# Amount of Substance

Amount of substance, n

**n = mass** /** Mr**

__Solution calculations__

The concentration of solutions are measured in mol dm^{-3}

**c= concentration in mol dm ^{-3} **

**V = volume in cm ^{3}**

**n = Vc / 1000**

Many people find equations like this difficult to remember. You may prefer to remember an easy, obvious sentence and then learn how to use it.

**Remember: ** "One litre of a one mole per litre solution contains one mole."

To find the number of moles in 15.6 cm^{3} of a 3.5 mol dm^{-3} solution.

1 dm^{3 }of a 1 mol dm^{-3 }solution contains 1 mole

1 cm^{3 }of a 1 mol dm^{-3 }solution contains 1/1000 mole

15.6 cm^{3 }of a 1 mol dm^{-3 }solution contains 15.6/1000 mole

15.6 cm^{3 }of a 3.5 mol dm^{-3 }solution contains 15.6 x 3.5/1000 mole

It is a bit longer but it is easier to follow......you decide what is best for you.

**The Ideal Gas equation**

Avogadro said that: providing that the pressure and temperature of gases are the same, equal volumes of two gases can be assumed to have the same number of moles.

At GCSE you may have used "1 mole of gas occupies 24 dm^{3 }". Strictly, this should have been quoted as being at 25^{o}C and 1 atmosphere pressure. ** **

The ideal gas equation takes all of this into account

**pV = nRT**

p is the pressure in Pa (normal atmospheri pressure is about 100 kPa)

V is the volume in m^{3 }(There are 1000 dm^{3} in 1 m^{3})

T is the temperature in Kelvin (0 ^{o}C is 273 K)

R= 8.31 JK^{-1} mol^{-1}