Question

Having issues with significant figures.

82.420g + 3.24g = 85.66g
Would it be 85.7g?

Answer 1

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

Answer 2

I was thinking that since 3.24 has 3 significant figures then the answer should have 3. Very confusing.

Answer 3

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.

Answer 4

That helps. Thanks so much!