VIRUS Calculus Tracking: UPS Take Test: 50 Points How to Homework 5c 1. Explain

Question

VIRUS Calculus Tracking: UPS Take Test: 50 Points How to Homework 5c 1. Explain in terms of thermodynamic stability (see figure) why pure substance exists as a solid at low temperature but changes to liquid and then to vapor phase at high temperature. (2) Briefly explain why impurity lowers the freezing point but increases boiling point of a solvent. Pure liquid 2. Calculate the freezing point of a glass of water of volume 250 cm3 sweetened with 7.5 g of sucrose (M=342·3gmol.l) Vapour No Shared Links The cryoscopic constant Click "Subscriptions" to select the social network, app, and website feeds you want to appear here. Freezing point Boiling point 3. The osmotic pressure of an aqueous solution at 288K is 99.0 kPa. Calculate the freezing point of the solution. The cryoscopic constant K1.86Kkgmol Answers: -0.1 6°C -0.077

Explanation / Answer

Osmotic pressure is a colligative property, and it's calculated using the formula:

Pos = C*R*T

(i)

Putting values into the above formula we have:

250000 = C*8.314*300

Solving we get:

C = 100.233 moles/m3

(ii)

Assuming that the density of the aqueous solution is the same as that of pure water, we have:

Density, d = 1000 kg/m3

So, mass of 1m3 or solution = 1000*1 = 1000 kg

Molality, b = moles of solute/mass of solvent in kgs

So,

b = 100.233/1000 = 0.1 moles/kg

(iii)

Using the equation for depression of freezing point, we have:

dT = -Kf*m

Putting values we get:

(T-0) = -1.86*0.1 = -0.186

So,

Freezing point of solution, T = -0.1860C = -0.180C

Hope this helps !

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