Going from grams to moles for urea means dividing by its molar mass, 60.056 g/mol. One gram is 0.0166511 mol, or 16.6511 mmol; 100 mg is 1.6651 mmol; and a 60.056 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, CH4N2O, which is where these figures come from.
The figure comes from the formula. Urea is 12.011 (C) + 4 × 1.008 (H) + 2 × 14.007 (N) + 15.999 (O), which is 60.056 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Nitrogen makes up the largest share of the mass: 2 of the 8 atoms in a formula unit are nitrogen, and they account for 28.014 g of the 60.056 g, or 46.65% 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 0.1665 mmol of urea; on a three-decimal balance it is 1 mg, or 16.65 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 nitrogen fertiliser with the highest nitrogen content, a skin humectant, and the compound the body uses to excrete nitrogen — reported as BUN. At 60.056 g/mol it is heavier than 3 of the 21 organic compounds covered here, and that ranking matters more than it looks: acetone has a molar mass of 58.08 g/mol, so a gram of it contains 3.4% more formula units than a gram of urea. Weigh by mass and you are not weighing equal amounts of substance.
Purity is the usual gap between the calculation and the balance. A reagent sold at 98% means 1.2011 g of a nominal 60.056 g is something else, so for exact work you divide the weighed mass by the assay figure on the certificate. For most purposes the difference is smaller than the error in reading the meniscus, but it is systematic rather than random, so it does not average out.
The formula
m- The mass you weighed, in grams
60.056- The molar mass of urea 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 60.056 g/mol, the molar mass of urea.
- 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 urea in moles
0.25 ÷ 60.056 = 0.00416278 mol, or 4.1628 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 16.65 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of urea in moles
2 ÷ 60.056 = 0.0333023 mol, or 33.302 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 16.65 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of urea in moles
25 ÷ 60.056 = 0.416278 mol, or 416.28 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 16.65 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 urea (CH4N2O)?
It is 60.056 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 12.011 (C) + 4 × 1.008 (H) + 2 × 14.007 (N) + 15.999 (O). One mole of urea therefore weighs 60.056 g, and a gram of it is 16.651 mmol.
What percentage of urea is nitrogen?
46.65% by mass. Each formula unit contains 2 nitrogen atoms contributing 28.014 g of the 60.056 g total, so 100 g of urea contains 46.65 g of nitrogen and a kilogram contains 466.46 g of it.
How do I convert grams of urea to moles?
Divide the mass in grams by 60.056 g/mol. So 1 g is 0.0166511 mol, 10 g is 0.166511 mol and 100 g is 1.66511 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of urea?
1.66511 mol, which is 1665.11 mmol. Read the other way, one mole of urea is 60.056 g, so 100 g is 1.665 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of urea is 16.651 µmol, 10 mg is 166.51 µmol and 100 mg is 1.6651 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 urea, which is 0.0163181 mol rather than 0.0166511 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 urea
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Nitrogen (N) | 2 | 14.007 | 28.014 | 46.65% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 26.64% |
| Carbon (C) | 1 | 12.011 | 12.011 | 20% |
| Hydrogen (H) | 4 | 1.008 | 4.032 | 6.714% |
| Total — one mole of urea | 60.056 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 60.056 g/mol.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000017 | 0.0166511 | 16.6511 |
| 5 mg | 0.000083 | 0.0832556 | 83.2556 |
| 10 mg | 0.000166511 | 0.166511 | 166.511 |
| 25 mg | 0.000416278 | 0.416278 | 416.278 |
| 50 mg | 0.000832556 | 0.832556 | 832.556 |
| 100 mg | 0.00166511 | 1.66511 | 1665.11 |
| 250 mg | 0.00416278 | 4.16278 | 4162.78 |
| 500 mg | 0.00832556 | 8.32556 | 8325.56 |
| 1 g | 0.0166511 | 16.6511 | 16,651.1 |
| 2 g | 0.0333023 | 33.3023 | 33,302.3 |
| 5 g | 0.0832556 | 83.2556 | 83,255.6 |
| 10 g | 0.166511 | 166.511 | 166,511 |
| 25 g | 0.416278 | 416.278 | 416,278 |
| 50 g | 0.832556 | 832.556 | 832,556 |
| 100 g | 1.66511 | 1665.11 | 1665112.561609 |
| 250 g | 4.16278 | 4162.78 | 4162781.404023 |
| 500 g | 8.32556 | 8325.56 | 8325562.808046 |
Every row is the mass divided by 60.056 g/mol. A balance reading to 1 mg resolves 16.65 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 |
|---|---|---|---|
| Methanol | CH4O | 32.042 | 31.209 |
| Ethanol | C2H6O | 46.069 | 21.707 |
| Acetone | C3H6O | 58.08 | 17.218 |
| Urea (this page) | CH4N2O | 60.056 | 16.651 |
| Isopropyl alcohol | C3H8O | 60.096 | 16.64 |
| Ethylene glycol | C2H6O2 | 62.068 | 16.111 |
| Glycine | C2H5NO2 | 75.067 | 13.321 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.