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PERCENTAGE COMPOSITION OF COMPOUNDS:

Law of Definite Proportions:  Compounds are consistent chemical combinations of atoms that can be expressed as:
(i)  Ratio of masses
(ii) Ratio of atoms
(iii) Ratio of moles of atoms
(iv) Percent composition
ILLUSTRATIVE EXAMPLE (1): Calculate the percent Nitrogen in Dinitrogen (N2O) Monoxide?
SOLUTION:
ILLUSTRATIVE EXAMPLE (2): What is the % composition of each element in (Mg(OH)2) magnesium hydroxide?
SOLUTION:
ILLUSTRATIVE EXAMPLE (3): Haemoglobin contains 0.33% of Iron by weight. The molecular weight of it is approx. 67200. The numbers of iron atoms (Atomic wt of Fe=56 u) present in one molecule of Haemoglobin are?
SOLUTION:
ILLUSTRATIVE EXAMPLE (4): The hydrated salt, Na2SO4.nH2O undergoes 55.9% loss in weight on heating and becomes anhydrous. The value of n will be?
SOLUTION:
ILLUSTRATIVE EXAMPLE (5): Air contain 20 % O2 by volume. How many cm³ of air will be required for oxidation of 100 cm³(ml) of acetylene?
SOLUTION:
Since air contain 20 % oxygen by volume then amount of air required to react with 100cc/ml of C2H2
Try yourself:
Exercise (4): Two oxides of metal contain 72.4 and 70 of metal respectively if formula of 2nd oxide is M2O3 find that of the first.

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