Question

(b) The density of platinum is 23.4 g/cm3 at 20°C. Calculate the mass of 77.0 cm3 of platinum at this temperature.

(c) The density of magnesium is 1.74 g/cm3 at 20°C. What is the volume of 275 g of this metal at this temperature?

*what I am stuck on the most is that the answer has to be in the correct of SIGNIFICANT FUGURES.<--I know what they are but I am not getting answer right at all! Please help, thanks:)

Answer 1

Good. Then you know how to work the density part of the problem. Here is a good web site that will explain significant figures. Feel free to post back here if you have questions from the link.
http://www.chemteam.info/SigFigs/SigFigs.html

Here is the first one to help you get started.
mass = volume x density
mass = 77.0 cc x 23.4 g/cc = 1801.80 grams.
In multiplication and division, you are allowed as many s.f. in the answer as you have in the smallest number of s.f. used in the calculation.
77.0 has 3 s.f.
23.4 has 3 s.f.
Therefore, you are allowed 3 s.f. in the answer. With an answer of 1801.80, the first 1,8, and 0 are significant. The second 1, second 8 and last 0 are not. So you write the answer as
1.80E3 grams.

Answer 2

Thanks a million "DrBob222" your example and website you provided me helped me out alot!!!:)