Michelle N.

# Question about how significant digits work in chemistry.

I'm doing my online chemistry summer class and significant digits are included, of course. But I'm confused about them because everything I believe is being contradicted. Here's what I mean...

For one assignment we have to add and subtract measurements and "Be sure each answer you enter contains the correct number of significant digits." Well, here are the given questions and answers:
(1) 15.90 mL - 7.770 mL = 8.13 mL
(2) 4.8 mL + 9.927 mL = 14.7 mL
(3) 0.9 mL + 11.870 mL = 12.8 mL
Because of this assignment, I have the impression that to write the final answer in significant digits, it would be by using one of the original numbers with the least amount of numbers after the decimal. What I mean is when I add the problem from question (2), I get 14.727. But I need to put that answer into the correct significant digits. The two original numbers in the equation for (2) are 4.8 and 9.927. I see that for the first number, 4.8, there is only one number left after the decimal place and for the second number, 9.927, there are three numbers after the decimal place. Therefore, I would write my answer, 14.727, with only one number left after the decimal place like 4.8 because it has the least amount of numbers after the decimal place compared to 9.927. My final answer, therefore, would be 14.7. So, that's my mindset with significant digits and I get the rest of the problems correct.

I'm confused.

-Sorry for the large amount of text. I was typing out my mindset so that you could, hopefully, see what is wrong with my mindset.-

## 2 Answers By Expert Tutors

By: Tutor
New to Wyzant

Recent graduate studying for medical school

Michelle N.

No, it was 4.8 without a zero at the end. There were other similar problems like 9.9 mL + 11.80 mL = 21.7 mL. There were other assignments too that did not follow the 3 rule, like "What average volume should the chemical engineer report? Be sure your answer has the correct number of significant digits." And the given volumes were 0.602 L, 7.04 L, and 5.139 L, with the final answer being 4.260 L. And this is why I've been confused about significant figures for two years.
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07/20/18

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