# Maxwell Distribution of speeds

## Homework Statement

A gas consisting of 15,200 molecules, each of mass 2.00 x 10-26 kg, has the following distribution of speeds, which crudely mimics the Maxwell distribution:

Number of Molecules - Speed (m/s)
1600 - 220
4100 - 440
4700 - 660
3100 - 880
1300 - 1100
400 - 1320

(a) Determine vrms for this distribution of speeds.

## Homework Equations

vrms = √(2 * Ek/m)
Ek = (1/2) * n * M * v2

## The Attempt at a Solution

Weighted Average
((1600 * 220) + (4100 * 440) + (4700 * 660) + (3100 * 880) + (1300 * 1100) + (400 * 1320)) / 15200
= (9944000 m/s) / 15200
= 654.32 m/s

vrms = √(2 * Ek/m)
Ek = (1/2) * n * M * v2
n = number of moles
M = molar mass
m = mass of single molecule

n = 15200 molecules / 6.02x1023 molecules/mole
= 2.525x10-20 mole

M = 2.00x10-26 kg / molecule * 6.02x1023 molecules / mole
= 0.01204 kg / mol

Ek = (1/2)(2.525x10-20moles)(0.01204 kg / mole)(654.21 m/s)2
= 6.506x10-17kg⋅m/s

vrms = √(2 * (6.506x10-17 kg⋅m/s) / 2.00x10-26 kg)
= 80659.78 m/s ...

The correct answer is 710 m/s so obviously I am either grossly overcomplicating this, or I am using the wrong equation.

vela
Staff Emeritus
Homework Helper
RMS means "root mean square". It's the square root of the mean of the square, so take the velocities and square them, find the weighted average of the squares, and then take the square root of the average.

I did what you said: I squared the velocities, then I calculated the weighted average (same way as I did above), and then took the square root of the weighted average. I ended up getting 82914.03 m/s, which is close to the answer I originally got but not the correct answer.

What am I missing?

vela
Staff Emeritus