Moles to grams for paracetamol is a multiplication by 151.165 g/mol. A mole weighs 151.165 g, 100 mmol weighs 15.1165 g, 10 mmol weighs 1.51165 g and a millimole is 151.165 mg. Those four figures cover most of what gets weighed out in practice, and the table below extends them from a micromole up to ten moles.
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.
Two habits make the weighed mass match the calculation. Weigh into a container you have tared rather than onto paper, because what stays behind on paper is a real loss — on a 100 mg weighing it is easily a percent. And when the target is under about 20 mg, weigh ten times that and dilute, since the absolute error of the balance does not shrink with the sample.
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
n- The amount of paracetamol you need, in moles
151.165- The molar mass of paracetamol in g/mol, derived from its formula
m- The mass to weigh out, in grams
How it works, step by step
- Enter the amount you need, in moles.
- It is multiplied by 151.165 g/mol.
- The result is the mass to weigh out, also given in milligrams.
- Weigh into a tared container; anything left in the boat is a real loss from the amount you calculated.
Worked examples
5 mmol of paracetamol in grams
5 mmol × 151.165 g/mol = 0.755825 g, which is 755.825 mg. Doubling the amount doubles the mass to 1.51165 g, because the relationship is a plain proportion once the molar mass is fixed.
250 mmol of paracetamol in grams
250 mmol × 151.165 g/mol = 37.7912 g, which is 37,791.2 mg. Doubling the amount doubles the mass to 75.5825 g, because the relationship is a plain proportion once the molar mass is fixed.
2 mol of paracetamol in grams
2 mol × 151.165 g/mol = 302.33 g, which is 302,330 mg. Doubling the amount doubles the mass to 604.66 g, because the relationship is a plain proportion once the molar mass is fixed.
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 many grams is one mole of paracetamol?
151.165 g, by definition of the molar mass. Two moles is 302.33 g and half a mole is 75.5825 g.
How many grams is 0.1 mol, and how many milligrams is 1 mmol?
0.1 mol of paracetamol is 15.1165 g. One millimole is 151.165 mg and one micromole is 151.165 µg, which is the range most bench work sits in.
How do I weigh out an amount too small for my balance?
Weigh a larger mass and dilute it. To get 1.512 mg of paracetamol, weigh 15.12 mg into 100 mL, then take 10 mL of that: the absolute error of the balance stays the same while the amount delivered falls by a factor of ten.
Should I use the hydrate or the anhydrous molar mass?
This page is the anhydrous form, C8H9NO2, at 151.165 g/mol. If your jar is the other form the molar mass differs and so will every mass you weigh, so check the label before using the figure. Salts that are sold as hydrates weigh more per mole because the water is part of the crystal.
Moles to grams 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.
| Wanted | Weigh (g) | Weigh (mg) | With 2% over (g) |
|---|---|---|---|
| 1 µmol | 0.000151165 | 0.151165 | 0.000154188 |
| 10 µmol | 0.00151165 | 1.51165 | 0.00154188 |
| 100 µmol | 0.0151165 | 15.1165 | 0.0154188 |
| 1 mmol | 0.151165 | 151.165 | 0.154188 |
| 10 mmol | 1.51165 | 1511.65 | 1.54188 |
| 50 mmol | 7.55825 | 7558.25 | 7.70941 |
| 100 mmol | 15.1165 | 15,116.5 | 15.4188 |
| 250 mmol | 37.7912 | 37,791.2 | 38.5471 |
| 500 mmol | 75.5825 | 75,582.5 | 77.0941 |
| 750 mmol | 113.374 | 113,374 | 115.641 |
| 1 mol | 151.165 | 151,165 | 154.188 |
| 2 mol | 302.33 | 302,330 | 308.377 |
| 5 mol | 755.825 | 755,825 | 770.941 |
| 10 mol | 1511.65 | 1511650.000000 | 1541.88 |
Weigh is the amount multiplied by 151.165 g/mol. The last column carries a 2% allowance, which is what you take when some of the solid will be left in the weighing boat.
| 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.