One litre of hard water contains $2.0 \times 1 0^{- 3}$ mol of $Ca X^{2 +}$ . Sodium palmitate, $C X_{15} H X_{31} COONa$ (molar mass 278 g/mol), is added until every calcium ion is precipitated as calcium palmitate. The mass of sodium palmitate used up per litre is

Answer: B

Each

Ca X^{2 +}

ion needs two palmitate ions:

2 C X_{15} H X_{31} COO X^{-} + Ca X^{2 +} (C X_{15} H X_{31} COO) X_{2} Ca

. Moles of soap =

2 \times 2.0 \times 1 0^{- 3} = 4.0 \times 1 0^{- 3}

mol. Mass =

4.0 \times 1 0^{- 3} \times 278 = 1.112 g