Question

A two litre flask contains $22$ g of carbon dioxide and $1$ g of helium at ${20}^0$ C. Calculate the partial pressure exerted by $C{O}_2$ and $He$ if the total pressure is $3$ atm.

• A. $2.25$ atm, $0.75$ atm
• B. $1.5$ atm, $1.5$ atm
• C. $2.5$ atm, $0.5$ atm
• D. $2$ atm, $1$ atm

Answer 1

Answer: (D)

$2$ atm, $1$ atm

The molar masses of carbon dioxide and helium are 44 g/mol and 4 g/mol respectively.
The number of moles is the ratio of mass to molar mass.
22 g of carbon dioxide $=\frac{22}{44}=0.5$ moles.
1 g of helium $=\frac{1}{4}=0.25$ moles.
The mole fraction of carbon dioxide $=\frac{0.5}{0.5+0.25}=0.667$
The mole fraction of helium $=\frac{0.25}{0.5+0.25}=0.333$
The partial pressure of a gas is the product of its mole fraction and total pressure.
The partial pressure of carbon dioxide $=0.667\times 3=2$ atm.
The partial pressure of helium $=0.333\times 3=1$ atm.