Question

Percent composition of a compound (molar mass= 60.06) 39.99% C, 6.727% H, 53.33% O, What is the molecular formula of the compound?

Answer 1

You are calculating the empirical formula of this chemical compound, which is the question with moles, molar mass, and number of moles.

We assume the total mass of this compound is 100g. Because 39.99 +6.727+53.33 =100.047is about 100g. That's the easiest way for us to do it.

First you divide the mass of carbon by its molar mass(relative formula mass)because there is a formula about moles state: number of moles=mass/molar mass.

So, 40/12 is about 3.3. Then, the RFM of H is 1, so the number of mole is 6.7/1=6.7. The RFM of O is 16, whihc means the molar mass od oxygen is 16g, so 53.3/16, which is about 3.3 too.Next, you get the number of moles in order is; 3.3, 6.7, 3.3. Now we need to look at the ratio between these numbers. 3.3 and 3.3 has a common factor of 1, so the subscript of them are both 1. next use 6.7/3.3, and the greatest common factor is 2, so the empirical formula is CH2O.

BUT CH2O is only the empirical formula. what we need to do is calculate the RFM of CH2O first.

So, 12+2+16=30.

Then we find out how many times 30 goes into 60.06; 60.06/30 is 2.002. we simplify it as 2.

Finally, all the subscript need to times 2. therefore the molecular formula is C2H4O2.

To solve this kind of  questions, there are many steps:Know what you are calculating about, it's about the MOLECULAR formula, so you need to find out the number of moles of each elements, then the empirical formula, then the molecular formula.

) Find the RFM of the element, because that is the molar mass(mass of 1 mole) of this element.

2) number of moles= mass/molar mass. use this formula to help you get the number of moles of each element in this compound

3) look at the relationship between the number of moles of each elements. find out the ratio between them.

4)use the molar mass of the whole compound to find out the molecular formula of this compounds. calculate the RFM of the compound in empirical formula form, and then divide the molar mass by the RFM.

if i got anything wrong, please tell me :)