## Formulas

jlinwashington1B
Posts: 65
Joined: Fri Sep 28, 2018 12:22 am

### Formulas

Can someone please explain how we find the number to multiply by when making the molecular formula?

Jeril Joseph 1B
Posts: 55
Joined: Mon Jun 17, 2019 7:23 am
Been upvoted: 1 time

### Re: Formulas

In order to find the molecular formula, the actual molar mass of the substance must be given in the problem. If that's not given, you don't solve for the molecular formula. If so, divide that value, which should be g.mol^-1, by the molar mass computed by adding up the atomic weight of the elements in the substance. After, you use the quotient(should be a whole number) of that division and multiply that by each of the subscripts in the empirical formula. Hope this helps.

Brian Kwak 1D
Posts: 88
Joined: Fri Sep 28, 2018 12:17 am

### Re: Formulas

As previously stated the molecular formula can not be solved without knowing what the molar mass is. If you have the molar mass then you can divide the molar mass by the molar mass of the empirical formula, if you calculated the empirical formula. Now if you have the molar mass of the molecule and the percentages of each element then you can multiply the percentages by the molar mass to get the # of moles of each element in the molecular formula. If the question gives you the sample mass and the mass of each element in the sample you can calculate the percentages and then use the molar mass to find the # of moles of each element in the molecular formula as well. It’s important to note that without the molar mass you can not find the molecular formula, but if the question asks to find the molecular formula it will most likely be given.

Daniel Kim 1D
Posts: 38
Joined: Wed Oct 03, 2018 12:17 am

### Re: Formulas

You first divide by the smallest mole then multiply the number that will make the empirical formula into all whole numbers. After that, you use the grams/mol that is given and divide the molar mass by that to see what is the factor that your moles have to be multiplied by.

Haorui Li 1A
Posts: 30
Joined: Mon Jun 17, 2019 7:23 am

### Re: Formulas

After you get the empirical formula, you get the molar mass of the empirical formula of the compound. If you need to find the molecular formula, the question will give you the molar mass of the molecular formula. Now you divide the molar mass of the molecular formula by the molar mass of the empirical formula, you get the number to multiply by.

Junxi Feng 3B
Posts: 17
Joined: Sat Sep 14, 2019 12:17 am

### Re: Formulas

First of all, I think the actual molecular mass of the compound must be given. And then you simply divide the actual molecular mass by the molecular mass of its empirical formula. The number you get will be the answer.

Harry Zhang 1D
Posts: 16
Joined: Sat Sep 14, 2019 12:16 am

### Re: Formulas

In order to find the number to multiply on a empirical formula to figure out the molecular formula, you must be given the molar mass of the compound and then divide it by the mass of the empirical formula you obtain to see the proportional relationship between them. If you get a number of 2, simply multiply each atom in the empirical formula by 2 and you will have your molecular formula.

Jialun Chen 1A
Posts: 20
Joined: Sat Sep 07, 2019 12:16 am

### Re: Formulas

To find the molecular formula, you have to find the molar mass empirical formula first. Then, use the molar mass of the molecular formula to divide the molar mass of the empirical formula, that would give you a whole number (or nearly). Finally, use the whole number to multiply the # of atoms in empirical formula (e.g. If the whole number is 6 and the empirical formula is CH2O, the molecular formula would be C6H12O6).

Minh Ngo 4B
Posts: 16
Joined: Thu Jul 25, 2019 12:17 am

### Re: Formulas

The easiest way is to get whatever number you have to moles. If you have grams then convert them to moles using molar mass. If you have percentages, then imagine you have 100g of that substance so the percentage would then become grams. After you have everything in term of moles, you would divide those numbers by the smallest number you have. If the results come out as whole numbers, you are done for finding the empirical formula. But if not, you have to multiply by any integer you think would get your mole to a whole number. Remember, if you do end up multiplying, you have to multiply all the number of moles you have by that integer, not just one of them.
If they tell you to find the molecular formula, they would have to give you the molecular mass. Find the molecular mass of your empirical formula. Then divide the given molecular mass by the molecular mass of your empirical formula to get the ratio. The next step is just multiplying the number you have by that ratio!
Hope this helps!

Grecia Velasco 4D
Posts: 17
Joined: Thu Jul 11, 2019 12:17 am

### Re: Formulas

In order to find the molecular formula, you need to follow these steps: calculate the mass percentage composition, find the empirical data, and then calculate the molecular formula. I'm going to assume you know all of these steps until the molecular formula step. In order to find
find the number to multiply by when making the molecular formula
you need to overall molar mass (most likely given) and divide that by the molar mass of the empirical data. You should get a number that you then multiply to the subscripts of the empirical formula.
Example: you the number 2.00 from the (given molar mass)/(molar mass from empirical formula) formula, just multiply 2.00 to your empirical formula (C3H403) becomes (C6H8O6).

:) Hope this helps!