## Mass Percent Composition

MichelleKaku1H
Posts: 21
Joined: Fri Sep 29, 2017 7:04 am

### Mass Percent Composition

When finding empirical formulas, do you always have to find the mass percent composition? How do you know when you need to find it? For example, in problem F.13 I didn't need to find it.

Amanda Mac 1C
Posts: 58
Joined: Fri Sep 29, 2017 7:05 am

### Re: Mass Percent Composition

How I approached F13 was to find the mass percent composition first. So 4.14g P/27.8g solid * 100 comes out to be 14.89% which means Cl makes up 85.11% of the compound. With this you can assume, 100 grams and find the moles and then the atomic ratio from there. Finding empirical formulas almost always require finding mass percent composition and then finding the moles. Looking at this problem, that was my first instinctual step. Empirical formula/molecular formula problems will usually give you enough information (individual masses of elements found in the compound) to find the mass percent composition so I guess that's an indicator. Hope this helps even a little!
Last edited by Amanda Mac 1C on Thu Oct 05, 2017 2:00 pm, edited 1 time in total.

Michele Shi 1K
Posts: 33
Joined: Thu Jul 27, 2017 3:00 am

### Re: Mass Percent Composition

I have the same question. For problem F21, the solution manual didn't find the mass percentage composition for each element. They just went directly from the given grams of each element to moles. I thought it would change the number of moles if you didn't assume the 100g sample.

Mary Becerra 2D
Posts: 53
Joined: Fri Sep 29, 2017 7:06 am

### Re: Mass Percent Composition

But wouldn't you have to find the mass percentage composition in order to go through the next steps of dividing what you find by the smallest number and then finding out whether it is the empirical or molecular function?

Abigail Yap 2K
Posts: 51
Joined: Fri Sep 29, 2017 7:07 am

### Re: Mass Percent Composition

It is not necessary to calculate the percent compositions or use a 100g sample. After converting the given masses to moles, you will then divide each one by the smallest value. This will give you a ratio of the atoms relative to each other, which is all you need to write an empirical formula. Hope this helps.