alleryStart learning →A 0.494 g sample of chloroform vapour fills 129 cm^3 at 99.6 degrees C and 742.1 mm Hg. What is its approximate molar mass?
A60 g/mol
B120 g/mol
C240 g/mol
D84 g/mol
Answer & Solution
Correct answer: B. 120 g/mol
1. Use molar mass equals mRT over PV.
2. Convert the temperature to kelvin: 99.6 + 273.2 = 372.8 K.
3. Convert the pressure to atmospheres: 742.1 / 760 = 0.976 atm.
4. Convert the volume to litres: 129 cm^3 = 0.129 L.
5. Build the numerator: 0.494 g x 0.08206 L atm/mol K x 372.8 K = 15.11 g L atm per mol.
6. Build the denominator: 0.976 atm x 0.129 L = 0.1259 L atm.
7. Divide: 15.11 / 0.1259 = 120 g/mol.
8. Leaving the volume in cubic centimetres would shrink the denominator a thousandfold and inflate the answer, which is the unit trap here.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 9 "Gases", section 9.3 Stoichiometry of Gaseous Substances, Mixtures, and Reactions_
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