Notesale: Turn your study into money

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Module: BIOM – 1007

Lecturer: Dr Weldon

Date: 27/10/16

Moles, Molar Masses and Concentrations
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A mole is a number equal to 6
...
02 x 1023 molecules

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One mole of any element has the same number of atoms, therefore;
 1 mol of C = 6
...
02 x 1023 molecules

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One mole of 2 different elements has the same number of atoms but different masses
 The mass of one mole of a substance is the molar mass:
 Measured in g mol-1 or grams per mole
 The molar mass of an element is the same as its mass number:
 Potassium, for example, has mass number 39, and so has a molar mass of 39 g mol-1
 The molar mass of a compound is just the sum of all the mass numbers in the molecule
 For example, potassium hydroxide (KOH) will be 39+16+1 = 56g mol-1

o

You can calculate the amount (mol) of a substance if you know:
 The mass of the sample (g)
 The molar mass of the elements involved
 The formula is the following:

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o

For example, how many moles of fluorine are in 95g of this element?
 It would be 95/19 = 5 moles

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Another example, how many molecules of glucose are in 500g of glucose (C6H12O6)?
 It would be 500/((6x12)+(1x12)+(6x16)) = 2
...

 This is known as the concentration
 The concentration is considered as the number of moles of a substance per litre of solution
 This is known as molarity
 This is measured in mol L-1 or mol dm-3 or M
 The formula for concentration is:



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For example, the amount of 500ml sodium chloride (NaCl) in a 2mol L-1 concentration will be:
 2x0
...
So how do you prepare
2L of a 2M solution of NaCl?
 The moles needed are 2 x 2 = 4
 4 moles = mass needed/molar mass (58)
 Therefore, you will need 232g of NaCl to make the solution

o

Areas of science require solutions made by percentage weight calculations
 Therefore, a 2% glucose solution in 400ml = 400x0

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