1000 L of water contains $Ca (HCO X_{3}) X_{2}$ at 0.0100 mol/L and no other hardness. By Clark's method it is softened with the least amount of pure quicklime, which is first slaked. The mass of $CaO$ needed is (Ca 40, O 16)

Answer: A

CaO + H X_{2} O Ca (OH) X_{2}

and

Ca (HCO X_{3}) X_{2} + Ca (OH) X_{2} 2 CaCO X_{3} + 2 H X_{2} O

are both 1 : 1. Moles of bicarbonate are

1000 \times 0.0100 = 10

, so 10 mol

CaO

is needed, which is

10 \times 56 = 560

g. Using 2 mol per mole would wrongly treat each

HCO X_{3} X^{-}

separately.