The molar mass of HCl (hydrogen chloride) is 36.46 g/mol. It is the atomic mass of hydrogen (1.008) plus the atomic mass of chlorine (35.45). In GCSE and CBSE-style questions the relative formula mass (Mr) of HCl is usually written as 36.5, using H = 1 and Cl = 35.5.
How to calculate the molar mass of HCl
| Element | Atoms | Atomic mass (g/mol) | Total |
|---|---|---|---|
| Hydrogen (H) | 1 | 1.008 | 1.008 |
| Chlorine (Cl) | 1 | 35.45 | 35.45 |
| Molar mass of HCl | 36.46 g/mol | ||
So one mole of HCl, which contains 6.022 × 10²³ molecules, has a mass of 36.46 g. The molecular mass of a single HCl molecule is 36.46 u.
Why is chlorine’s atomic mass 35.45 and not a whole number?
Natural chlorine is a mixture of two isotopes: about 75.8% chlorine-35 and 24.2% chlorine-37. The weighted average is:
(0.758 × 34.97) + (0.242 × 36.97) ≈ 26.51 + 8.95 = 35.45
That is why HCl’s molar mass comes out as 36.46 and not a round number. It also explains a detail you may see in mass spectrometry: HCl shows two molecular peaks, at 36 and 38, in roughly a 3:1 ratio.
Molar mass, molecular mass and Mr: are they different?
- Molar mass = mass of one mole = 36.46 g/mol.
- Molecular mass = mass of one molecule = 36.46 u (atomic mass units).
- Relative molecular mass (Mr) = the same number with no unit, 36.46 (or 36.5 with rounded values).
People sometimes search for “HCl atomic mass”, but HCl is a molecule, not an atom, so it has a molecular mass. The atomic masses involved are H = 1.008 and Cl = 35.45.
Worked examples
1. Grams to moles. How many moles are in 73 g of HCl? 73 ÷ 36.46 = 2.0 mol.
2. Moles to grams. What is the mass of 0.25 mol of HCl? 0.25 × 36.46 = 9.12 g.
3. Percentage composition. Chlorine makes up 35.45 ÷ 36.46 × 100 = 97.2% of HCl by mass; hydrogen is the remaining 2.8%.
4. Molarity of concentrated hydrochloric acid. Lab-grade concentrated HCl is about 37% HCl by mass with a density of 1.19 g/mL. One litre weighs 1,190 g and contains 0.37 × 1,190 = 440 g of HCl. Moles = 440 ÷ 36.46 = 12.1, so concentrated HCl is about 12 M.
5. Making 1 L of 1 M HCl. Using C₁V₁ = C₂V₂: V₁ = (1 × 1,000) ÷ 12.1 ≈ 83 mL of concentrated acid, added slowly to water and made up to 1 L. Always add acid to water, never water to acid.
Quick facts about HCl
| Formula | HCl |
|---|---|
| Molar mass | 36.46 g/mol |
| State at room temperature | Colourless gas with a sharp smell; its solution in water is hydrochloric acid |
| Boiling point (gas) | −85 °C |
| Acid strength | Strong acid, fully ionised in water: HCl → H⁺ + Cl⁻ |
| Equivalent weight | 36.46 (basicity 1), so normality = molarity |
Frequently asked questions
What is the molar mass of HCl?
The molar mass of HCl is 36.46 g/mol, from H (1.008) + Cl (35.45).
What is the Mr of HCl?
The relative molecular mass (Mr) of HCl is 36.5 when you use rounded atomic masses H = 1 and Cl = 35.5.
How many grams are in one mole of HCl?
One mole of HCl has a mass of 36.46 grams.
What is the molarity of concentrated HCl?
Concentrated hydrochloric acid (about 37% w/w, density 1.19 g/mL) is roughly 12 mol/L.
Why is the molar mass of HCl not a whole number?
Chlorine’s atomic mass, 35.45, is an average of its two natural isotopes, chlorine-35 and chlorine-37, so HCl’s molar mass is 36.46 rather than a whole number.
References
- PubChem – Hydrochloric Acid
- IUPAC Commission on Isotopic Abundances and Atomic Weights, standard atomic weight of chlorine.