Which inorganic precipitate acts as a semipermeable membrane,or what is the chemical composition of a semipermeable membrane?

Calculate the osmotic pressure of a $20\%$ (w/v) anhydrous $CaCl_2$ solution at $0^\circ C$. Assume $100\%$ ionization. (in $\text{atm}$)

Which of the following pairs of solutions is isotonic?

The osmotic pressure $(\pi)$ of a solution is given by the formula:

The osmotic pressure of a living cell is $12 \ atm$ at $300 \ K$. The strength of sodium chloride solution that is isotonic with the living cell at this temperature is $...... \ g \ L^{-1}$. (Nearest integer)
Given : $R = 0.08 \ L \ atm \ K^{-1} \ mol^{-1}$
Assume complete dissociation of $NaCl$
(Given : Molar mass of $Na$ and $Cl$ are $23$ and $35.5 \ g \ mol^{-1}$ respectively.)

$A$ $5.25\%$ solution of a substance is isotonic with a $1.5\%$ solution of urea (molar mass $= 60\ g\ mol^{-1}$) in the same solvent. If the densities of both the solutions are assumed to be equal to $1.0\ g\ cm^{-3},$ the molar mass of the substance will be .............. $g\ mol^{-1}$.