Question

Which of the following solutions has the highest osmotic pressure?

(a) 100 g of a polymer with MW=1,000 g/mol in 1 L of water. (b) 30 g of glucose (MW=180 g/mol) in 1 L of water. (c) 6 g of NaCl (MW=58 g/mol) in 1 L of water. (d) 3 g of Ethanol (MW=46 g/mol) in 1 L of water.

Answer 1

Osmotic pressure is the pressure which needs to be applied on the pure solvent to just stop the flow of solvent particles coming from the impure solution.

Osmotic pressure P for a dilue solution of concentration C can be given as P = iCRT where R is the universal gas constant R = 0.0821 L atm mole-1 K-1 and T is the temperature in Kelvin, i is the vant hoff factor of the solute particles

Here we are comparing all the solutions at the same temperature so, P can be compared by looking at the i*C for each solution

a) we have moles of polymer = mass of polymer/molecular weight of polymer = 100g / 1000g/mole = 0.1 mole and its in 1L solution so, molarity = no. of moles/volume in L = 0.1moles/1L = 0.1 M

Polymer does not dissciate into ions so, i = 1

so, we have i*C = 1*0.1 = 0.1

b)

we have moles of glucose = mass of glucose/molecular weight of glucose= 30g / 180g/mole = 0.1667 moles and its in 1L solution so, molarity = no. of moles/volume in L = 0.1667moles/1L = 0.1667 M

glucose does not dissciate into ions so, i = 1

so, we have i*C = 1*0.1667 = 0.1667

c)

we have moles of NaCl= mass of NaCl/molecular weight of NaCl= 6g / 58g/mole = 0.103 moles and its in 1L solution so, molarity = no. of moles/volume in L = 0.103 moles/1L = 0.103 M

NaCldoes dissciate into ions as NaCl -> Na+ + Cl- so, i = 2 (2 ions produced by 1 molecule of NaCl)

so, we have i*C = 2*0.103 = 0.206

d)

we have moles of ethanol = mass of ethanol/molecular weight of ethanol= 3g / 46g/mole = 0.065 moles and its in 1L solution so, molarity = no. of moles/volume in L = 0.065 moles/1L = 0.065 M

glucose does not dissciate into ions in water so, i = 1

so, we have i*C = 2*0.065 = 0.13

The i*C value is highest for NaCl and thus it has the highest osmotic pressure.