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.

To determine the empirical formula for a compound using the mass composition, you need to follow a few steps:

1. Convert the mass of each element to moles using their molar masses. To do this, divide the mass of each element by its atomic weight. The atomic weight of copper (Cu) is 63.55 g/mol, and the atomic weight of fluorine (F) is 18.99 g/mol.

Moles of Copper = 0.537g / 63.55 g/mol
Moles of Fluorine = 0.321g / 18.99 g/mol

2. Calculate the mole ratio between the elements by dividing the moles of each element by the smallest number of moles obtained. This step ensures that you have the simplest, most reduced ratio possible.

Cu: F = Moles of Copper / Moles of Fluorine

3. If necessary, round the ratio to the nearest whole number. Since the ratio might not be a whole number due to experimental errors, it is essential to round it to obtain a whole number ratio.

4. Write the empirical formula using the mole ratio. The empirical formula represents the simplest whole number ratio of atoms in a compound, and it does not give the actual number of atoms present.

Let's apply these steps to the given problem:

1. Moles of Copper = 0.537g / 63.55 g/mol = 0.00846 mol
Moles of Fluorine = 0.321g / 18.99 g/mol = 0.0169 mol

2. Cu: F = 0.00846 mol / 0.0169 mol = 0.5

3. The ratio of 0.5 can be rounded to a whole number, which is 1.

4. The empirical formula for the compound is CuF.