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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: cvc121 on October 05, 2016, 12:12:18 AM

Title: Adding and Subtracting Significant Figures
Post by: cvc121 on October 05, 2016, 12:12:18 AM

Hi,

To add or subtract sig figs, it is to my understanding that the answer is reported so that it reflects the least precise number.

For the example 43.4 - 22 - 3, would I report my answer as 18 or 20? The least precise number is 3 but I am not sure if I would report my answer with 1 or 2 sig figs. I am guessing that my answer should be 20 since it has only one significant figure as well.

Thanks. Any help is very much appreciated!

Title: Re: Adding and Subtracting Significant Figures
Post by: Borek on October 05, 2016, 02:33:11 AM

Definitely 18.

Rules for adding and subtracting are a bit different and they reflect position of the digits.

123+0.01 is 123, despite the fact 0.01 has only 1 SD. However, 123*0.01 is 1.

Title: Re: Adding and Subtracting Significant Figures
Post by: KungKemi on October 05, 2016, 04:28:24 AM

I'm pretty certain that with addition and subtraction you round the answer to the least number of decimal places (out of the components)...

For example, if the expression is 13.1 + 14.56 + 15, then the answer would be 43, and if the expression is 1.0250 + 2.56 + 3.000, then the answer would be 6.59.

As for multiplication and division, you always round the answer to the least number of significant figures...

For example, if the expression is 13.1 × 14.56 × 15, then the answer would be 2900, and if the expression is 1.025 × 2.56 × 3.000, then the answer would be 7.87.

I hope this helps,
KungKemi