Question

The formula of hydrogen phosphate of a metal is MHPO₄.Give the formula of the Metal Sulphite.

Answer 1

• The molecular formula of the compound is always electrically neutral.
• That is the positive and negative valencies of the ions or the radicals present in the chemical compounds add upto zero.
• One of the method used to write molecular formula is criss-cross method.

In criss-cross method,

• Write the symbols side by side, usually the one with positive valency first.
• Write the valency of each atom on the top of the symbol.
• To get simplest ratio, divide the valency numbers by their highest common factor(H.C.F). Ignore the (+) and (-) symbols of the radicals.
• Interchange the valencies of the radicals.
• Write the interchanged valency numbers to the lower right of the radicals.
• If the radical is a group of atoms and receives a valency number more than 1, enclose it within brackets.

From MHPO₄

• We know that Oxidation state of Hydrogen phosphate is -1 (HPO₄ ^-1)
• Then the valency of Hydrogen phosphate is 1.
• Therefore the oxidation state of metal is +1
• Then the valency of Metal is 1

Then the the formula of the Metal Sulphite can be calculated by Criss-cross method.

Element Metal Sulphite

Symbol M SO₃

Valency 1 2

Interchanging valency 2 1

Therefore the formula of Metal sulphite is M₂SO₃