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Structure of matter seminar: moles and molarity

University of Lincoln presentation

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The moleDefinition:

1 mole of any substance contains6.022x1023atoms and/or moleculesAmadeo Carlo Avogadro (1776-1856)

6.022 x 1023

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For Elements:

The relative atomic mass,Ar = mass of 1 mole of atoms in

grams (g)

Ar

(g) mass moles of Number

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For Molecules:

The relative molecular mass,Mr = sum of all Ar in the molecule

= mass of 1 mole of molecules (g)

Mr

(g) mass moles of Number

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Molar Concentrations, M

A molar solution (1M) is a solution containing 1 mole of substance (solute) in every litre of solvent

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Molarity

The molarity of a solution is the concentration of the solution expressed as:

the number of moles per litre –M or mol L-1 or mol dm-3

(all of these are the same)

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Molarity

How to calculate the molarity:

1. Calculate how many moles there are in solution

2. Work out how many moles there are per ml

3. X 1000 to give the number of moles per litre

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Example 1

22 g of CaCO3 was dissolved in water and made up to give a total volume of 200 cm3. C

Calculate the concentration of the solution in mol dm-3.

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Example 1

1. No mols of CaCO3 in solution =

22g/100 = 0.22 mols

2. No mols/ml = 0.22/200 = 0.0011 (= 1.1 x 10-3)

3. 1.1 x 10-3 x 1000 = 1.1M

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Example 2

If we take 15ml of our 1.1M CaCO3 solution and make it up to 250ml with H2O, what is the concentration of the new dilution?

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Example 2

1. No. mols in 15 ml of 1.1M solution == 1.1 x 15 = 0.0165 mols 1000

2. This is put into 250 ml. No. mols per ml = 0.0165/250 = 6.6 x 10-5

3. 6.6 x 10-5 x 1000 = 0.066M

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Alternatively

You can use the following formula:

V1 C1 = V2 C2

15 ml x 1.1 M = 250 ml x C2

C2 = 15 ml x 1.1 M = 0.066 M 250 ml

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Acknowledgements

• JISC• HEA• Centre for Educational Research and Developmen

t• School of natural and applied sciences• School of Journalism• SirenFM• http://tango.freedesktop.org