Empirical and molecular formulae

Question 1

What is the empirical formula?

Answer 1

The simplest whole number ratio of the atoms of each element present in a compound.

Question 2

How do you calculate the empirical formula of a compound?

Answer 2

Step 1: Find the masses of each of the elements present in the compound
Step 2: Work out the number of moles of atoms of each element
Step 3: Convert the number of moles of each element into a whole number ratio (you may have to divide all the numbers by the smallest ratio).

Question 3

Step 2 of working out the empirical formula requires you to calculate the number of moles of atoms of each element. What formula would you use to calculate this?

Answer 3

Number of moles= mass of element / mass of 1 mole of element

Question 4

10.01g of a white solid contains 4.01g of calcium, 1.02g of carbon, and 4.80g of oxygen. What is the empirical formula of calcium carbonate?

Answer 4

CaCO3 (if confused check pg 30)

Question 5

0.795g of black copper oxide is reduced to 0.635 of copper when heated in a stream of hydrogen. What is the empirical formula of copper oxide? Ar Cu = 63.5, Ar 0 = 16.0.

Answer 5

CuO (if confused check pg 31)

Question 6

What is the molecular formula?

Answer 6

The actual number of atoms of each element in one molecule of the compound

Question 7

How do you work out the molecular formula of a compound?

Answer 7

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

Question 8

ethene is found to have a relative molecular mass of 28.0 but it’s empirical formula, CH2, has a relative mass of 14.0. What is the molecular formula for ethene?

Answer 8

C2H4 (if confused check pg 32)

Question 9

An organic compound containing only carbon, hydrogen, and oxygen was found to have 52.17% carbon and 13.04% hydrogen. What is the molecular formula if Mr = 46.0?
100.00g of this compound would contain 52.17g carbon, 13.04g hydrogen and 34.79g oxygen.

Answer 9

C2H60 (if confused check pg 33)