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The law of multiple proportions is illustrated by the pair of compounds:
(A) sodium chloride and sodium bromide
(B) water and heavy water
(C) sulphur dioxide and sulphur trioxide
(D) magnesium hydroxide and magnesium oxide

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Hint: The compounds to obey law of multiple proportion the compounds must have the same type of element present in the two compounds, also the isotopes should not be different to obey law of multiple proportion.

Complete answer: The law of multiple proportions states that when two elements combine with each other to form more than one compound, the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers. It was given by John Dalton in 1803.
Consider carbon dioxide and carbon monoxide. Carbon dioxide has exactly twice the amount of oxygen as it is present in carbon monoxide. Therefore, the ratio of oxygen in the two compounds is 1:2, a small whole number ratio.
Now we will consider each of the given options, and see where the law is applicable.
Considering option (A), sodium chloride and sodium bromide. Sodium chloride and sodium bromide do not obey the law of multiple proportions because it has different types of atoms involved in the given two molecules.
Considering option (B), water and heavy water. Here also the law of multiple proportions is not applicable since the atoms in the two given compounds are different. The atoms must be the same to obey the law of multiple proportions.
Considering option(C), sulphur dioxide and sulphur trioxide. Here the law of multiple proportions is applicable. Sulphur dioxide has 2 atoms of oxygen whereas sulphur trioxide has 3 atoms of oxygen.The molar mass of Sulphur dioxide is 64g/mol and molar mass of Sulphur trioxide is 80g/mol. In the two compounds, oxygen atom is in the ratio of 2:3.
Considering option(D), magnesium hydroxide and magnesium oxide. Here, the law of multiple proportions is not applicable. Magnesium hydroxide has one hydrogen atom present other than magnesium and oxygen. For the law of multiple proportions to be applicable, types of atoms in the molecule should be the same.
So, the correct answer is “Option C”.

Note: The other laws of Dalton are, Law of definite proportions, Conservation of mass, Law of Definite proportions, Dalton's atomic theory. No matter how much mass of two elements will react with each other, the product will contain the masses of its elements in a certain ratio only.



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