Maharashtra BoardEnglish MediumSTD 12 ScienceChemistrySolutions2 Marks
Question
Isotonic solutions
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Answer
The solutions having the same osmotic pressure at a given temperature are called isotonic solutions.
Explanation : If two solutions of substances $A$ and $B$ contain $n_A$ and $n_B$ moles dissolved in volume $V$ (in $dm ^3$ ) of the solutions, then their concentrations are, $C _{ A }=\frac{n_{ A }}{V}$ (in mol $dm ^{-3}$ ) and $C _B=\frac{n_{ B }}{V}\left(\right.$ in $\left.mol dm ^{-3}\right)$ If the absolute temperature of both the solutions is $T$, then by the van't Hoff equation, $\pi_A=C_A R \operatorname{Tand} \pi_B=C_B R T$ where $\pi_A$ and $\pi_B$ are their osmotic pressures. For the isotonic solutions, $\begin{aligned} & \pi_A=\pi_B \\ & \therefore C_A=C_B \\ & \therefore \frac{n_A}{V}=\frac{n_B}{V} \\ & \therefore n _A= n _{ B } \end{aligned}$ Hence, equal volumes of the isotonic solutions at the same temperature will contain equal number of moles (hence, equal number of molecules) of the substances.
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