Molar mass of ammonia
The molar mass of ammonia (NH₃) is 17.031 g/mol: the sum of the atomic weights 14.007 + 3 × 1.0080. School problems round it to 17 g/mol (N = 14, H = 1).
Breakdown by element
M(NH3) = 14.007 + 3 × 1.0080 = 17.031 g/mol
The result is rounded to the precision of the least precise atomic weight. Atomic weights from IUPAC (CIAAW). Agrees with PubChem: 17.031 g/mol. PubChem CID 222
What is ammonia?
Ammonia is a colourless gas with a sharp smell, lighter than air. It dissolves very well in water; the solution is sold as household ammonia.
Ammonia is used to make nitrogen fertilisers and nitric acid. Industry makes it from nitrogen and hydrogen in the Haber–Bosch process.
Properties
- State at 25 °C
- Gas
- Melting point
- −77.7 °C
- Boiling point
- −33.3 °C
- Solubility in water
- soluble
Sources: PubChem (HSDB, NIOSH), Wikipedia (CRC Handbook), NIST Chemistry WebBook; each value agrees in two of them.
Molar mass problems with ammonia
For school problems (N = 14, H = 1): M(NH₃) = 17 g/mol
Problem 1
How many moles are in 68 g of NH₃?
n = m ÷ M = 68 g ÷ 17 g/mol
Answer: 4 mol
Problem 2
What is the mass of 0.5 mol of NH₃?
m = n × M = 0.5 mol × 17 g/mol
Answer: 8.5 g
Problem 3
What is the mass percentage of N in NH₃?
w(N) = M(N) ÷ M(NH₃) = 14 ÷ 17 ≈ 0.824
Answer: 82.4%
Problem 4
How many molecules are in 0.2 mol of NH₃?
N = n × NA = 0.2 mol × 6.02 × 10²³ mol⁻¹
Answer: 1.204 × 10²³
Problem 5
What volume does 1.7 g of NH₃ take up at STP?
n = m ÷ M = 1.7 g ÷ 17 g/mol = 0.1 mol
V = n × Vm = 0.1 mol × 22.4 L/mol
STP here: 0 °C and 1 atm (101.3 kPa).
Answer: 2.24 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.