at onyorans of ethanol, CatboH0), must be added to 500.0g of water to make a sol

Question

at onyorans of ethanol, CatboH0), must be added to 500.0g of water to make a solution that The molal froezing point depression constant for water is 1.86 "C.kg/mole. Setup m = _n.cefrole-.78 rrale 3.11 70 Lo 0 3) 3.92 g of a nondissociating compound are dissolved in 400. g of liquid benzene. The freezing point of the solution is 5.19 "C. Calculate the molar mass of the compound. (The freeing point of pure benzene is 5.50 and its m olal freezing point depression is $12 c.kgimole). Setup; Answer 10-

Explanation / Answer

Q2

Apply Colligative properties

This is a typical example of colligative properties.

Recall that a solute ( non volatile ) can make a depression/increase in the freezing/boiling point via:

dTf = -Kf*molality * i

dTb = Kb*molality * i

where:

Kf = freezing point constant for the SOLVENT; Kb = boiling point constant for the SOLVENT;

molality = moles of SOLUTE / kg of SOLVENT

i = vant hoff coefficient, typically the total ion/molecular concentration.

At the end:

Tf mix = Tf solvent - dTf

Tb mix = Tb solvent - dTb

T = 0 F --> -17.7778 °C

then

-17.7778 = 1.86 * molality  

molality = 17.7778 /1.86

molality = 9.557

mol of S = 9.557 per kg

kg of wate r= 0.5

mol of S = 9.557*0.5 = 4.7785 mol of ethanol

mass = mol*M;W = 4.7785*46 = 219.811 g of ethanol required

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