Empirical and molecular formula

Question 1

What is the empirical formula?

Answer 1

The simplest whole number ratio of elements in a compound.

Question 2

What is the molecular formula?

Answer 2

The formula that’s used and is the actual ratio of elements in a compound.

Question 3

How can you find empirical and molecular formulae for a compound?

Answer 3

From composition by mass.

Question 4

What is the first step to find the empirical formula?

Answer 4

Divide the mass of each element by its Ar to find the number of moles of each element present.

Question 5

What do you do after finding the number of moles for each element in the empirical formula calculation?

Answer 5

Divide by the smallest number of moles.

Question 6

What is the next step after dividing by the smallest number of moles in the empirical formula calculation?

Answer 6

Multiply up to get whole numbers for each element.

Question 7

How do you find the molecular formula from the empirical formula?

Answer 7

Divide the molecular mass (Mr) by the mass of the empirical formula, and multiply the empirical formula by the result.

Question 8

EXAM TIP: If given percentage by mass for a compound, what is the easiest way to approach the problem?

Answer 8

Assume you have 100g of the compound and use the percentages as masses.