Reporting Correct Data

1. Sep 23, 2012

frozonecom

Significant Figures in Data

Suppose I have two recorded Molarity of a certain solution.
And the data I have are:

0.092 M and 0.095 M --- which both has 2 significant figures.

If I were asked to get the AVERAGE MOLARITY of my substance, then I'd do:

(0.092 M + 0.095 M)/2 = My AVG. MOLARITY.

Since 2 is an exact number, I will not base the SF of my final answer to that number.
Instead, I'd base it to my Molarities, but I know I should do the addition first, since they're enclosed in a parenthesis. (following PEMDAS)

0.092 M + 0.095 M = 0.187 M ----- (It should have 3 decimal places, since that is my least decimal place in my data)

Then, dividing my answer by 2, should I report it as:

0.0935 M ----- since 0.187 has 3 SF
OR
0.094 M ------- since my initial working data only has 2 SF?

Which one is more correct? Which one should I follow and why?

Last edited: Sep 23, 2012
2. Sep 23, 2012

frozonecom

Anyone??

3. Sep 23, 2012

Staff: Mentor

I would quote (0.0935 +- 0.0004) M, where the 4 comes from the assumption that both initial values have an uncertainty of 0.0005 each and the remaining part is error propagation.
If you cannot give an explicit uncertainty for some reason, I think I would prefer 0.094 M.