3.11 Molecular and Empirical Formulas

Question 1

What does the molecular formula of a substance show?

Answer 1

The actual number of atoms of each element present in a compound or molecule.

Question 2

What is the empirical formula of a substance?

Answer 2

The simplest, whole number ratio of atoms of each element in a compound. This means that it doesn’t tell you exactly how many atoms there are, just the ratio of the atoms of each element.

Question 3

Describe how to find the molecular formula from the empirical formula and Mr with the example of “An unknown compound has an empirical formula of CH3, and an Mr of 30. Find the molecular formula of the compound”. (3)

Answer 3

• First, find the Mr of the empirical formula. CH3 contains 1xC and 3xH, so 1x12 + 3x1, which equals 15.
• See how many times the Mr of the empirical formula (15) goes into the unknown compound’s Mr (30). 30/15=2. So the empirical formula’s Mr goes into the unknown compound’s Mr 2 times.
• Look at the empirical formula: CH3, and multiply all the numbers by the multiple you just found (2): multiplying all the numbers in CH3 by 2 gives you C2H6. So the molecular formula is C2H6.

Question 4

Describe how to find the empirical formula from masses or percentages with the example of “A compound is found to contain 50.05% sulfur and 49.95% oxygen by weight. What is the empirical formula for this compound?” (4)

Answer 4

1) Assume 100g of the compound is present. This changes the percentages given in the question into grams: Sulfur (S): 50.05% -> 50.05g, Oxygen (O): 49.95% -> 49.95g.
2) Convert these masses to moles (moles = mass / Mr):
moles of sulfur (S) = 50.05g / 32 = 1.564, moles of oxygen (O) = 49.95g / 16 = 3.122.
3) Divide both the mole values you just calculated (1.564 and 3.122) by the lower number of the two (1.5640625), this gives you the smallest whole-number ratio between the two elements (you usually need to round the answer to the nearest whole number). Sulfur (S): 1.564 / 1.564 = 1. Oxygen (O): 3.122 / 1.564 = 1.996 (round this to the nearest whole number) = 2.
4) So now you know the ratio of sulfur to oxygen in this compound is 1:2. Use this ratio to write out the empirical formula for the compound: SO2.