Question

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The enthalpy of solution of KBr in water is about +198 kJ/mol (endothermic). Nevertheless, the solubility of KBr is relatively high. Explain why. A sample consisting of various gases contains 3.5*10^-6 mole fraction of radon. This gas at a total pressure of 36 atm is shaken with water at 30 degree C. Assume that the solubility of radon in water with 1 atm pressure of the gas over the solution at 30 degree C is 7.27*10^-3M. Calculate the molar concentration of radon in the water. Caffeine (C_8H_10N_4O_2) is a stimulant found in coffee and tea. A solution of caffeine in the solvent chloroform (CHCI_3) has a concentration of 0.0520m. a) Calculate the percentage of caffeine by mass in the solution. b) Calculate the mole fraction of caffeine in the solution.

Answer 1

Problem-2: Given that when the atmospheric pressure is 1 atm, the concentration of radon in water is 7.27x10^-3 M

Now applying Henry's law

P = K_H x C

=>  K_H = P / C = 1 atm / 7.27x10^-3 M = 137.6 atm / M

Now when P = 36 atm, the molar concentration of radon in water can be calculated as

P = K_H x C

=> C = P /K_H = 36 atm / 137.6 atm / M = 0.262 M (answer)

Problem-3:(a): Given molality of the solution, m = 0.0520 m

i.e 0.0520 mol of caffeine is present in 1000 g of solvent chloroform.

molar mass of caffeine = 194.19 g/mol

Hence mass of caffeine present in 1000 g of solvent chloroform = 0.0520 mol x 194.19 g/mol = 10.098 g

Hence percentage of caffaine by mass in the solution = [10.098 g / 1000 g] x 100 = 1.01 % (answer)

(b): molar mass of chloroform = 119.3776 g/mol

mass of chloroform = 1000 g

Hence moles of solvent chloroform = mass / molar mass = 1000 g / 119.3776 g/mol = 8.377 mol

mole fraction caffeine in the solution = moles of caffeine / (moles of caffeine + moles of chloroform)

= 0.0520 mol / (0.0520 mol + 8.377 mol)

= 0.00617 (answer)