**Chemical
Formulae**

To
work out the number of moles in an element/compound, just divide the mass by
the Ar or the Mr.

For
example:

“How many moles are there in 32g of oxygen?”

Ar of oxygen is 16

So 32/16 = number of moles

Number of moles = 2

To
work out the mass, multiply the number of moles by the Ar
or Mr.

For
example:

“What is the mass of 6 moles of Carbon?”

Ar of Carbon is 12

So 12 x 6 = Mass

Mass= 72g

To work out the number of particles, just multiply the number of moles by
Avagadro's constant

“How many particles are there in 2 moles of carbon?”

2 x 6.02 x10^{23}

= 1.2 x 10^{24}

To work out the empirical formulae of a substance,
the mass is needed.

For
example:

“0.020g of Hydrogen combine with 0.168g
of Oxygen. What is the empirical formula of Water?”

Mass: 0.020g H 0.168g O

Number of Moles: 0.20/1 0.168/16

Ratio: 0.02 :
0.01 = 2:1 so the formula is H_{2}O

To
work out the molecular formula, divide the Molecular Mass by the sum of the
Atomic masses of the empirical formula.

For
example:

“The empirical formula of methane is CH_{3},
the formula mass of methane is 30, work out the molecular formula”

CH_{3 }= 12 + (3x1)

= 15

30/15= 2

so the molecular formula is twice the empirical formula.

C_{2}H_{6}_{}

Revise AS Chemistry for Salters (OCR) (Salters Advanced Chemistry)