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Doubt & Answer Explanation

Doubt

how to find the molecular weight by combustion method?

Answer Explanation

The substance whose molecular weight is to be calculated is burnt in air. The resulting products are weighed.
Let us assume that the substance contains only carbon, hydrogen and oxygen. Then, carbon dioxide and water vapour will be produced.

If the mass of carbon dioxide is x, then the mass of carbon in the compound is x/44 x 12 (because x/44 represents the moles of carbon dioxide formed). If the mass of water vapour is y, then the mass of hydrogen in the compound is y/18 x 2 (because y/18 represents the moles of water formed). The mass of oxygen in the compound is 32x/44 + 16x/18 grams because 32 grams of oxygen are present in 44grams of carbon dioxide and 16 grams of oxygen are present in 18 grams of water.

The net molecular weight of the compound is the sum of the masses of oxygen, carbon and hydrogen.

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