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EL - ELEMENTS OF LIFE

Moles

The Periodic Table

Atomic Structure

Chemical Bonding

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Amount of a Substance

Chemical Formulæ

Concentrations of Solutions

Titration Calculations

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Amount of a Substance

There are more atoms of hydrogen in the human body than atoms of any other element but hydrogen contributes far less than carbon or oxygen to the mass.  Chemists are interested in the number of atoms present more than the mass.

Chemists can convert masses of elements into a measure of the number of atoms they contain by using the mole.  One mole of any substance is that amount of substance which contains as many particles as there are atoms of carbon-12 in 12 grams of carbon-12.

Atoms are very small, hence the number of atoms in one mole is very large: there are 6.02 x 10²³ atoms in 12g of carbon-12.  This number is called the Avogadro Number, L. Since the relative atomic mass in grams of all elements contains 6.02 x 10²³ atoms, 1 mole of atoms of an element equals the relative atomic mass of the element.

• 1 mole of molecules contains 6.02 x 10²³ molecules and equals the relative molecular mass.
• 1 mole of carbon weighs 12g.
• Therefore 6 g of carbon contains 6/12 moles = 0.5 moles.

Number of Moles =	Mass in Grams
	Mass of 1 Mole

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Chemical Formulæ

Moles can be used to work out chemical formulæ.  We can find the simplest formula of any compound if we know the amounts of each element present in it.

For example, an oily liquid was found to contain 0.2g of hydrogen, 3.2g of sulphur and 6.4g of oxygen.

Element	Number of Moles	Simplest Ratio
Hydrogen	0.2/1 = 0.2	2
Sulphur	3.2/32 = 0.1	1
Oxygen	6.4/16 = 0.4	4

Hence the simplest formula is H₂SO₄.

The chemical formula can also be determined if the percentage mass of each element is known.

For example methane contains 75% by mass of carbon and 25% by mass of hydrogen.

Element	Moles	Simplest Ratio
Carbon	75/12  =  6.25	1
Hydrogen	25/1  =  25	4

Hence the simplest formula is CH₄.

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Concentrations of Solutions

Chemical reactions are often carried out in solution.  When you are using a solution, it is important to know how much of the solute is dissolved in a particular volume of solution.

Concentrations are usually measured in grams per cubic decimetre or moles per cubic decimetre.  To convert from grams per cubic decimetre to moles per cubic decimetre, you need to know the molar mass of the substance.

For example, the molar mass of sodium hydroxide, NaOH, is 40 g.

A solution containing 80 g dm^-3 has a concentration of

80	= 2 mol dm-3
40

Sometimes concentration is referred as molarity.  A one molar solution (1M) contains one mole in 1 dm³ of solution.  If you know the concentration and volume of a solution you can easily work out the number of moles present.

Number of Moles =	M   x   V
	1000

[ M  = concentration in mol dm^-3,   V  = volume in cm³ ]

Concentrations of solutions can be determined by a technique called titration.  In this technique, a solution of concentration to be determined is reacted with a solution of known concentration.  The volumes of each solution which exactly react together can be determined often by using an indicator.

Iron is a trace element found in the human body.  Iron carries out a vital role in the body: as part of the substance hæmoglobin, present in the blood it is responsible for the transport of oxygen.  The amount of iron present in the blood can be accurately determined by titration.

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Titration Calculations

Example 1

Determine the concentration of sulphuric acid if 25 cm³ of 0.1M sodium hydroxide solution is neutralised by 20 cm³ of sulphuric acid.

25 cm3 of 0.1M NaOH contains				25 x 0.1	moles
				1000
				= 0.0025 moles

The equation for the reaction is:

2NaOH (aq)  +  H₂SO₄ (aq) Na₂SO₄ (aq)  +  2H₂O (l)

Hence 2 moles of NaOH reacts with 1 mole of H₂SO₄

Number of moles of H2SO4 in 20 cm3=			0.0025	=  0.00125
			2

Number of moles of H2SO4 in 1000 cm3=				1000 x 0.00125	=  0.0625
				20

Hence concentration is 0.0625 mol dm^-3.

Example 2

An impure sample of iron of mass 2.55g was dissolved in dilute sulphuric acid and the solution made up to 250 cm³.  The solution contained iron (II) ions together with the impurities.  25 cm³ samples of the solution were titrated with potassium manganate (VII) solution of concentration 0.02 mol dm^-3.  The average volume required to completely react with the iron (II) ions was 28.5 cm^-3.  What is the percentage purity of the iron?

Number of moles of MnO4- used =			28.5 x 0.02	=  5.7 x 10-4
			1000

The equation for the reaction is:

MnO₄^- (aq) + 5Fe²⁺ (aq)  + 8H⁺ (aq) Mn²⁺ (aq) + 5Fe³⁺ (aq)  + 4H₂O (l)

Hence 1 mole of MnO4-reacts with 5 moles of Fe2+.
Number of moles of Fe2+		= 5 x 5.7 x 10-4
		= 2.85 x 10-3
This is contained in 25 cm3 of sample.
In 250 cm3 there are 10 x 2.85 x 10-3 moles of Fe2+.
1 mole of Fe = 56g
Hence mass of iron in 250 cm3		= 56 x 10 x 2.85 x 10-3
		= 1.596 g
Percentage Purity =		1.596 x 100	=  62.6
		2.55

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