JEE Main · 2019 · Shift-IIeasyMOLE-154

What would be the molality of 20\% (mass/mass) aqueous solution of KI? (Molar mass of KI = 166\,g/mol)

Some Basic Concepts (Mole Concept) · Class 11 · JEE Main Previous Year Question

Question

What would be the molality of 20%20\% (mass/mass) aqueous solution of KI? (Molar mass of KI =166 g/mol= 166\,\text{g/mol})

Options
  1. a

    1.351.35

  2. b

    1.511.51

    ✓
  3. c

    1.081.08

  4. d

    1.481.48

Correct Answerb

1.511.51

Detailed Solution

🧠 Take 100 g100\,\text{g} of solution 20%20\% (w/w) means 20 g20\,\text{g} of KI and 80 g80\,\text{g} of water in every 100 g100\,\text{g}.

🗺️ Two steps Moles of KI =20/166≈0.1205 mol= 20 / 166 \approx 0.1205\,\text{mol}. Mass of water =0.080 kg= 0.080\,\text{kg}. m=0.12050.080≈1.51 m.m = \frac{0.1205}{0.080} \approx 1.51\,\text{m}.

⚠️ Common mistake Dividing by 0.1 kg0.1\,\text{kg} (the whole solution). Molality always uses solvent mass.

Answer: (b)\boxed{\text{Answer: (b)}}

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