**Diatomic molecules **

For the test, we must first memorize diatomic molecules. Some elements can not exist in nature on their own, and therefore form diatomic molecules. These molecules are: O2, I2, H2, N2, Cl2, F2, Br2.

**Formula weight ( Molar mass)**

We can calculate formula weight or molecular weight of a molecule by adding up the weight of each element, taken from the periodic table time the number of atoms of each element.

For example: H2O

H = 2*1 = 2

O = 1*16 = 16

2+16 = 18 g/mol

**% Composition**

(Mass of part/ total mass) * 100%

For example percent mass of Hydrogen in water is (2/18)*100%

**Converting moles to atoms/molecules **

When converting between moles and atoms or molecules, we must use Avogadro's number (6.02* 10^23 atoms or molecules per mole).

We always start with what is given to us. Then the units we want to get rid of go in the denominator to cancel and units we want to get go in the numerator.

For example, how many molecules of water are in 3 moles?

Lets set it up: 3mols * (6.02* 10^23 molecules/ 1 mole)

Thus moles cancel and we end up with molecules.

**Converting grams to moles**

The conversion between grams and moles is similar to the one described above. The only difference is that instead of using Avogadro's number, we use molar mass (found from the periodic table).

Thus if we want to convert 5g of water to moles, we start with what is given 5g * (1mol/18g). Therefore grams will cancel giving us the moles we are looking for.

**Empirical Formula**

Empirical formula is the formula with the lowest whole number ratios. In other words if you try to divide all the subscripts in empirical formula you can not divide anymore.

For example, what is the empirical formula for C4H10. We can divide both number by 2, giving us C2H5. 2 and 5 can not be divided any further and therefore, this is the empirical formula for this compound.

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