Amount Of Substance - types of formulae

Question 1

What is the definition of the empirical formula?

Answer 1

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

Question 2

empirical formula - example

What are the steps?

(5)

Answer 2

Mass
Relative atomic mass
Moles
Divide through smaller number
Whole number ratio

Question 3

empirical formula - example

6.2g of P is combined with O2 to form 14.2g of phosphorus oxide

Answer 3

Mass
P = 6.2g
14.2 - 6.2 = 8g oxygen

Relative atomic masses (from periodic table)
Phosphorus (P) = 31
Oxygen = 16

Moles
n = m/molar m
6.2/31 = 0.2
8/16 = 0.5

Divide through by smallest number

0. 2/0.2 = 1
1. 5/0.5 = 2.5

Whole number ratio
1 : 2.5
2 : 5

empirical formula = P2 O5

Question 4

What is the definition of molecular formula?

Answer 4

The number and type of atoms of each element in a molecule

Question 5

molecular formula - example

A compound has an empirical formula of CH2, but a relative molecular mass of 224, find the molecular formula

Answer 5

CH2 = C + H + H
= 12 + 1 + 1
= 14 mass per CH2

224/14 = 16 CH2 units

molecular formula = C16H32

Question 6

What is the difference between empirical and molecular formula?

(Using ice cream)

Answer 6

The empirical formula tells us what flavour we have

The molecular formula tells how many scoops we have