Molar mass of carbon dioxide
The molar mass of carbon dioxide (CO₂) is 44.009 g/mol: the sum of the atomic weights 12.011 + 2 × 15.999. School problems round it to 44 g/mol (C = 12, O = 16).
Breakdown by element
M(CO2) = 12.011 + 2 × 15.999 = 44.009 g/mol
The result is rounded to the precision of the least precise atomic weight. Atomic weights from IUPAC (CIAAW). Agrees with PubChem: 44.009 g/mol. PubChem CID 280
What is carbon dioxide?
Carbon dioxide is a colourless, odourless gas about 1.5 times as heavy as air. It forms when living things breathe, when fuels burn and when sugars ferment, and plants take it in during photosynthesis.
At atmospheric pressure CO₂ is never a liquid: solid carbon dioxide, dry ice, turns straight into gas at −78.5 °C. It is an acidic oxide: in water it forms weak carbonic acid, and it turns limewater, Ca(OH)₂, milky, which is how it is identified.
Properties
- State at 25 °C
- Gas
- Density
- 1.977 g/L 0 °C
- Melting point
- −56.6 °C only under pressure
- Boiling point
- −78.5 °C sublimes
- Solubility in water
- slightly soluble
Sources: PubChem (HSDB, NIOSH), Wikipedia (CRC Handbook), NIST Chemistry WebBook; each value agrees in two of them.
Molar mass problems with carbon dioxide
For school problems (C = 12, O = 16): M(CO₂) = 44 g/mol
Problem 1
How many moles are in 88 g of CO₂?
n = m ÷ M = 88 g ÷ 44 g/mol
Answer: 2 mol
Problem 2
What is the mass of 0.2 mol of CO₂?
m = n × M = 0.2 mol × 44 g/mol
Answer: 8.8 g
Problem 3
What is the mass percentage of C in CO₂?
w(C) = M(C) ÷ M(CO₂) = 12 ÷ 44 ≈ 0.273
Answer: 27.3%
Problem 4
How many molecules are in 1.5 mol of CO₂?
N = n × NA = 1.5 mol × 6.02 × 10²³ mol⁻¹
Answer: 9.03 × 10²³
Problem 5
What volume does 22 g of CO₂ take up at STP?
n = m ÷ M = 22 g ÷ 44 g/mol = 0.5 mol
V = n × Vm = 0.5 mol × 22.4 L/mol
STP here: 0 °C and 1 atm (101.3 kPa).
Answer: 11.2 L
Common substances
What is molar mass?
Molar mass M is the mass of one mole of a substance, about 6.022 × 10²³ of its particles. It is measured in grams per mole (g/mol).
To find it, add up the atomic weights of all the atoms in the formula: multiply each element’s atomic weight by its subscript, the number of its atoms. In g/mol, the molar mass equals the relative molecular mass Mr.
For water: M(H₂O) = 2 × 1.0080 + 15.999 = 18.015 g/mol.