Chemistry
posted by Chopsticks .
Sometimes, instead of percentages compostion, you will have the composition of a sample by mass. Using the actual mass of the sample, determine the empirical formula for compounds that have the following analyses.
A.
a .858g sample of an unkown substance is composed of .537g of Copper, and .321g of Florine
How would you do this problem? When doing empirical formula, I only know how to do it if its a percentage. Should I divide the amount of copper over the total of the unknown substance to get a percentage and then convert to moles? Help.

That's exactly how you do it. First determine percent Cu, then percent fluorine. Let's just suppose these come out to be 70% and 30% (made up numbers). Then take a 100 g sample which will give you 70 g Cu and 30 g F. Find moles from that and convert to ratios.

Hey, but how far should I round the percentage to?

You have three significant figures in each of the masses; I like to carry one extra place and round at the end when I do the ratios. That means carry the percentages to the hundredths place or two places to the right of the decimal.