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Solutions — Chemistry STD 12 Science — Question

Gujarat 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 containing $10.50\ g$ of $MgBr_2$ in $200\ g$ of water. (Molar mass of $MgBr_2 = 184 g$)
$(K_f$ 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. $\Delta\text{T}_{f}=7.5^{o}C$
$\Delta\text{T}_{f}=\text{iK}_{f}\text{m}$
$\text{T}_{f}^{0}-\text{T}_{f}=3\times1.86^0\text{C kg mol}^{-1}\times\frac{10.50g}{\text{184 gmol}^{-1}}\times\frac{\text{1000}}{\text{200 kg}}$
$0^oC-T_f = 1.59^oC$
$T_f = –1.59^oC$ or $271.41\ K.$

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