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Solutions — Chemistry STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 ScienceChemistrySolutions5 Marks
Question
  1. Differentiate between molarity and molality for a solution. How does a change in temperature influence their values?
  2. Calculate the freezing point of an aqueous solution containing10.50g of MgBr2 in 200 g of water. (Molar mass of MgBr2 = 184 g)

(Kf for water = 1.86 K kg mol–1)

✓

Answer

  1. Molality (m) is the number of moles of the solute per kilogram (kg) of the solvent whereas Molarity is the number of moles of solute present in one litre (or one cubic decimeter) of solution at a particular temperature. 

Molality is independent of temperature whereas Molarity is function. of temperature because volume depends on temperature and the mass does not or Molarity decreases with increase of temperature.

  1. $\Delta\text{T}_{f}=7.5^{o}C$

$\Delta\text{T}_{f}=\text{iK}_{f}\text{m}$

$\text{T}_{f}^{0}-\text{T}_{f}=3\times1.86^0\text{C kg mol}^{-1}\times\frac{10.50g}{\text{184 gmol}^{-1}}\times\frac{\text{1000}}{\text{200 kg}}$

0oC-Tf = 1.59oC
Tf = –1.59oC or 271.41 K.

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