Moles and ConcentrationA Teaching Resource

Dr Jennie Litten-Brown Dr Colin Litten-BrownBecky Morgan Contact: j.c.litten-brown@reading.ac.uk

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Progress South Central Lifelong Learning Networkwww.progresssouthcentral.org.uk

The University of Readingwww.reading.ac.uk

This document has been prepared where possible using guidelines provided by the British Dyslexia Association.

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The purpose of this presentation is to introduce the concept of moles in relation to concentrations of solutions.

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Teaching Aims

Teaching Aims• To understand the principle of moles.

• To introduce the basic measurements of concentration

and to illustrate how an understanding of moles is

important in expressing concentration.

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Why learn about moles?

• By ‚moles‛ we are, of course, referring to

the unit of measurement of substances,

not the cute furry animals found in the

garden.

• So, what is a mole? Here is a definition:

‘The mole is the amount of substance of

a system which contains as many

elementary entities as there are atoms

in 12g of carbon 12; its symbol is ‚mol‛.’

Moles

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Moles• In 12 grams of Carbon-12 there are:

– 6.02 x 1023 atoms.

– This is called Avogadro’s number

• One mole of any other substance

also contains this number of atoms.

• One mole of a pure substance has

a mass in grams equal to its

molecular weight.

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• In a periodic table of the chemical

elements, the molecular weight is

normally given or can be calculated.

• For example, oxygen has an atomic

weight of 15.9994 g.mol-1 (usually

rounded up to 16 for convenience).

• As oxygen exists as O2, the molecular

weight is 31.9988 (or 32).

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Periodic Table

Molecular Weight• By using the periodic table and the atomic

weights of elements, the molecular weight

of more complex molecules can be

calculated by adding their constituent

atoms together.

• For example, a molecule of sodium chloride

of NaCl (salt) is composed of an atom of

sodium (atomic weight 23g.mol-1) and one

of chlorine (atomic weight 35.5g.mol-1).

• The molecular weight of salt is therefore

approximately 23 + 35.5 = 58.5 g.mol-1. 11

Hypothesis Testing• So, why are moles useful?

• Understanding moles is useful when you are looking

at the concentration of one substance in another.

• When mixing two substances there is always a solute

(the substance being added e.g. salt) and a solvent

(the substance into which the solute is added

e.g. water).

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Concentration• There are many ways of expressing

concentration but they are all a measure

of the amount of solute in solvent.

• Concentrated solutions are those in

which the level of solute is increased

(and solvent decreased).

• Dilute solutions are those in which the

level of solute is decreased (and solvent

increased).

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ConcentrationSome ways of expressing concentration are:

– Mass percentage (solute & solvent are mixed by weight)

– Mass-volume percentage (mass of solute in a given volume of solvent)

– Volume-volume percentage (solute & solvent are mixed by volume)

– ‘Parts-per’ (e.g. parts per million – compares amount of solute in solvent

using the same units e.g. 1g solute in 1million g solvent = 1ppm).

– Molarity (number of mols of solute in a given volume of solution e.g. mol/l)

– Molality (number of mols of solute in a given mass of solvent e.g. mol/kg)

– Mole fraction (number of mols of solute as a proportion of the total

number of mols in a solution)

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Concentration• In scientific applications it is

common to refer to solutions

in terms of molarity.

• Often when performing laboratory-

based analyses, the reagents used

will be supplied in a specific

concentration e.g. a molar solution.

• We will now look at how you go

about calculating molarity.

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Molarity• Molarity is given by the number of

moles of a solute in a given volume

of solvent.

• For example, in an earlier slide we

found that the molecular weight of

salt (NaCl) is about 58.5g.mol-1.

• If we were to dissolve 58.5g of salt in

water (made up to a total volume of 1

litre) we would have 58.5g or 1 mole

of salt in 1 litre of solution. This is a

MOLAR solution.16

Molarity• Molarity is given by the simple equation:

• So, if we were to add 117g of salt to our vessel and

make it up to 1 litre with water, we would have a

molarity of:

• Molarity is usually expressed by the symbol ‘M’.

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Moles of soluteLitres of solution

= Molarity of solution

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= 2M

Molarity• By way of contrast, molality is given by the simple

equation:

• So, if we were to add 58.5g of salt to 1kg of water,

we would have a molality of:

• Molality is usually expressed with the symbol ‘m’.

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Moles of soluteKilograms of solvent

= Molality of solution

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= 1m

Conclusion• So, you can see that by understanding moles

we can understand the concentration of one

substance in another.

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Answers1. What is the molecular weight of NaOH?

2. What molarity are the following solutions?

– 1 litre water containing 80g NaOH

– 1 litre water containing 186g KCl

– 2 litres water containing 80g NaOH

– 1 litre water containing 24.5g CuSO4.5H2O

3. What mass of NaOH is needed to make a 1M solution?

4. What mass of NaOH is needed to make 4 litres

of a 2.5M solution?

Answers1. What is the molecular weight of NaOH? = 40g.mol-1

2. What molarity are the following solutions?

– 1 litre water containing 80g NaOH = 2M

– 1 litre water containing 186g KCl = 2.5M

– 2 litres water containing 80g NaOH = 1M

– 1 litre water containing 24.5g CuSO4.5H2O = 0.1M

3. What mass of NaOH is needed to make a 1M solution?

= 40g

4. What mass of NaOH is needed to make 4 litres

of a 2.5M solution? = 400g

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