Amount of Substance

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  • Created by: Pope1912
  • Created on: 04-03-15 15:56

The Mole

A mole is the number of particles.

Amount of substance is measured using a unit called a mole, n.

Molar mass is the mass of one mole and is just the same as relative molecular mass.

Moles = Mass / Mr

Concentration is measured in Mol dm-3.

Number of moles = Concentration x Volume

All gases occupy the same volume when under the same conditions.

Room temperature and pressue = 24 dm3

Number of moles = Volume in dm3 / 24

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Ideal Gas Equation

pV = nRT

p = pressure (Pa)

V = volume (m3)        1cm3 = 1 x 10-6 m3     1dm3 = 1 x 10-3 m3

n = number of moles

R = 8.31 J K-1 mol-1

T = temperature (K)    K = C + 273

n = pV / RT

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Equations and Calculations

Balanced equations have the same number of each atom on both sides.

You can only balance by adding whole compounds.

In ionic equations the charges must balance.

State symbols:

s = solid

l = liquid

g = gas

aq = aqueous (solution in water)

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Allow you to find out exactly how much acid is needed to neutralise a quantity of alkali.

Pipette - measure only one volume of solution.

Burette - measure different volumes and allows you to add the solution drop by drop.

Find out what value you need to know.

Number of moles of acid = conc x vol / 100

Conc of alkali = moles of acid x 1000 / vol

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Empirical and Molecular Formula

Empirical Formula - gives the smallest whole number ratio of toms in a compound.

n = mass / mols

Divide all by the smallest number of moles.

Molecular Formula - gives the actual numbers of atoms in a molecule.

molecular mass / empirical mass

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Percentage Yield

The theoretical yield is the mass of the product that should be formed in a chemical reaction. It assumes no chemicals are "lost" in the process.

You can use the masses of the reactants and the balanced equation to calculate the theoretical yield.

Number of moles reacted = mass / molar mass

Theoretical yield = moles reacted x molar mass of products

For any reaction the actual mass of the product will always be less than the theoretical yield.

Reasons for this may be: reactants don't react fully, solution gets left on the filter paper, reactants are lost during transfers between containers.

Percentage yield = (actual yield / theoretical yield) x 100

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Atom Economy

% atom economy = (mass of desired product / total mass of reactants) x 100

Efficiency of a reaction is measured by percentage yield.

Percentage yield does not always represent how wasteful a reaction is.

It may be 100% efficient  but most of the atoms end up in by-products rather than the desired product.

Atom economy is a measure of the proportion of reactant atoms that become part of the desired product in the balanced chemical equation.

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