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.
|Number of Moles =||Mass in Grams|
|Mass of 1 Mole|
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.
|Carbon||75/12 = 6.25||1|
|Hydrogen||25/1 = 25||4|
Hence the simplest formula is CH4.
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|
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|
[ 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.
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|
|= 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|
|Number of moles of H2SO4 in 1000 cm3=||1000 x 0.00125||= 0.0625|
Hence concentration is 0.0625 mol dm-3.
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|
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|
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