Molar MassMolar mass of hydrogen fluoride (HF)
The molar mass of hydrogen fluoride, HF, is 20.01 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 20.01 grams.
20.01 g/mol
1.0080 + 18.998 = 20.006 · 2 atoms in the formula · also called hydrofluoric acid
About hydrogen fluoride
Hydrogen fluoride dissolves in water to give hydrofluoric acid. It is a weak acid in the chemical sense but very dangerous, and it attacks glass, so it is stored in plastic bottles and used to etch glass.
Other names: hydrofluoric acid.
The working
The formula HF has 1 hydrogen atom and 1 fluorine atom, 2 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Hydrogen (H) | 1 | 1.0080 | 1.008 | 5.04% |
| Fluorine (F) | 1 | 18.998 | 18.998 | 94.96% |
1.0080 + 18.998 = 20.006 g/mol
With the rounded atomic masses used in many school books (H = 1, F = 19) the answer is 20 g/mol. The difference from 20.01 is too small to matter in most school sums.
Percentage composition
By mass, HF is 5.04% hydrogen and 94.96% fluorine. The largest share comes from fluorine. In a 100 g sample there are 5.0 g of hydrogen and 95.0 g of fluorine.
Worked examples
Grams to moles. How many moles are in 20 g of HF? Divide the mass by the molar mass: 20 ÷ 20.01 = 0.9997 mol.
Moles to grams. What is the mass of 0.5 mol of HF? Multiply: 0.5 × 20.01 = 10 g.
Counting particles. 20 g contains 0.9997 mol × 6.022 × 1023 = 6.020 × 1023 molecules, and 1.204 × 1024 atoms in all.
As a gas. At 0 °C and 1 atmosphere one mole of an ideal gas occupies 22.4 litres, so the density of HF is about 20.01 ÷ 22.4 = 0.89 g per litre. Air averages about 29 g/mol, so this gas is lighter than air.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 2.001 g |
| 0.5 mol | 10 g |
| 1 mol | 20.01 g |
| 2 mol | 40.01 g |
| 5 mol | 100 g |
| Mass | Moles |
|---|---|
| 1 g | 0.04999 mol |
| 10 g | 0.4999 mol |
| 100 g | 4.999 mol |
| 1 kg | 49.99 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of HF?
The molar mass of hydrogen fluoride (HF) is 20.01 g/mol, or 20.006 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of HF?
100 ÷ 20.01 = 4.999 mol.
What is the mass of one molecule of HF?
Divide the molar mass by the Avogadro constant: 20.01 ÷ 6.022 × 1023 = 3.322 × 10-23 g.
What is the percentage of fluorine in HF?
Fluorine makes up 94.96% of the mass of HF: 1 × 18.998 ÷ 20.006 × 100.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.