Which of the following statements is not correct ?

Options

A. 5% aqueous solutions of NaCl and KCl are said to be isomolar
B. 1 M sucrose solution and 1 M glucose solution are isotonic
C. Molecular mass of acetic acid and benzoic acid is higher than normal mass in cryoscopic methods.
D. For the same solution, `(DeltaT_b)/(DeltaT_f)=K_b/K_f`

Explanation

To solve the question, we need to analyze each of the given statements and determine which one is not correct. Let's go through the options step by step.

### Step 1: Analyze the First Option
**Statement:** A 5% aqueous solution of sodium chloride (NaCl) and potassium chloride (KCl) are isomolar.

**Analysis:**
- A 5% solution means 5 grams of solute in 100 mL of solution.
- The molarity (M) is calculated using the formula:
\[ M = \frac{\text{mass of solute (g)}}{\text{molar mass of solute (g/mol)} \times \text{volume of solution (L)}} \]
- For NaCl, the molar mass is approximately 58.5 g/mol.
- For KCl, the molar mass is approximately 74.5 g/mol.
- Since both solutions have the same mass of solute (5 g) and the same volume (100 mL), we can calculate the molarity for both:
- Molarity of NaCl:
\[ M_{NaCl} = \frac{5 \text{ g}}{58.5 \text{ g/mol} \times 0.1 \text{ L}} \approx 0.856 \text{ M} \]
- Molarity of KCl:
\[ M_{KCl} = \frac{5 \text{ g}}{74.5 \text{ g/mol} \times 0.1 \text{ L}} \approx 0.672 \text{ M} \]
- Since the molarities are different, the statement that they are isomolar is incorrect.

### Step 2: Analyze the Second Option
**Statement:** One molar of sucrose and one molar of glucose solutions are isotonic.

**Analysis:**
- Isotonic solutions have the same osmotic pressure, which is dependent on the number of solute particles in solution.
- Since both solutions are 1 M, they contain the same number of moles of solute per liter, thus they are isotonic.

### Step 3: Analyze the Third Option
**Statement:** The molar mass of acetic acid and benzoic acid is higher than normal mass in the cryoscopic method.

**Analysis:**
- Acetic acid and benzoic acid can dimerize in solution, which affects the van 't Hoff factor (i).
- The effective molar mass observed in colligative properties like freezing point depression is higher due to this dimerization.
- Therefore, this statement is correct.

### Step 4: Analyze the Fourth Option
**Statement:** For the same solution, \(\Delta T_B / \Delta T_F = K_B / K_F\).

**Analysis:**
- The relationship between boiling point elevation and freezing point depression is given by:
\[ \Delta T_B = i K_B m \quad \text{and} \quad \Delta T_F = i K_F m \]
- If the same solution is used, the van 't Hoff factor (i) and molality (m) are the same, thus:
\[ \frac{\Delta T_B}{\Delta T_F} = \frac{K_B}{K_F} \]
- This statement is correct.

### Conclusion
After analyzing all the options, the statement that is not correct is:

**Final Answer:** The first option, which states that a 5% aqueous solution of NaCl and KCl are isomolar, is incorrect.

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