Unit 7: Chemical Formulas and Molar Mass

Subscripts

numbers written on the right bottom, apply to # of the element to the left, shows its amount

Parentheses

Subscript applies to the entire compound that is in the parenthesis, determines the number, amount of it

Coefficients

# in front of a compound

Applies to the entire compound, so it is the number.amount of the entire compound

Molar mass

the mass in grams of one mole of a substance

- 1 mole of a substance contains 6.02 x 10^23 of particles

Method of calculation of molar mass

- First find the # of each element in the compound

- Multiply the molar mass for each element

- Add all the masses together

- Units for molar mass g/mole

Molar mass as a conversion factor

- Molar mass can be used to convert moles to grams or grams to moles

For all the problems, you need to find the molar mass of the compound first

Converting moles to grams

Number of given moles x molar mass (g)/1 mol = grams

Converting grams to moles

Given grams x 1 mol/molar mass (g) = moles

percent composition formula

mass of the part/mass of whole x 100

empiracal formula

Writing compounds in the lowest terms, simplified

An empirical formula consists of the symbols for the elements combined in a compound, with subscripts showing the smallest whole number ratio of the different atoms in the compound

Ionic compounds and the empirical formula

For ionic compounds the formula unit is the same as the empirical formula

Covalent compounds and the empirical formula

For covalent compounds the molecular formula can be the same as the empirical formula or a multiple of the empirical formula

Calculating empirical formulas

1. Assume a 100g sample, so percentages become masses (only if percentages are given)

2. Convert all the masses to moles, by dividing by the molar mass

3. Divide each calculated mole value by the smallest calculated mole value

4. Round calculated numbers to the nearest whole number to generate formula

**If final answers are not whole numbers, you must multiply each element by the same number until you have all whole numbers (or very close)

Calculating molecular formulas (a multiple of the empirical formula)

1. Calculate the mass of the compound using the empirical formula that was just solved or given

2. Divide the molar mass by the empirical molar mass

3. Mulitply each subscript in your empirical formula by your answer in step 2