.764 | 330,764 |
| 100 g | 0.661529 | 661.529 | 661,529 |
| 250 g | 1.65382 | 1653.82 | 1653821.982602 |
| 500 g | 3.30764 | 3307.64 | 3307643.965204 |
Every row is the mass divided by 151.165 g/mol. A balance reading to 1 mg resolves 6.615 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 |
|---|---|---|---|
| Creatinine | C4H7N3O | 113.12 | 8.8402 |
| Octane | C8H18 | 114.232 | 8.7541 |
| Tris base | C4H11NO3 | 121.136 | 8.2552 |
| Paracetamol (this page) | C8H9NO2 | 151.165 | 6.6153 |
| Uric acid | C5H4N4O3 | 168.112 | 5.9484 |
| Glucose | C6H12O6 | 180.156 | 5.5507 |
| Aspirin | C9H8O4 | 180.159 | 5.5507 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.