Moles and Concentration A Teaching Resource

Dr Jennie Litten-Brown Dr Colin Litten-Brown Becky Morgan Contact: [email protected]

Sponsored by

Progress South Central Lifelong Learning Network www.progresssouthcentral.org.uk

The University of Reading www.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

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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? 7

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‛.’

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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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Periodic Table • 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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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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Concentration Some ways of expressing concentration are: –

Mass percentage (solute & solvent are mixed by weight)

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

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

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‘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).

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

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

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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 laboratorybased 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:

Moles of solute = Molarity of solution Litres of solution • 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:

2 = 2M 1 • Molarity is usually expressed by the symbol ‘M’. 17

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

Moles of solute = Molality of solution Kilograms of solvent • So, if we were to add 58.5g of salt to 1kg of water, we would have a molality of:

1 = 1m 1 • Molality is usually expressed with the symbol ‘m’.

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Conclusion • So, you can see that by understanding moles we can understand the concentration of one substance in another.

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Answers 1. 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?

Answers 1. 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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