Molar MassMolar mass of ethyne (C2H2)
The molar mass of ethyne, C2H2, is 26.04 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 26.04 grams.
26.04 g/mol
2 × 12.011 + 2 × 1.0080 = 26.038 · 4 atoms in the formula · also called acetylene
About ethyne
Ethyne is acetylene, the fuel of the oxy-acetylene welding flame. It has the same empirical formula, CH, as benzene.
Other names: acetylene.
The working
The formula C2H2 has 2 carbon atoms and 2 hydrogen atoms, 4 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Carbon (C) | 2 | 12.011 | 24.022 | 92.26% |
| Hydrogen (H) | 2 | 1.0080 | 2.016 | 7.74% |
2 × 12.011 + 2 × 1.0080 = 26.038 g/mol
With the rounded atomic masses used in many school books (C = 12, H = 1) the answer is 26 g/mol. The difference from 26.04 is too small to matter in most school sums.
Percentage composition
By mass, C2H2 is 92.26% carbon and 7.74% hydrogen. The largest share comes from carbon. In a 100 g sample there are 92.3 g of carbon and 7.7 g of hydrogen.
Worked examples
Grams to moles. How many moles are in 25 g of C2H2? Divide the mass by the molar mass: 25 ÷ 26.04 = 0.9601 mol.
Moles to grams. What is the mass of 0.1 mol of C2H2? Multiply: 0.1 × 26.04 = 2.604 g.
Counting particles. 25 g contains 0.9601 mol × 6.022 × 1023 = 5.782 × 1023 molecules, and 2.313 × 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 C2H2 is about 26.04 ÷ 22.4 = 1.16 g per litre. Air averages about 29 g/mol, so this gas is lighter than air.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 2.604 g |
| 0.5 mol | 13.02 g |
| 1 mol | 26.04 g |
| 2 mol | 52.08 g |
| 5 mol | 130.2 g |
| Mass | Moles |
|---|---|
| 1 g | 0.03841 mol |
| 10 g | 0.3841 mol |
| 100 g | 3.841 mol |
| 1 kg | 38.41 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of C2H2?
The molar mass of ethyne (C2H2) is 26.04 g/mol, or 26.038 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of C2H2?
100 ÷ 26.04 = 3.841 mol.
What is the mass of one molecule of C2H2?
Divide the molar mass by the Avogadro constant: 26.04 ÷ 6.022 × 1023 = 4.324 × 10-23 g.
What is the percentage of carbon in C2H2?
Carbon makes up 92.26% of the mass of C2H2: 2 × 12.011 ÷ 26.038 × 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.