1. Condition for isotonic solutions

Two solutions are isotonic at the same temperature if they have the same osmotic pressure.

$\pi = iCRT$

At the same temperature, isotonic condition is:

$i_1 C_1 = i_2 C_2$

where:

• $i$ = van’t Hoff factor
• $C$ = molarity

For strong electrolytes, assume complete dissociation.

2. Find van’t Hoff factor for each solute

• $\mathrm{NaCl} \to \mathrm{Na}^+ + \mathrm{Cl}^-$ $i = 2$

• Urea is non-electrolyte $i = 1$

• $\mathrm{CaCl_2} \to \mathrm{Ca}^{2+} + 2\mathrm{Cl}^-$ $i = 3$

• $\mathrm{KCl} \to \mathrm{K}^+ + \mathrm{Cl}^-$ $i = 2$

• $\mathrm{AlCl_3} \to \mathrm{Al}^{3+} + 3\mathrm{Cl}^-$ $i = 4$

• $\mathrm{Na_2SO_4} \to 2\mathrm{Na}^+ + \mathrm{SO_4}^{2-}$ $i = 3$

• $\mathrm{Al_2(SO_4)_3} \to 2\mathrm{Al}^{3+} + 3\mathrm{SO_4}^{2-}$ $i = 5$

3. Check each pair

Pair A: $1\,\mathrm{M}$ $\mathrm{NaCl}$ and $2\,\mathrm{M}$ urea

For $1\,\mathrm{M}$ $\mathrm{NaCl}$:

$iC = 2 \times 1 = 2$

For $2\,\mathrm{M}$ urea:

$iC = 1 \times 2 = 2$

So, pair A is isotonic.

Pair B: $1\,\mathrm{M}$ $\mathrm{CaCl_2}$ and $1.5\,\mathrm{M}$ $\mathrm{KCl}$

For $1\,\mathrm{M}$ $\mathrm{CaCl_2}$:

$iC = 3 \times 1 = 3$

For $1.5\,\mathrm{M}$ $\mathrm{KCl}$:

$iC = 2 \times 1.5 = 3$

So, pair B is isotonic.

Pair C: $1.5\,\mathrm{M}$ $\mathrm{AlCl_3}$ and $2\,\mathrm{M}$ $\mathrm{Na_2SO_4}$

For $1.5\,\mathrm{M}$ $\mathrm{AlCl_3}$:

$iC = 4 \times 1.5 = 6$

For $2\,\mathrm{M}$ $\mathrm{Na_2SO_4}$:

$iC = 3 \times 2 = 6$

So, pair C is isotonic.

Pair D: $2.5\,\mathrm{M}$ $\mathrm{KCl}$ and $1\,\mathrm{M}$ $\mathrm{Al_2(SO_4)_3}$

For $2.5\,\mathrm{M}$ $\mathrm{KCl}$:

$iC = 2 \times 2.5 = 5$

For $1\,\mathrm{M}$ $\mathrm{Al_2(SO_4)_3}$:

$iC = 5 \times 1 = 5$

So, pair D is isotonic.

4. Count the isotonic pairs

All four pairs satisfy the isotonic condition.

$\boxed{4}$

5. Comparison with stored correct answer

Stored correct answer = $4$

My derived answer = $4$

Hence, they agree.