Empirical formulae

Question 1

empirical formula

Answer 1

simplest whole-number ratio of atoms of each element in a compound

Question 2

How do you calculate the empirical formula, using this example:

a 10g sample of a compound X contains 8g of carbon and 2g of hydrogen

Answer 2

1) write the symbol of each element as a header
2) write down the mass of each element in g
3) write down the Ar of each element
4) for each element, mass/Ar
5) divide each answer by the smallest answer
6) if needed x the answers by 2, or round up/down

C: 8g, 12, 8/12 = 0.667, 0.667/0.667 = 1
H: 2g, 1, 2/1 = 2, 2/0.667 = 3

CH3

Question 3

How do you find the molecular formula of a compound from the empirical formula using the Mr?

Answer 3

method 1 (find what percentage the empirical formula is of the molecular):

• calculate the Mr of the empirical formula
• find the %: divide the empirical Mr by the molecular Mr
• divide the numbers in the empirical formula by the result to reach 100% and the molecular formula

method 2 (find what percentage the molecular formula is of the empirical):

• calculate the Mr of the empirical formula
• find the %: divide the molecular Mr by the empirical Mr
• multiply the numbers in the empirical formula by the answer to find the molecular formula

method 3 (finding the number of atoms by using the individual percentages of each element of the substance and the Mr):

• find the percentage amount of each element in the substance by using the amounts given, or putting the Ar into the ratio of atoms (empirical formula)
• use the percentages to find the total mass of each type of element (decimal % x Mr)
• individual Mr/Ar of each element to find the number of atoms in the Mr of each element

Question 4

Describe the experiment needed to determine empirical formula

Answer 4

a crucible containing a magnesium ribbon is heat by a bunsen burner

it is placed on a pipeclay triangle on a tripod, and the lid is kept open/opened periodically to allow oxygen to fully react with the magnesium

the empty crucible is weighed before, as well as the weight of the crucible with the magnesium

it is also weighed after, this allows you to find the weight of the oxygen contained within the magnesium oxide

Question 5

Use the information below to:

a) calculate the mass of magnesium used
b) calculate the mass of oxygen gained

empty crucible and lid: 19.06g
crucible, lid, Mg, before: 19.42g
crucible, lid, Mg, after: 19.66g

Answer 5

a) mass of Mg: 19.42 - 19.06
= 0.36g

b) mass of O: 19.66 - 19.42
=0.24g

Question 6

Use these masses to determine the empirical formula of magnesium oxide:

empty crucible and lid: 19.06g
crucible, lid, Mg, before: 19.42g
crucible, lid, Mg, after: 19.66g

(Ar: O = 16, Mg = 24)

Answer 6

Mg = 0.36/24 = 0.015
O = 0.24/16 = 0.015

Mg = 0.015/0.015 = 1
O = 0.015/0.015 = 1

MgO

Question 7

When determining empirical formula through an experiment, a lid is needed on the crucible.
The lid must be kept slightly open during heating.

Give two reasons why.

Answer 7

oxygen must be let in to react with the Mg

magnesium (oxide) must not be allowed to escape