Posted by **linda** on Monday, December 21, 2009 at 9:35pm.

Having issues with significant figures.

82.420g + 3.24g = 85.66g

Would it be 85.7g?

- chemistry -
**DrBob222**, Monday, December 21, 2009 at 9:46pm
I don't think so.

In addition, the result may have as many s.f. as the least precise value. Since 3.24 has only two places to the right of the decimal, the answer may have two places to the right of the decimal; therefore, you need not round the .66 to .7. I would keep the answer as 85.66. Here is a site that may be of some help but post again if you don't get it.

http://www.chemteam.info/SigFigs/SigFigs.html

- chemistry -
**linda**, Monday, December 21, 2009 at 9:58pm
I was thinking that since 3.24 has 3 significant figures then the answer should have 3. Very confusing.

- chemistry -
**DrBob222**, Monday, December 21, 2009 at 10:44pm
Not confusing if you keep up with the rules. The rules are different for multiplication and division versus addition and subtraction.

For M and D you count the number of s.f., just as you did, and the number of s.f. in the answer is the same as the smallest number of s.f. in the numbers being multiplied or divided. BUT in addition and subtraction, you count the number of places to the right of the decimal and the number of places to the right of the decimal in the answer is the same as the lowest number of places in any of the numbers added or subtracted. For example,

3.20 x 1.6 = 5.120. The 3.20 has three s.f. and 1.6 has two; therefore, the answer may have two so it would be rounded to 5.1.

Adding 5.22 + 1.1 + 5.127 = 11.777. The number 1.1 determines the rounding; therefore, we round to 11.8.

- chemistry -
**linda**, Monday, December 21, 2009 at 11:10pm
That helps. Thanks so much!

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