Worked Chemistry Problems: Ideal Gas Law

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You may wish to refer to the General Properties of Gases to review concepts and formulae related to ideal gasses.

Ideal Gas Law Problem #1

Problem

A hydrogen gas thermometer is found to have a volume of 100.0 cm3 when placed in an ice-water bath at 0°C. When the same thermometer is immersed in boiling liquid chlorine, the volume of hydrogen at the same pressure is found to be 87.2 cm3. What is the temperature of the boiling point of chlorine?

Solution

For hydrogen, PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the gas constant, and T is temperature.

Initially:

P1 = P, V1 = 100 cm3, n1 = n, T1 = 0 + 273 = 273 K

PV1 = nRT1

Finally:

P2 = P, V2 = 87.2 cm3, n2 = n, T2 = ?

PV2 = nRT2

Note that P, n, and R are the same. Therefore, the equations may be rewritten:

P/nR = T1/V1 = T2/V2

and T2 = V2T1/V1

Plugging in the values we know:

T2 = 87.2 cm3 x 273 K / 100.0 cm3

T2 = 238 K

Answer

238 K (which could also be written as -35°C)

Ideal Gas Law Problem #2

Problem

2.50 g of XeF4 gas is placed into an evacuated 3.00 liter container at 80°C. What is the pressure in the container?

Solution

PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the gas constant, and T is temperature.

P=?
V = 3.00 liters
n = 2.50 g XeF4 x 1 mol/ 207.3 g XeF4 = 0.0121 mol
R = 0.0821 l·atm/(mol·K)
T = 273 + 80 = 353 K

Plugging in these values:

P = nRT/V

P = 00121 mol x 0.0821 l·atm/(mol·K) x 353 K / 3.00 liter

P = 0.117 atm

Answer

0.117 atm