Molality of NaOH Solution Given Density – JEE Chemistry | Doubtify JEE

  

  🧲 Solutions | Chemistry | Doubtify JEE

---

🧮 Question:

The density of the NaOH solution is 1.2 g/cm³. What is the molality of the solution?

---

🖼️ Question Image:

---

⚗️ Concept Overview:

This is a fundamental problem from the Solutions chapter of Physical Chemistry. It tests your understanding of molality, density, and the method of converting between different concentration units. In many JEE Main and Advanced problems, students get confused between molality (mol/kg) and molarity (mol/L). This problem helps to eliminate that confusion and improve conceptual clarity.

---

🧠 Step-by-Step Explanation:

Let’s assume we are working with 1000 cm³ = 1 L of NaOH solution.

1. Density = 1.2 g/cm³
   So, total mass of 1 L solution =

   $$1.2 \, \text{g/cm³} \times 1000 \, \text{cm³} = 1200 \, \text{g}$$

2. Let’s say the solution is x% NaOH by mass (you may be given the percentage or molarity in the full version of the question – for now, let’s assume it's 10% NaOH by mass as a common JEE pattern).

   So, in 1200 g solution, NaOH = 10% of 1200 = 120 g

3. Molar mass of NaOH = 40 g/mol

   Moles of NaOH =

   $$\frac{120}{40} = 3 \, \text{mol}$$

4. Mass of solvent (water) = 1200 – 120 = 1080 g = 1.08 kg

5. Molality is defined as:

   $$\text{Molality} = \frac{\text{Moles of solute}}{\text{Mass of solvent in kg}} = \frac{3}{1.08} \approx 2.78 \, \text{mol/kg}$$

✅ Final Answer:

Molality = 2.78 mol/kg

This is an approximate value assuming 10% solution by mass. If the actual concentration is given in the original problem, the same steps apply with appropriate numbers.

---

🧠 Why This Question is Important:

This type of question frequently appears in JEE Main Chemistry sections. It emphasizes the relationship between molality and density, which many aspirants find tricky. Most students often memorize formulas without understanding unit conversions. This question forces you to understand the basics, making it a great concept-builder.

Also, problems like this build your numerical calculation accuracy, especially with unit handling (grams, liters, mol/kg), which is crucial for speed in the JEE exam.

---

🧠 Solution (Image):

---

🎥 Video Solution:

---

📩 Have a Doubt?

Mail us at doubtifyqueries@gmail.com or DM us on Instagram @doubtifyjee

Stay consistent. Watch – Pause – Solve – Repeat.
Team Doubtify is with you till the finish line! 💪