Can subtracting reduce the number of Significant Figures a number has?

Moderators: Chem_Mod, Chem_Admin

ColmConnolly3D
Posts: 100
Joined: Wed Sep 30, 2020 10:06 pm
Been upvoted: 1 time

Can subtracting reduce the number of Significant Figures a number has?

Postby ColmConnolly3D » Mon Jan 25, 2021 4:42 pm

Say I have to subtract then divide:

0.7552g - 0.7241g = 0.0311 g <- Here we keep 4 decimal places because we are subtracting.

For my next step, I would like to divide by 768.95 g/mol

Will my rounded answer be 4.044*10^-5 mol or 4.04*10^-5 mol? The question boils down to can subtraction reduce the accuracy of a measurement. Since the subtraction answer came from two numbers with 4 sf accuracy, why would subtracting them (which only cares about the # of places after a decimal) decrease its accuracy to 3 sf? Should I just stick to the original sf accuracy in this situation since the calculations are separate steps, or rather reevaluate the sig figs the number has after my first step.

This question pertains to my lab for 14BL, but I have changed the numbers around so you aren't doing the work for me :)

Hoping someone can help!
Colm

Chloe Little 3K
Posts: 103
Joined: Wed Sep 30, 2020 10:02 pm
Been upvoted: 1 time

Re: Can subtracting reduce the number of Significant Figures a number has?

Postby Chloe Little 3K » Mon Jan 25, 2021 5:25 pm

Your rounded answer should have 4 sig figs which would be 4.044*10^-5 mol because when you divide, the number of significant figures in the answer should match the number of significant figures of the least precisely known number used in the calculation, meaning the number with the fewest sig figs. Since 0.0311 g has 4 sig figs and 768.95 g/mol has 5 sig figs, your answer should have 4 sig figs.


Return to “Significant Figures”

Who is online

Users browsing this forum: No registered users and 3 guests