1.3 chemical calculations

moles, concentrations, gas equations.

Moles definition

One mole of any substance contains the same number of particles as there are atoms of C-12 in exactly 12g Carbon-12.

Formulae for actual number of particles.

amount in moles = number of particles/ Avogadro’s number.

formulae for moles

Moles = Mass/ Mr

formula for percentage composition.

% = total Ar of element/total Mr of compound x 100

formula for mass of element

mass of element = Ar of element/ Mr of compound x mass of compound

Formulae for % yield

= actual mass of product/ maximum mass of product x 100

Formula for atom economy

= mass required of product/mass of reactants or products x 100

Formula for moles in liquids

n = concentration (mol dm-3) x volume (dm-3)

The formula for moles in gases when conditions are:

25 C → 1 atm

0 C → 1 atm

n = volume / molar volume

Ideal gas equation

PV = nRT

What do the symbols in PV = nRT stand for?

• P = Pascals

• V = volume (m3)

• n = moles

• R = constant in data booklet

• T = temperature (K)

Combined Gas Law:

P1V1/T1 = P2V2/T2 (remove whatever the question states is constant.)