(a) A solution contains 5.85 g NaCl (Molar mass `=58.5" g mol"^(-1)`) per litre of solution. It has an osmotic pressure of 4.75 atm at `27^(@)C`. Calculate the degree of dissociation of NaCl in this solution.
[Given : R = 0.082 L atm `K^(-1)mol^(-1)`]
(b) State Henry's law. Why is air diluted with helium in the tanks used by scuba divers ?

Explanation

(a) No. of moles of `NaCl=(5.85)/(58.5)=0.1` mol
Apply the equation for osmotic pressure
`pi=(n_(2)RT)/(V)`
Here, `n_(2)=0.1mol,R=0.082" L atm K"^(-1)mol^(-1)`
Substituting the values in the above equation, we get
`pi=0.1xx0.082" atm"=2.46" atm"`
Thus calculated osmotic pressure = 2.46 atm
Observed osmotic pressure = 4.75 atm
vant Hoff factor, `i=("Observed osmotic pressure")/("Calculated osmotic pressure")`
or `" "i=(4.75)/(2.46)`
`NaCl to Na^(+)+Cl^(-)`
If x is the degree of dissociation, then, total number of moles will be
`1-x+x+x=1+x`
or `" "i=(1+x)/(1)`
From (1) and (2), we have
`1+x=(4.75)/(2.46)` or x = 0.93%
Degree of dissociation = 0.93 or Percentage dissociation = 93%
(b) Henry.s Law : At a constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of the liquid or solution.
Scuba divers have to cope with the high solubility of atmospheric gases at high pressure under4 water. When the diver comes out, the dissolved gases come out, bubbles of nitrogen are formed in the blood which are painful.
To overcome this difficulty, air is diluted with helium and the mixture contains smaller percentage of nitrogen.