Unit 1 Section 2 Formulas

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Molecular Formula

Molecular Formula: Gives the actual numbers of atoms in a molecule

If you know the empirical formula and molecular mass, you can calculate the molecular formula:

1. Find the mass of the empirical formula(just like relative formula mass).

2. Divide molecular mass by empirical mass. This shows how many multiples of the empirical formula are in the molecular formula.

3. Multiply the empirical formula by that number to find the molecular formula.

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Empirical Formula

Empirical formula: The smallest whole number ratio of atoms in a compound.

To work out the empirical formula from percentages of elements:

1. Assume 100g of the compound. You can now turn the percentages into masses and work out how many moles of each element are in 100g of the compound.

2. Divide each number of moles by the smallest number of moles you found in step 1. This gives the ratio of elements in the compound.

3. Apply the numbers from the ratio to the formula of the compound to give the empirical formula.

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