# Molecular Weight Determination

### Determination of molecular weight (formula weight)

Example: What's the molecular weight of CH3COOH (acetic acid) knowing that C=12g/mole,O=16g/mole and H=1g/mole?

Add all elements:  2*C + 2*O + 4*H  -->  2*12g/mole + 2*16g/mole + 4*1g/mole = 60 g/mole

CHEMIX Molecular Calculator solution:

Insert CH3COOH in the formula text field and Calculate

### Determination of Weight % Composition From Formulas

Formulas allow us to determine molecular weight of a compound as well as the weight % composition of each element represented in the compound.

The Molecular Weight is determined by taking the subscript for each element represented in the formula and multiplying it by the atomic weight of the element to get a mathematical product. Then the products are all added up for the molecular weight. For example:

Example 1 : Determine the molecular weight of H2O (water) knowing that: H=1 g/mole  and  O=16 g/mole .

There are 2 H's in the formula so the weight of the hydrogen has to be 2 grams (or a.m.u). There is only one O in the formula so the weight of O has to be 16 grams.

The molecular weight of H2O = 2*1 grams + 16 grams = 18 grams

Example 1 - The CHEMIX molecular calculator solution

Insert H2O in Formula text field and Calculate

Example 2 : Determine the weight percent of hydrogen and oxygen in H2O knowing that H2O = 18 gram/mole

18 grams = 100 % so the

weight % of hydrogen has to be :  18 grams/ 2 grams * 100% = 11.19 %

weight % oxygen = 18 grams / 16 grams * 100% = 88.81 %

Example 2 - The CHEMIX molecular calculator solution

Solution: Insert H2O in Formula text field and Calculate

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