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Some Basic Concepts of Chemistry question

2025 · 22 Jan · Shift 2 · Q10
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Some Basic Concepts of Chemistry question

2025 · 22 Jan · Shift 2 · Q10

JEE MainChemistrySome Basic Concepts of ChemistryMCQ+4 / −1
Density of 3 M NaCl solution is 1.25 g/mL1.25 \mathrm{~g} / \mathrm{mL}1.25 g/mL. The molality of the solution is :
  1. A
    2.79 m
  2. B
    2 m
  3. C
    1.79 m
  4. D
    3 m
View written solutionFree

Correct answer: A

  1. Given data
  • Molarity of NaCl solution =3 M= 3\,M=3M
  • Density of solution =1.25 g mL−1= 1.25\,\text{g mL}^{-1}=1.25g mL−1
  • We need to find molality.
  1. Take 1 litre of solution

Since molarity is 3 M3\,M3M, in 1 L1\,\text{L}1L of solution:

moles of NaCl=3\text{moles of NaCl} = 3moles of NaCl=3

  1. Mass of 1 litre of solution

Density =1.25 g mL−1= 1.25\,\text{g mL}^{-1}=1.25g mL−1

1 L=1000 mL1\,\text{L} = 1000\,\text{mL}1L=1000mL

So mass of 1000 mL1000\,\text{mL}1000mL solution is:

mass of solution=1.25×1000=1250 g\text{mass of solution} = 1.25 \times 1000 = 1250\,\text{g}mass of solution=1.25×1000=1250g

  1. Mass of solute (NaCl)

Molar mass of NaCl:

58.5 g mol−158.5\,\text{g mol}^{-1}58.5g mol−1

Mass of 333 moles NaCl:

3×58.5=175.5 g3 \times 58.5 = 175.5\,\text{g}3×58.5=175.5g

  1. Mass of solvent

mass of solvent=mass of solution−mass of solute\text{mass of solvent} = \text{mass of solution} - \text{mass of solute}mass of solvent=mass of solution−mass of solute

=1250−175.5=1074.5 g= 1250 - 175.5 = 1074.5\,\text{g}=1250−175.5=1074.5g

=1.0745 kg= 1.0745\,\text{kg}=1.0745kg

  1. Calculate molality

Molality is moles of solute per kg of solvent:

m=31.0745m = \frac{3}{1.0745}m=1.07453​

m≈2.79 mm \approx 2.79\,mm≈2.79m

  1. Match with options
  • A: 2.79 m2.79\,m2.79m ✅
  • B: 2 m2\,m2m ❌
  • C: 1.79 m1.79\,m1.79m ❌
  • D: 3 m3\,m3m ❌

Therefore, the correct answer is Option A.

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