Chemistry
An element X forms a chloride with the formula XCl3, and an oxide with the formula X2O3. This oxide dissolves in water to form a solution that turns blue litmus red.
(a) Identify the element.
(b) Is X a metal or a non-metal ? Justify your answer.
(c) Write the chemical formula for the sulphate of element X.
Answer
(a) The element is phosphorus.
Reason — In XCl3, chlorine has valency 1. Since one atom of X combines with three chlorine atoms, the valency of X is 3.
But its oxide turns blue litmus red which means it's a non-metal. Phosphorus is a non-metal with valency 3. Formula for phorphorus oxide is P2O3 . Boron and aluminium also have valency 3 but boron is a metalloid and aluminium is a metal.
(b) X is a non-metal because its oxide forms an acidic solution in water (turns blue litmus red), which is a characteristic property of non-metals.
(c) The sulphate of phosphorus is P2(SO4)3.
Note — Although the formula P2(SO4)3 can be written using valency rules, this compound does not actually exist in reality. This is because sulphates are formed by metals (which give cations), whereas phosphorus is a non-metal and does not form P3+ cations under normal conditions. Hence, P2(SO4)3 is only a theoretical formula based on valency.
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