Empirical & Molecular Formulae

Question 1

The molecular formula is the

Answer 1

formula that shows the number and type of each atom in a molecule

E.g. the molecular formula of ethanoic acid is C2H4O2

Question 2

The empirical formula is the

Answer 2

simplest whole number ratio of atoms of each element present in one molecule or formula unit of a compound

E.g. the empirical formula of ethanoic acid is CH2O

Question 3

Empirical formula is calculated from knowledge of the ratio of masses of each element in the compound

STEPS TO FIND EMPIRICAL FORMULA GIVEN THE MASS

Answer 3

Note the mass of the element

divide the masses by the atomic masses to find moles

divide by the lowest figure to obtain the ratio

Question 4

DO PRACTICE OF EMPIRICAL FORMULA QUESTIONS

Question 5

Determine empirical formula given percentages

Answer 5

note percentage mass of each element

divide the percentage by atomic masses

divide by the lowest figure to obtain ratio

Question 6

The molecular formula can be found by

Answer 6

by dividing the relative formula mass of the molecular formula by the relative formula mass of the empirical formula

Multiply the number of each element present in the empirical formula by this number to find the molecular formula

Step 1: Calculate relative mass of the empirical formula

Relative empirical mass = (C x 4) + (H x 10) + (S x 1)
Relative empirical mass = (12 x 4) + (1 x 10) + (32 x 1)
Relative empirical mass = 90
Step 2: Divide relative formula mass of X by relative empirical mass

Ratio between Mr of X and the Mr of the empirical formula = 180/90
Ratio between Mr of X and the Mr of the empirical formula = 2
Step 3: Multiply each number of elements by 2

(C4 x 2) + (H10 x 2) + (S x 2) = (C8) + (H20) + (S2)
Molecular Formula of X is C8H20S2