need help calculating rate of diffusion?

Question

Following a practical I have plotted concentration in mol dm-3 against time on a graph.

I need to calculate the rate of diffusion using the figures I have.

If I select two points on the graph

ie at 15 minutes the concentration value is 1.57 x 10 to power -6

and at 105 minutes the conc. is 5.5 x 10 to power -6


would I work out the rate of diffusion per minute in the following way?

(5.5 - 1.57) divided by (105 - 15)

I'm not great with using powers either so could someone please explain

thankyou!

If you need any other info please let me know

Answer 1

Diffusion rate is generally directly proportional to the concentration gradient; in simple terms, to the difference in concentration across a membrane. This means the rate tends to decrease in a closed system where the concentrations are tending toward equality. You could get an average rate using your method, but the answer would depend on when your samples werre taken. There is a "rate constant" associated with diffusion. I couldn't find a definition on the web, but if it's similar to other rate constants it's expressed as the time it takes for the concentration gradient (or the diffusion rate) to decrease by a factor of 1/e, e being the base of natural logarithms, and 1/e roughly equal to 0.3679. Thus a shorter time constant means a faster rate constant. So you might consider determining the the time constant by finding two points where the concentrations are in that ratio and reporting the time between them.