The molar mass of hydrochloric acid, HCl, is 36.458 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 36.458 g, and a gram of it is 1.652e+22 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. Hydrochloric acid is 1.008 (H) + 35.45 (Cl), which is 36.458 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: 1 of the 2 atoms in a formula unit is chlorine, and they account for 35.45 g of the 36.458 g, or 97.24% 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 hydrochloric acid is 36.458 mg and one micromole is 36.458 µg, so a balance reading to a milligram places you within 27.43 µmol. A single formula unit weighs 6.0540e-23 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
Muriatic acid, sold as a percentage solution rather than a solid. Concentrated acid is about 37% w/w, which works out near 12 mol/L. At 36.458 g/mol it is the lightest of the 12 acids covered here, and that ranking matters more than it looks: formic acid has a molar mass of 46.025 g/mol, so a gram of it contains 20.8% fewer formula units than a gram of hydrochloric 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 36.458 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 of hydrochloric acid in grams
36.458- The molar mass of hydrochloric acid 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 hydrochloric acid in grams.
- It is divided by the molar mass, 36.458 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 hydrochloric acid as a molecule count
0.5 g divided by 36.458 g/mol is 0.0137144 mol, and multiplying by the Avogadro constant gives 8.259e+21 formula units of hydrochloric acid. The same mass is 13.714 mmol, which is the figure a reaction is actually planned in.
5 g of hydrochloric acid as a molecule count
5 g divided by 36.458 g/mol is 0.137144 mol, and multiplying by the Avogadro constant gives 8.259e+22 formula units of hydrochloric acid. The same mass is 137.14 mmol, which is the figure a reaction is actually planned in.
50 g of hydrochloric acid as a molecule count
50 g divided by 36.458 g/mol is 1.37144 mol, and multiplying by the Avogadro constant gives 8.259e+23 formula units of hydrochloric acid. The same mass is 1371.4 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 hydrochloric acid (HCl)?
It is 36.458 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 1.008 (H) + 35.45 (Cl). One mole of hydrochloric acid therefore weighs 36.458 g, and a gram of it is 27.429 mmol.
What percentage of hydrochloric acid is chlorine?
97.24% by mass. Each formula unit contains 1 chlorine atom contributing 35.45 g of the 36.458 g total, so 100 g of hydrochloric acid contains 97.24 g of chlorine and a kilogram contains 972.35 g of it.
How many molecules are in one gram of hydrochloric acid?
About 1.652e+22. One gram is 0.0274288 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 36.458.
How much does a single molecule of hydrochloric acid weigh?
6.0540e-23 g, which is 36.458 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 hydrochloric acid it is 36.458. Molar mass carries grams per mole: 36.458 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 hydrochloric acid comes out at 36.458 g/mol rather than 36.
Molar mass, composition and mole equivalents of hydrochloric acid
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Chlorine (Cl) | 1 | 35.45 | 35.45 | 97.24% |
| Hydrogen (H) | 1 | 1.008 | 1.008 | 2.765% |
| Total — one mole of hydrochloric acid | 36.458 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 36.458 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000036 | 0.036458 | 6.022e+17 |
| 10 µmol | 0.00036458 | 0.36458 | 6.022e+18 |
| 100 µmol | 0.0036458 | 3.6458 | 6.022e+19 |
| 1 mmol | 0.036458 | 36.458 | 6.022e+20 |
| 10 mmol | 0.36458 | 364.58 | 6.022e+21 |
| 50 mmol | 1.8229 | 1822.9 | 3.011e+22 |
| 100 mmol | 3.6458 | 3645.8 | 6.022e+22 |
| 250 mmol | 9.1145 | 9114.5 | 1.506e+23 |
| 500 mmol | 18.229 | 18,229 | 3.011e+23 |
| 750 mmol | 27.3435 | 27,343.5 | 4.517e+23 |
| 1 mol | 36.458 | 36,458 | 6.022e+23 |
| 2 mol | 72.916 | 72,916 | 1.204e+24 |
| 5 mol | 182.29 | 182,290 | 3.011e+24 |
| 10 mol | 364.58 | 364,580 | 6.022e+24 |
Mass is the amount multiplied by 36.458 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 |
|---|---|---|---|
| Hydrochloric acid (this page) | HCl | 36.458 | 27.429 |
| Formic acid | CH2O2 | 46.025 | 21.727 |
| Acetic acid | CH3COOH | 60.052 | 16.652 |
| Boric acid | H3BO3 | 61.831 | 16.173 |
| Nitric acid | HNO3 | 63.012 | 15.87 |
| Oxalic acid | C2H2O4 | 90.034 | 11.107 |
| Lactic acid | C3H6O3 | 90.078 | 11.101 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.