Ok I really need some help with Chemistry PLEASE!?

Question

My teacher gave us a sheet mainly about significant figures, I totally get the basis for what we're doing and everything. He introduced this today but there is one problem I just don't understand, well actually 2 but they are similar.

Instructions for this question: When you multiply or divide numbers, your answer can have no more significant figures than the least number of significant figures in the problem.

100.0 g x 1.00 cal/g x Celcius x 28.6Celcius =


Instructions for this question: Round your answers to the proper number of significant figures.

2.00 x 10(squared) g x 1 cal/g x Celcius x 28.6Celcius =


If you could work it out and explain how you did it that would be awsome! Thanks so much for all answers!

Answer 1

1.) 100.0 g x 1.00 cal/g x Celcius x 28.6 Celcius =

OK, the key here is that you need to account for the units as well as the numbers.

Let's re write it:
100.0 * 1.00 * 28.6 = 2860
(g * cal * celcius)/(g * celcius) = cal
2860 cal is your solution. You don't need to round, because the lowest # of significant digits is 3 (28.6 and 1.00), and your answer is already 3 significant digits.

2.) 2.00 x 10(squared) g x 1 cal/g x Celcius x 28.6Celcius =

Rewrite:
2.00 * 10^2 * 1 * 28.6 = 5720
(g * cal * celcius)/(g * celcius) = cal
5720 cal

The lowest # of significant digits in your questtion is 1 (1 cal/g*celcius is 1 significant digit), so your answer is 6000 cal or 6 kcal.

Answer 2

100.0*1.00*28.6=286.
2.00*10^2*1*28.6=572=600