# I need help with Subscripts for empirical formula, how do i know which number to multiply so that I get a whole number?

##### 2 Answers

When you have to calculate a compound's empirical formula from its percent composition, there are a few tricks to use to help you deal with decimal mole ratios between the atoms that comprise your compound.

Now, I assume you know how to get to this point, so I won't show you the whole approach. Let's assume you have a compound containing

In such cases it is very useful to use *mixed fractions*. Mixed fractions are a combination of a whole number and a regular (or proper) fraction.

In this case, **2 and 1/3**, or **7/3**, and **1 and 2/3**, or **5/3**. This makes the ratios equal to

Now multiply all of them by **3** to get rid of the denominator and you'll get the empirical formula

If you get enough practice with empirical formulas you'll be able to "see" the answer faster. For example, if you have a compound comporised of

It will become obvious in time that you have to multiply all of them by **3** to get all-whole numbers and an empirical formula of

Notice that the mixed fractions method is useful in this case as well, since **1.33** is actually **1 and 1/3**, or **4/3**.

As a conclusion, it takes a little practice to be able to determine which numbers can be written in a useful way as mixed fractions, so spend some time on getting this skill down.

**SIDE NOTE** *I assume you know how to get around mixed fractions, so I won't detail how I got 7/3 or 4/3.*

After you divide by the smallest amount of moles if you end up with a number ending in .25 then multiply all numbers by 4. If you end up with a number ending in .33 then multiply all numbers 3. If you end up with a number ending in .20 then multiply all numbers by 5. If you end up with a number ending in .5 then multiply all numbers by 2.