What is a Chemical Formula? A molecule, which is a cluster of atoms, can be described by the use of atomic symbols. Chemical formulas name the atoms that have joined and their ratios.  Chlorine gas is di-atomic. So we write the formula of chlorine molecule as Cl2. 1. The molecular formula in the figure shows 2 molecules of water. 2. The structural formula of CCl4 shows that the carbon atom is combined to four atoms of chlorine by co-valent bonds.Each short line depicts a pair of electrons. Fig.1. Three different ways of showing Formulas. 2 H2O is the most common way to show the molecule of water. A number in front gives the number of molecules. The subscript no .2 shows that there are 2 H atoms in each molecule. How to calculate the percentage of the elements in a compound? ( according to mass) Refer to the Periodic Table for the Relative Atomic masses. Use the Relative atomic masses. H = 1.08, O = 16. Formula mass of H2O = 2x1.08 + 16, = 2. 16 + 16 = 18.16, The percentage of Hydrogen = (2.16 / 18.16) x 100. = = 11.89 % The percentage of Oxygen = 16/ 18.16 x 100 =88.1 % The formula of a compound is found by working this the other way. How to determine the formula of a compound? Let us take and examples: Example.1 Fig.2 Magnesium burning inside a crucible. When magnesium burns, a white coloured solid, magnesium oxide is formed. The formula is shown here. The equation for the reaction: Magnesium + Oxygen = Magnesium oxide
Explanation. Let us take an example.  mass 10 g.  mass 2 gJohn has bought 500 g. of strawberries and 500 g. of grapes to be packeted and distributed among some children..
The number ratio of fruits strawberries : grapes = 50 : 250 = 1: 5 If we use S for strawberry and G for grapes, The formula of a packet will be S1G5 This can be given as S G5 This is similar to a chemical formula. If you can get the mass ratio of the ingredients of a pure compound and their molecular masses, it is a simple process to get the formula. Let us take and example:
Example 2. Finding the percentage composition of Ammonium sulphate. Molecular mass of (NH4)2SO4…………………………………= 132
Use this Periodic Table to get the Relative atomic mass.  Some in alphabetical order for your convenience. (Without decimals)
Example 3. A chemical consisting of three elements had the following percentages by mass. Na = 43.40 % C = 11.30 % O = 45.3 % Find the formula of the compound.
Formula of the chemical is = Na2 C1 O3 (We don’t have to write the 1 for C. ) Na2CO3 This is the formula of Sodium carbonate Verification The total of percentages must add up to 100. Nitrogen… = 21.21 Hydrogen…= 6.06Sulphur ..= 24.24 Oxygen……= 48.48 Total = 99.99 gets rounded up to 100.  1.0 Determine the percentage composition of these compounds.
2x15=30 marks 2.0 Find the formulae of these; Mass ratio or the percentages by mass are given.
3x10 = 30 . 3. On heating 50g of mercury oxide 46 g of mercury was obtained. Derive the formula of the mercury oxide. 4. On heating 1.6g of copper 1.8g of a copper oxide was formed. Find the formula of the oxide. 5. By reducing 2.39 mg of lead 2.07 mg of lead was obtained. Find the empirical formula of the oxide. 6.A compound has the following percentage composiion. Derive the empirical formula. Carbon = 40% Hydrogen = 6.7 % Oxygen= 53.3 % 8x4 =32 7. These are the 3 common isotopes of Hydrogen. Although the water forming from them would be chemically the same mass ratios will not be the same. Calculate the Oxygen percentage in the 3 types of water.
සැලකිිය යුතුයි:- සාමාන්ය හයිඩ්රජන් සැම්පලයක H-1 සමස්ඵානික 99.99% ක්ම තිබෙන බැව් සැලකිිය යුතුය. 2x4= 8 Marks
Answers:1. HgO, 2. H1O1, (There is no compound like this. This compound really is H2O2,) 3. Cu2O, 4. Ca O2H2 (This really is Ca(OH)2 , 5. PbO. Answers
3. Steps for the calculation. Mass ratio of Hg:O is ---:----- Atom ratio is ----/201 : -----/16 = ----- : ------ Get the approximate ratio leaving for experimental errors. Therefore the formula must be --------- |
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