Question

Two 1-L beakers, A and B, each containing a different aqueous solution of urea (a nonvolatile nonelectrolyte with MW = 60.06 g/mol) are placed together in a box, which is then sealed. The temperature remains constant at 20 °C. Initially, there is 270 mL of solution in A and 690 mL of solution in B. As the solutions stand in the sealed box, their volumes change slowly for a while. When they stop changing, beaker A contains 490 mL and beaker B contains 470mL. It is then determined that the solution in A is 3.88 M in urea and has a density of 1.06 g mL^-1.

(a) What is the molar concentration of urea in the solution in beaker B at the end?  M

(b) Calculate the concentration of urea in the solution in A at the start.  M

(c) Calculate the concentration of the urea in the solution in B at the start.  M

(d) The vapor pressure of pure water at 20 °C is 17.5 torr.
What is the pressure of water vapor in the box at the end, after the volumes have stopped changing?  torr

Answer 1

(a) What is the molar concentration of urea in the solution in beaker B at the end? : 3.88 M

(b) Calculate the concentration of urea in the solution in A at the start. 7.04 M

(c) Calculate the concentration of urea in the solution in B at the start. 2.64 M

(d) pressure of water vapor in the box at the end = 16.1 torr

Explanation

(a) The volumes in beaker A and beaker B will stop changing when both beakers have same concentration of urea.

Hence, concentration of urea in beaker B = concentration of urea in beaker A

concentration of urea in beaker B = 3.88 M

(b) moles of urea in beaker A = (concentration of urea in beaker A at the end) * (volume of beaker A at the end)

moles of urea in beaker A = (3.88 M) * (490 mL)

moles of urea in beaker A = 1901.2 mmol

Concentration of urea in beaker A at the start = (moles of urea in beaker A) / (volume of beaker A at the start)

Concentration of urea in beaker A at the start = (1901.2 mmol) / (270 mL)

Concentration of urea in beaker A at the start = 7.04 M