Ch. 3.2 : Determination of Formulae

Question 1

What is empirical formula ?

Answer 1

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

Question 2

What is relative molecular mass?

Answer 2

It compares the mass of a molecule with the mass of an atom of carbon -12 (Mr)

Question 3

What is relative formula mass?

Answer 3

It compares the mass of a formula unit with the mass of an atom of carbon - 12

Question 4

How do you find empirical formula ?

Answer 4

Element : 
Mass / Percentage (g) : 
Mass number : 
Moles : 
Ratio : 
Formula :

Question 5

How do you work out molecular formula ?

Answer 5

• work out the relative mass of the empirical formula
• divide the relative molecular mass of the compound by it
• multiply your answer by the empirical formula

Question 6

Define the terms hydrated and anhydrous

Answer 6

Hydrated - water molecules are part of the structure

Anhydrous - no water within the structure

Question 7

What is water of crystallisation ?

Answer 7

Water molecules part of a crystals crystalline structure

Question 8

How do you remove water from crystals ? How does it’s appearance change ?

Answer 8

When it is heated , the bonds holding the water within the crystal are broken and the water is driven off.

This leaves a white anhydrous power, as the crystalline structure is lost.

Question 9

What are the assumptions made during the experiment of removing water from the crystals ?

Answer 9

1. All the water has been lost. A good solution is to heat till constant mass.
2. No further decomposition. Many salts decompose further when heated.