## Empirical and Molecular Formulas

604744616
Posts: 34
Joined: Sun Jun 25, 2017 3:00 am

### Empirical and Molecular Formulas

I am having trouble understanding on how exactly to go from an empirical to a molecular formula and vice versa? I do not exactly understand what we need to know or how we get from one to the other? I went over the notes and the videos but it is just the hardest concept for me. Is anyone willing to break this concept down for me? Please and thank you

Chem_Mod
Posts: 18902
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 734 times

### Re: Empirical and Molecular Formulas

As I just discussed in my Monday 2-3pm review session:

You have to know the molecular mass to know if you have the empirical or molecular formula.

When you determine your 'formula', calculate its mass per mole.

If it matches the given molecular mass, then you know you have the molecular formula.

If it is less than the given molecular mass, then you have the empirical formula.

CameronJohari1J
Posts: 66
Joined: Fri Sep 29, 2017 7:06 am
Been upvoted: 1 time

### Re: Empirical and Molecular Formulas

To go from the empirical formula to the molecular formula, the molar mass of the molecular formula must be known. Divide the molecular formula molar mass by the empirical formula molar mass to get a natural number. Then multiply all the coefficients in the empirical formula by that number to get the molecular formula.
Ex:
empirical formula: CH; molar mass 13.02g/mol
Molar mass of molecular formula: 78.11g/mol
78.11/13.02= 6
Molecular formula C6H6
Hope this helps.

Jingyi Li 2C
Posts: 56
Joined: Fri Sep 29, 2017 7:06 am

### Re: Empirical and Molecular Formulas

The empirical formula is just the ratio of the atoms. The molecular formula shows the actual number of atoms. That is the molecular formula is the multiple of the empirical formula. In order to get the empirical formula from the molecular formula, you need to know the molar mass of the molecular formula. Then you add up all the atomic mass of the empirical formula and use that to divide the molar mass of the molecular formula. The whole number you get is then used to multiply the coefficients in the empirical formula.
In short,
Molar mass of the molecular formula/molar mass of the empirical formula=whole number
whole number * empirical formula=molecular formula