Water Quality Standards Questions and Answers

Answer: A

The carbonate and non-carbonate hardness are 150 and 200 mg/L.

Carbonate hardness is the part of total hardness balanced by alkalinity (bicarbonate and carbonate). When alkalinity is less than total hardness, carbonate hardness equals the alkalinity, 150 mg/L, and the rest is non-carbonate hardness: 350 - 150 = 200 mg/L.

The options pairing 150 or 200 with 350 treat total hardness as one of the two parts, but the two parts must add up to the total of 350 mg/L, and 100 does not equal the alkalinity.

Answer: C

Alkalinity is due to excess of \(\text{HCO}_{3}^{-}\) or \(\text{CO}_{3}^{--}\).

Alkalinity is the capacity of water to neutralise acid. In natural waters it is caused mainly by bicarbonate (HCO₃^-) and carbonate (CO₃^2-) ions, and at high pH by hydroxide (OH^-), usually of calcium, magnesium and sodium.

Nitric acid and sulphuric acid are acids, which would reduce alkalinity, not cause it. Calcium carbonate is only sparingly soluble; the alkalinity it contributes is through the carbonate and bicarbonate ions it releases, which is what the correct option names.