How is osmotic pressure of a solution determined ? If the membrane used was slightly leaky, how will it influence the measured value of osmotic pressure?
Osmotic pressure of a solution containing 7 g of a protein per 103 ml of solution is 25 mm Hg at 310 K. Calculate the molecular mass or the protein. (R = 0.0821 L atm K–1 mol).

(a) Measurement of Osmotic Pressure. Different methods are employed for the measurement of osmotic pressure in the laboratory but Berkley and Hartley's method gives the best results. The apparatus consists of a porous pot containing copper ferrocyanide deposited in its wall (acts as semi-permeable membrane) and fitted into a bronze cylinder to which is fitted a piston and a pressure gauge (to read the applied pressure).
The pot is fitted with a capillary indicator on left and water reservoir on right. Pot is filled with water while the cylinder is filled with a solution whose osmotic pressure is to be measured. Water tends to pass into the solution through the semipermeable membrane with the result that the water level in the indicator falls down. External pressure is now applied with piston so as to maintain a constant level in the indicator. This external pressure is osmotic pressure.
If the membrane used was a slightly, leaky, then the measured valued of osmotic pressure will not be definite.

(a) Measurement of Osmotic Pressure. Different methods are employed f

Fig. Berkley and Hartley's apparatus.
(b) straight M subscript straight B space equals space fraction numerator straight m subscript straight B RT over denominator Vπ end fraction straight m
space space space space space space space equals space fraction numerator 7 cross times 0.0821 cross times 310 over denominator begin display style 100 over 1000 end style cross times begin display style 25 over 760 end style end fraction cross times 54094 space straight g space mol to the power of negative 1 end exponent.

164 Views

A solution containing 8 g of a substance in 100 g of diethyl ether boils at 36.860C, whereas pure ether boils at 35.600C. Determine the molecular mass of the solute. (For ether Kb= 2.02 K kg mol–1) .

Answer;

We have given that

Tb = (36.86-35.60) = 1.26 KMB = ?,    WB = 8g,  WA = 100 gkb = 2.202 K kg mol-1


kb = Tb×MBWAWb×1000


Therefore
         2.02 = 1.26×MB×1008×1000

or         MB = 2.02×8×10001.26×100=128.25.
199 Views

Calculate the molarity of 1 litre solution of 93% H2SO4 (weight/volume). The density of solution is 1.84 gm L–1.

(i) Volume of solution = 1 L = 1000 mL
Density of solution = 1.84 g mL–1
Mass of solution = V x d
= (1000 mL) x 1.84 mL–1
= 18540 g
Mass of H2SO4 = 93g/100mL
 93 x 10 = 930 g
Mass of solvent (water)
= 1840 – 930 = 910 g.
(ii) Molarity (m)
   
   = Mass of H2SO4/Molar mass of H2SO4Mass of water in kg
  Mass of H2SO4                         = 930 g, Molar mass of H2SO4                        = 98 g mol-1Mass of water = 910 g1000 = 0.91 kg


Molarity (m) = 930g/98 mol-1(0.91 kg)                      = 10.43 mol kg-1 = 10.43 m.

2681 Views

Define the term “osmotic pressure”. Describe how the molecular mass of a substance can be determined on the basis of osmotic pressure measurement.

Osmotic pressure: Osmotic pressure is the minimum pressure that should be applied to the more concentrated solution to prevent osmosis.
Determination of osmotic pressure: Barkley and Hartley’s method: The apparatus consists of a porous pot containing copper ferrocyanide deposited in its wall (acts as semipermeable membrane) and fitted into a bronze cylinder to which is fitted a piston and a pressure gauge (to read the applied pressure).
The pot is fitted with a capillary indicator on left and water reservoir on right. Pot is filled with water while the cylinder is filled with a solution whose osmotic pressure is to be measured. Water tends to pass into the solution through the semipermeable membrane with the result that the water level in the indicator falls down. External pressure is now applied with piston so as to maintain a constant level in the indicator. This external pressure is osmotic pressure.
If the membrane used was a slightly, leaky, then the measured valued of osmotic pressure will not be definite.

Osmotic pressure: Osmotic pressure is the minimum pressure that shoul

Fig. Barkley and Hartley’s apparatus.

888 Views

Advertisement

Heptane and Octane form an ideal solution at 373 K. The vapour pressures of the pure liquids at this temperature are 105.2 K Pa and 46.8 K Pa respectively. If the solution contains 25 g of heptane and 28.5 g of octane, calculate
(i) Vapour pressure exerted by heptane.
(ii) Vapour pressure exerted by solution.
(iii) Mole fraction of octane in the vapour phase.


Components A (Heptane, C7H16)
poA = 105.2 kpaWA = 25 gMA = (12×7) + (16×1) = 100
Components of B (octane, C8H18)
           poB = 46.8 kpaWA = 28.5 gMA = (12×8) + (18×1) = 114
No. of moles of heptane,
              nA = 25100=0.25
No. of moles of octane,
                nB =WBMB=28.5114=0.25
Total moles in solution,
               nA+nB = 0.25+0.25  = 0.50
(i) Vapour pressure exerted by heptane
                 pA = poA×A
where XA is mole fraction of component A
                   pA = 105.2×nAnA+nB = 105.2×0.250.50     = 52.6 k pa
(ii) Vapour pressure of octane
              pB = poB×BpB = 46.8 × nBnA+nB = 46.8×0.250.50       = 23.4 kpa
   Total vapour pressure of solution
                  p =pA+pB = 52.6+23.4 = 76 kpa
(iii) Mole fraction of octane
                              = nBnA+nB=0.250.50=0.5.

370 Views

Advertisement
Advertisement