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JEE Advanced levelSingle correct optionDifficulty 3 of 3pb090

Hard water contains 100 mg of $Ca X^{2 +}$ per litre (Ca = 40, Na = 23). It is softened by passing it through sodium zeolite: $Na X_{2} Z + Ca X^{2 +} CaZ + 2 Na X^{+}$ . The mass of $Na X^{+}$ released into 1 L of the water is

A172 mg
B57.5 mg
C230 mg
D115 mg

Answer: D

n (Ca X^{2 +}) = 100/40 = 2.5

mmol. Each

Ca X^{2 +}

displaces 2

Na X^{+}

, so 5.0 mmol of

Na X^{+}

is released. Mass

= 5.0 \times 23 = 115

mg. The zeolite can be regenerated with brine.

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