Which of the following salt solutions would coagulate the colloid solution formed when $\mathrm{FeCl}_{3}$ is added to NaOH solution, at the fastest rate?

(A) 10 mL of $0.2 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{AlCl}_{3}$

(B) 10 mL of $0.1 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{Na}_{2} \mathrm{SO}_{4}$

(C) 10 mL of $0.1 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2}$

(D) 10 mL of $0.15 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{CaCl}_{2}$

JEE Main · 2023 · 29 Jan 2023 · Shift 1 · Surface Chemistry

Which of the following salt solutions would coagulate the colloid solution formed when

FeCl_{3}

is added to NaOH solution, at the fastest rate?

(A)10 mL of $0.2 mol dm^{- 3} AlCl_{3}$ ✓ Correct
(B)10 mL of $0.1 mol dm^{- 3} Na_{2} SO_{4}$
(C)10 mL of $0.1 mol dm^{- 3} Ca_{3} (PO_{4})_{2}$
(D)10 mL of $0.15 mol dm^{- 3} CaCl_{2}$

Answer: (A)

Solution

Adding

FeC l_{3}

to an excess of

NaOH

gives a sol of

Fe (OH)_{3}

that has adsorbed hydroxide ions and so carries a negative charge. A negative sol is coagulated by the positive ions of the added electrolyte, and by the Hardy--Schulze rule the coagulating power rises steeply with the charge on that cation, so the order of effectiveness is

A l^{3 +} > C a^{2 +} > N a^{+}

. The electrolyte supplying the triply charged cation,

0.2 mol d m^{- 3} AlC l_{3}

, coagulates the sol fastest.

JEE Main · 2023 · 29 Jan 2023 · Shift 1 · Single correct option

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JEE Main · 2023 · 29 Jan 2023 · Shift 1 · Surface Chemistry

Solution

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