Question

Iodomethane is a common liquid used in the laboratory. It vaporizes readily. If the pressure of iodomethane vapor in a flask is 336 mm Hg at 25.0 °C and the density of the vapor is 2.95 g/L, what is the molar mass of iodomethane?

Molar mass =  g/mol

Answer 1

Given:

Vapor pressure of Iodomethane = 336 mmHg , T = 25.0 ⁰C = 25.0 + 273.15 = 298.15 K

Density = 2.95 g /L

Solution:

Lets start with ideal gas law:

pV = nRT

p is pressure in atm, V is volume in L , n is number of moles, R is gas constant ( 0.08206 L atm / (mol K )

T is in K

since we are given density , pressure and temperature we have to substitute for moles and then we use density = mass / volume.

n = mass in g / molar mass

pV = ( m / M ) RT

M is molar mass and m is mass in g.

By rearranging we get

pM = (m / V ) RT

since d = m / V

we get,

pM = d RT

M = d RT / p

Lets convert pressure into atm

Pressure in atm = 336 mmHg x 1 atm / 760.0 mmHg

= 0.4421 atm

Lets plug in the values.

M = 2.95 g L^-1 x 0.08206 L atm (K mol ) ^‑1 x 298.15 K / 0.4421 atm

= 163.25 g / mol

Answer : molar mass of the iodomethane = 163.25 g /mol