a. What does `[H_(9)O_(4)]^(o+)` stand for ? Draw its structures.
b. Can sodium bicarbonate make water hard?
c. Hard water is softened before using in boilers. Why?
d. What is sequestration? How is hard water made soft by sequestration?

Explanation

(a) `[H_(9)O_(4)]^(o+)` stands for hydrated proton. In `[H_(9)O_(4)]^(o+)` is tetrahedrally surrounded by four water molecules.

(b) Hardness of water can be created due to sodium bicarbonate because soaps themselves are sodium salts of fatty acids which are soluble in water.
(c) Hard water on boiling gives white percipates of `MgCO_(3),kCaCO_(3)` and `CaSO_(4)` which form scales in boilers. the formation of these boiler scales causes rapid deterioration of the boiler due to over heating. further, boiler scales are non-conducting. heating. further, boiler scales are non-conducting. hence, more fuel is consumed. therefore, to prevent formation of boiler scales, hard water is softened before using in boilers.
(d) 'Sequestration' means to render ineffective. the hardness of water is due to the presence of `Ca^(2+)` and `Mg^(2+)` ions. these ions are rendered ineffective by treating with calgon to hard water, the `Ca^(2+)` and `Mg^(2+)` ions present in it, combines with calgon to form soluble compounds of calcium and magnesium salts.
`[2CaCl_(2)+underset("hexametaphosphate")underset("Sodium")(Na_(2)]Na_(4)(PO_(3))_(6)]tounderset("Complex")(Na_(2)[Ca_(2)(PO_(3))_(6)]+4NaCl`
`2MgSO_(4)+Na_(2)[Na_(4)(PO_(3))_(6)]tounderset("Complex")(Na_(2)[Mg_(2)(PO_(3))_(6)]+2Na_(2)SO_(4)`
These complex calcium and magnesium ions do not form any percipitate with soap and hence readily produces lather with soap solution. in this way, hard water is softned by sequestration.