# Homework Help: Find the Diffusion coefficient

1. Nov 29, 2011

### HelpaSista

1. The problem statement, all variables and given/known data

A student working 3.0 m away from you in the chemistry lab creates a bad odour. The odour takes a while to get to you (assume that air currents are negligible). You then move 2.0 m further away, but the odour catches up with you in another 60 seconds. What is the diffusion coefficient of the molecules you smell?

hint: the root mean square distancee and the time must be measured from the source of the diffusing molecule.

2. Relevant equations

t= R^2 / 6D

3. The attempt at a solution

I first tried to find time from 0-3m by using the ratio between the two distances

2^2 / 6*s X 6T/^3 = 1 (1 since D is the same)

I found that t = 135s

I then added 135 to 60 seconds and used that as my total time and 5 as my total R and solved for D

D = R^2 / 6*T

= 25 / 6*195

=0.02136

However, my textbook says the answer is 0.044 and I don't see what I am doing wrong. :(

Any help will be much appreciated, thank you!

2. Nov 30, 2011

### Redbelly98

Staff Emeritus
Welcome to Physics Forums.
It is not at all clear how you got this equation. What does it have to do with the ratio of the two distances? What is the "T/^3" supposed to mean, that combination of symbols makes no sense?