Empirical & Molecular Formulas

Empirical Formula =) shows the ratio of the atoms presents in their lowest terms. ( canceled down to the smallest whole number )

Molecular Formula =) shows the actual number of each type of atom press in one molecule.

Calculating EF from moles

  1. Using the given number of mole in the elements, divide them to the smallest ratio.

Eg : 6 mol of potassium & 3 mol of oxygen

EF of the compound =) K2O

Calculating EF from mass

  1. Find out the number of mole from the mass given and the Mr

  2. Then divide the num of mole into the smallest ratio

Eg : 6g of CO2 reacts with 32g of S

  • mole of C =) 0.5 & mole of S =) 1

  • Ratio 0.5 : 1 → 1 : 2

  • CS2

Calculating molecular formula

  1. Need to find out the Mr and the empirical formula first

  2. Divide Mr by E to get a scale factor

  3. Multiply EF by scale factor to get MF

Eg : EF = CH2 & Mr 43

Mr of ( CH2 ) = 12 + 1 × 2 = 14

42 / 14 = 3

MF =) CH2 × 3 = C3 H6