A diagram of a U tube for measuring osmotic pressure is shown below. The height

Question

A diagram of a U tube for measuring osmotic pressure is shown below. The height different between the two columns of liquid is 10.00 Cm. The dashed line represent a semi-permeable membrane through which solvent molecules can pass, but solute molecules can not. i) On which side (right or left) is the solute located? ii) Calculate the osmotic pressure in atmospheres. Hint this is actually easy: The pressure difference is given by rho gH. Where rho is the density of the solution. You can assume it is 1.000 grams per cm^3, g is the acceleration due to gravity, 9.8 M/S^2 and H is the difference in height. Be Careful of Units iii) Calculate the concentration of the solute.

Explanation / Answer

Ans. Osmosis is the movement of water (solvent) from its (water’s) higher to lower concentration across plasma membrane.

A solution with higher solute concertation has lower water (solvent) concertation.   

Or, a solution with higher water concertation has lower solute concertation.

Thus, during osmosis, water flows from a solution of lower solute concertation (has higher solvent concertation) to a solution of higher solute concertation (has lower solvent concertation).

Result of experiment: Increase in volume at left side by 10.0 cm indicates that water has flower from right to left (or, lower to higher solute concertation).

Thus, solute (higher concentration of solute) is located on left- to which water moves into.

#2. Density of water, (r) = 1.000 g/ cm3 = 1000 kg/ m3             ; [in SI unit]

            g = 9.8 m/ s2             ; [SI unit]

            h = 10 cm = 0.10 cm ; [SI unit]

Now, Pressure, p = (r)gh

                        = (1000 kg/ m3) (9.8 m s-2) (0.1 m)

                        = 98000 kg m-1 s-2

                        = 98000 Pa                                                    ; [1 Pa = 1 kg m-1 s-2]

                        = 0.96 atm                                                     ; [1 Pa = 9.869 x 10-6 atm]

Thus, osmotic pressure, P = 0.96 atm

#3. Osmotic pressure, P = i x MRT

                        i = van’t hoff factor = 1 for non-ionizing solutes

                        M = molarity of solution

                        R = gas constant = 0.0821 atm L mol-1K-1

                        T = temperature in kelvin

            [The temperature is assumed to be 250C = 298 K. The solute is assumed to be non-ionizing]

Comparing #2 and #3,

            P = 0.96 atm = i MRT

            Or, 0.96 atm = 1 x M x (0.0821 atm L mol-1K-1) x 298 k             

            Or, 0.96 atm = M x 24.4658 atm L mol-1  

            Or, M = 0.96 atm / (24.4658 atm L mol-1) = 0.03923 mol/ L = 0.039 M

Thus, concertation of solute in left-side solution = 0.039 M

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