The molar mass of magnesium chloride, MgCl2, is 95.205 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 95.205 g, and a gram of it is 6.325e+21 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. Magnesium chloride is 24.305 (Mg) + 2 × 35.45 (Cl), which is 95.205 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Chlorine makes up the largest share of the mass: 2 of the 3 atoms in a formula unit are chlorine, and they account for 70.9 g of the 95.205 g, or 74.47% 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 magnesium chloride is 95.205 mg and one micromole is 95.205 µg, so a balance reading to a milligram places you within 10.5 µmol. A single formula unit weighs 1.5809e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
Used for de-icing, tofu coagulation and magnesium supplementation. The hexahydrate is what shops sell, and it weighs more than twice as much. At 95.205 g/mol it is heavier than 8 of the 34 salts covered here, and that ranking matters more than it looks: sodium nitrate has a molar mass of 84.99377 g/mol, so a gram of it contains 12% more formula units than a gram of magnesium chloride. 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 1.9041 g of a nominal 95.205 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 of magnesium chloride in grams
95.205- The molar mass of magnesium chloride 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 magnesium chloride in grams.
- It is divided by the molar mass, 95.205 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 magnesium chloride as a molecule count
0.5 g divided by 95.205 g/mol is 0.00525183 mol, and multiplying by the Avogadro constant gives 3.163e+21 formula units of magnesium chloride. The same mass is 5.2518 mmol, which is the figure a reaction is actually planned in.
5 g of magnesium chloride as a molecule count
5 g divided by 95.205 g/mol is 0.0525183 mol, and multiplying by the Avogadro constant gives 3.163e+22 formula units of magnesium chloride. The same mass is 52.518 mmol, which is the figure a reaction is actually planned in.
50 g of magnesium chloride as a molecule count
50 g divided by 95.205 g/mol is 0.525183 mol, and multiplying by the Avogadro constant gives 3.163e+23 formula units of magnesium chloride. The same mass is 525.18 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 magnesium chloride (MgCl2)?
It is 95.205 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 24.305 (Mg) + 2 × 35.45 (Cl). One mole of magnesium chloride therefore weighs 95.205 g, and a gram of it is 10.504 mmol.
What percentage of magnesium chloride is chlorine?
74.47% by mass. Each formula unit contains 2 chlorine atoms contributing 70.9 g of the 95.205 g total, so 100 g of magnesium chloride contains 74.47 g of chlorine and a kilogram contains 744.71 g of it.
How many molecules are in one gram of magnesium chloride?
About 6.325e+21. One gram is 0.0105037 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 95.205.
How much does a single molecule of magnesium chloride weigh?
1.5809e-22 g, which is 95.205 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 magnesium chloride it is 95.205. Molar mass carries grams per mole: 95.205 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. Chlorine is quoted as 35.45 rather than a whole number for that reason, and the same applies to the other elements here, which is why magnesium chloride comes out at 95.205 g/mol rather than 95.
Molar mass, composition and mole equivalents of magnesium chloride
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Chlorine (Cl) | 2 | 35.45 | 70.9 | 74.47% |
| Magnesium (Mg) | 1 | 24.305 | 24.305 | 25.53% |
| Total — one mole of magnesium chloride | 95.205 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 95.205 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000095 | 0.095205 | 6.022e+17 |
| 10 µmol | 0.00095205 | 0.95205 | 6.022e+18 |
| 100 µmol | 0.0095205 | 9.5205 | 6.022e+19 |
| 1 mmol | 0.095205 | 95.205 | 6.022e+20 |
| 10 mmol | 0.95205 | 952.05 | 6.022e+21 |
| 50 mmol | 4.76025 | 4760.25 | 3.011e+22 |
| 100 mmol | 9.5205 | 9520.5 | 6.022e+22 |
| 250 mmol | 23.8013 | 23,801.3 | 1.506e+23 |
| 500 mmol | 47.6025 | 47,602.5 | 3.011e+23 |
| 750 mmol | 71.4038 | 71,403.8 | 4.517e+23 |
| 1 mol | 95.205 | 95,205 | 6.022e+23 |
| 2 mol | 190.41 | 190,410 | 1.204e+24 |
| 5 mol | 476.025 | 476,025 | 3.011e+24 |
| 10 mol | 952.05 | 952,050 | 6.022e+24 |
Mass is the amount multiplied by 95.205 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 |
|---|---|---|---|
| Ammonium nitrate | NH4NO3 | 80.043 | 12.493 |
| Sodium bicarbonate | NaHCO3 | 84.00577 | 11.904 |
| Sodium nitrate | NaNO3 | 84.99377 | 11.766 |
| Magnesium chloride (this page) | MgCl2 | 95.205 | 10.504 |
| Calcium carbonate | CaCO3 | 100.086 | 9.9914 |
| Potassium nitrate | KNO3 | 101.1023 | 9.891 |
| Sodium carbonate | Na2CO3 | 105.9875 | 9.4351 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.