A liquid compound X of molecular mass 18 u can be obtained from a number of natural sources. All the animals and plants need liquid X for their survival. When an electric current is passed through 200 grams of pure liquid X under suitable conditions, then 178 grams of gas Y and 22 grams of gas Z are produced. Gas Y is produced a the positive electrode whereas gas Z is obtained at the negative electrode. Moreover, gas Y supports combustion whereas gas Z burns itself causing explosions.
(a) Name (i) liquid X (ii) gas Y, and (iii) gas Z.
(b) What is the ratio of the mass of element Z to the mass of element Y in the liquid X ?
(c) Which law of chemical combination is illustrated by this example?
(d) Name two sources of liquid X.
(e) State an important use of Y in our life.

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Solution

Chemical equation: X $e l e c t r i c i t y - ------ \to$ Y + Z

(a) X is a natural source, all the animals and plants need liquid X for their survival. Molar mass of X is 18 amu or 18 grams.
So the compound X is water, that is $H_{2} O$
The decomposition of $H_{2} O$ produces $O_{2}$ and $H_{2}$ gas. Gas $O_{2}$ is produced at the positive electrode whereas gas $H_{2}$ is obtained at the negative electrode.
Therefore, compound Y is oxygen gas, that is $O_{2}$ and the compound Z is Hydrogen gas, that is $H_{2}$ . As $O_{2}$ supports combustion whereas gas $H_{2}$ burns itself causing explosions.
Balanced equation: $2 H_{2} O ⟶ O_{2} + 2 H_{2}$ ( in the presence of electricity).

(b). The molar mass of $H_{2} O$ is 18 g/mol. In that,
The mass of $H_{2}$ is 2 g/mol. (Because Atomic mass of H is 1 g/mol)
The atomic mass of O = 16 g/mol.
The mass ratio of Z: Y or H: O in the liquid X or $H_{2} O$ is
2: 16 or 1: 8

(c). The Law of Definite Proportions, statement that every chemical compound contains fixed and constant proportions (by mass) of its constituent elements.

(d). River, Rain

(e).All living things need oxygen (air) to survive. During the respiration process, all the living things take oxygen from the air and give out carbon dioxide. It is vital that all living things inhale oxygen in order to make the living cells function properly.

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