Calculate Root Mean Square Velocity of Gas Particles

This example problem demonstrates how to calculate the root mean square (RMS) velocity of particles in an ideal gas. This value is the square root of the average velocity-squared of molecules in a gas. While the value is an approximation, especially for real gases, it offers useful information when studying kinetic theory.

Root Mean Square Velocity Problem

What is the average velocity or root mean square velocity of a molecule in a sample of oxygen at 0 degrees Celsius?

Solution

Gases consist of atoms or molecules that move at different speeds in random directions. The root mean square velocity (RMS velocity) is a way to find a single velocity value for the particles. The average velocity of gas particles is found using the root mean square velocity formula:

μ_rms = (3RT/M)^½
μ_rms = root mean square velocity in m/sec
R = ideal gas constant = 8.3145 (kg·m²/sec²)/K·mol
T = absolute temperature in Kelvin
M = mass of a mole of the gas in kilograms.

Really, the RMS calculation gives you root mean square speed, not velocity. This is because velocity is a vector quantity that has magnitude and direction. The RMS calculation only gives the magnitude or speed. The temperature must be converted to Kelvin and the molar mass must be found in kg to complete this problem.

Step 1

Find the absolute temperature using the Celsius to Kelvin conversion formula:

• T = °C + 273
• T = 0 + 273
• T = 273 K

Step 2

Find molar mass in kg:
From the periodic table, the molar mass of oxygen = 16 g/mol.
Oxygen gas (O₂) is comprised of two oxygen atoms bonded together. Therefore:

• molar mass of O₂ = 2 x 16
• molar mass of O₂ = 32 g/mol
• Convert this to kg/mol:
• molar mass of O₂ = 32 g/mol x 1 kg/1000 g
• molar mass of O₂ = 3.2 x 10^-2 kg/mol

Step 3

Find μ_rms:

• μ_rms = (3RT/M)^½
• μ_rms = [3(8.3145 (kg·m²/sec²)/K·mol)(273 K)/3.2 x 10^-2 kg/mol]^½
• μ_rms = (2.128 x 10⁵ m²/sec²)^½
• μ_rms = 461 m/sec

Answer

The average velocity or root mean square velocity of a molecule in a sample of oxygen at 0 degrees Celcius is 461 m/sec.