Acrylic resins are polymeric materials used to make warm yet lightweight garment
ID: 884122 • Letter: A
Question
Acrylic resins are polymeric materials used to make warm
yet lightweight garments. The osmotic pressure of a solution
prepared by dissolving 47.7 g of an acrylic resin in enough water
to make 500. mL of solution is 0.325 atm at 25°C.
(a) What is the
average molar mass of the polymer?
(b) How many monomers
constitute the “average” molecule? The repeating unit of this
acrylic resin has the formula -CH2
CH(CN)-. (c) What would be
the vapor pressure of the solution if the vapor pressure of pure
water at 25°C is 0.0313 atm? (Assume that the density of the
solution is 1.00 gcm-3
(d) Which approach (osmometry or the
lowering of vapor pressure) would you prefer for the determination
of very high molar masses such as those of acrylic resins? Why?
Explanation / Answer
(a) What is the average molar mass of the polymer?
Mwt = wRT/PV = ((47.7*0.0821*298)/(0.325*0.5))
average molar mass of the polymer = 7181.653 g/mol
(b) How many monomers constitute the “average” molecule? The repeating unit of this
acrylic resin has the formula -CH2
Mass of repeating unit = 53
No of monomers = 7181.653 / 53 = 135.5 units
c) (P0-P/p0) = xB
no of moles of polymer = 47.7/7181.653 = 0.0066 mole
No of moles of water = 500/18 = 27.7 mole
Total no of moles = 27.7+0.0066 = 27.7066
molefraction of solute xB = 0.0066/27.7066 = 0.000238
((0.0313-p)/0.0313) = 0.000238
Vapour pressure of solution p = 0.03129 atm
d) osmometry wouldbe preferable method for determination of high molar masses.
because lowering of vapor pressure is verry less .so that its not suitable.