## Molecular formula

205192823
Posts: 100
Joined: Wed Sep 18, 2019 12:19 am

### Molecular formula

If you were given the total molecular mass and given the mass percentage compositions of all the elements, how would you solve for the molecular formula using the total mass or would you just disregard it?

Ashley Nguyen 2L
Posts: 103
Joined: Sat Aug 17, 2019 12:18 am
Been upvoted: 1 time

### Re: Molecular formula

If you were given the total molecular mass and the mass percentage composition of each element, you can assume that you have 100. g of the substance and convert each percentage to its respective mass (ie converting 45.6% carbon to 45.6 g of carbon). Next, you would find the moles of each element by dividing each by its molar mass. Once you have the moles of each element, divide all the masses by the lowest mol amount. You want to get all whole numbers, so if one of your products is not close to a whole integer, multiply all of the products by a factor that would give you only whole numbers. The final products is the number of moles of each element in your compound. The molar mass is used to determine if you have found the empirical formula or molecular formula. If the total molar mass of the substance you found does not match the total molar mass you were given at the start of the problem, try multiplying all of the moles of your elements by a whole number integer to obtain the same molar mass you were given.

ALegala_2I
Posts: 102
Joined: Thu Jul 11, 2019 12:17 am

### Re: Molecular formula

To clarify, when you divide by the least number of moles. The answer you are getting is a molar ratio of atoms in the molecule. This is relative to the other atoms in the molecule. We want this ratio to be a whole number because we can only have a whole number of atoms. To find the molecular formula, you divide the given molar mass by the molar mass of the empirical formula to get another ratio. You use this number and multiply the empirical formula by it to get the molecular formula.

Grecia Velasco 4D
Posts: 56
Joined: Thu Jul 11, 2019 12:17 am

### Re: Molecular formula

First solve for empirical formula. Once you have that, calculate the empirical formula's molar mass. Because you have the molecular formula's molar mass, use the formula: molecular formula molar mass/ empirical formula molar mass=#. Use this # and multiple it with the empirical formula to get the molecular formula.

Diana Chavez-Carrillo 2L
Posts: 122
Joined: Fri Sep 28, 2018 12:18 am

### Re: Molecular formula

Everyone above is correct but I personally like to see actual examples to better understand a concept so I provided you with a step-by-step example I solved from a cool website that provides practice problems like this https://chem.libretexts.org :

Determine the empirical and molecular formula for chrysotile asbestos. Chrysotile
has the following percent composition: 28.03% Mg, 21.60% Si, 1.16% H, and
49.21% O. The molar mass for chrysotile is 520.8 g/mol.

STEP 1: CONVERT % TO MASS
Mg: 28.03 g
Si: 21.60g
H: 1.16g
O: 49.21g

STEP 2: CONVERT MASS TO MOLES
Mg: 28.03 g / 24.305g/mol = 1.153
Si: 21.60g / 28.086g/mol = 0.7690
H: 1.16g / 1.008g/mol = 1.15
O: 49.21g / 15.999g/mol = 3.076

STEP 3: DIVIDE BY SMALL #
Mg: 1.153 / 0.7690 = 1.499 * 2 = 3
Si: 0.7690 / 0.7690 = 1 *2 = 2
H: 1.15 / 0.7690 = 1.49 *2 = 3
O: 3.076 / 0.7690 = 4 * 2 = 8
EMPIRICAL FORMULA = Mg3Si2H3O8

STEP 4: MULTIPLY MOLAR MASS WITH COEFFICIENT
Mg3Si2H3O8
Mg: 24.305g/mol * 3 = 72.915
Si: 28.086g/mol * 2 = 56.172
H: 1.008g/mol * 3 = 3.024
O: 15.999g/mol * 8= 127.992

STEP 5: ADD EM’ ALL UP
72.915 + 56.172 + 3.024 + 127.992 = 260.103 g/mol Mg3Si2H3O8

STEP 6: DIVIDE MOLECULAR FORMULA BY EMPIRICAL FORMULA
520.8 g/mol. / 260.103 g/mol = 2

STEP 7: MULTIPLY QUOTIENT TO EMPIRICAL FORMULA
2 * ( Mg3Si2H3O8 ) = Mg6Si4H6O16
MOLECULAR FORMULA = Mg6Si4H6O16

I hope that helps you better understand the process of solving a question like that! :)
Last edited by Diana Chavez-Carrillo 2L on Sat Oct 12, 2019 6:30 pm, edited 1 time in total.

Mellanie Gamero 4D
Posts: 10
Joined: Tue Oct 01, 2019 12:16 am

### Re: Molecular formula

If this information is given, this also means that you must find the empirical formula beforehand, and then find the molecular formula.

Charisse Vu 1H
Posts: 101
Joined: Thu Jul 25, 2019 12:17 am

### Re: Molecular formula

You would first need to find the empirical formula using the mass percentages. You would assume that there is 100g of the molecule and convert the grams of each atom into moles. Then, you would divide the moles by the smallest number of moles to find the ratio of atoms in the molecule. To find the molecular formula, you would then find the mass of the molecule you just found and divide the given molar mass by the mass of the empirical formula you found. Then, you would multiply the empirical formula subscripts by that number to find the molecular formula.