Well water has dissolved particles, ions and molecules together, at a total concentration of 0.25…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Well water has dissolved particles, ions and molecules together, at a total concentration of at . A reverse osmosis unit is to make fresh water from it. Find the least pressure that the pump must supply, in kPa. Then find the height of a column of fresh water that has this pressure at its base, in metres. Does a pump that supplies work?
Need a reminder? The key ideaShow
The osmotic pressure of a dilute solution is , with in mol/L and in kelvin. gives atm, and gives kPa. Isotonic solutions have equal . A column of height of a liquid of density gives a pressure .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. Reverse osmosis needs a pressure greater than the osmotic pressure of the solution.
Step 1. The osmotic pressure. .
Step 2. The column. .
Step 3. The pump. A pump at gives a pressure well above , so fresh water flows out through the membrane.
Answer: the pressure must be above , which is the pressure at the foot of a column of water. The pump works.
Watch out. The pump must press on the solution. If it pressed on the fresh-water side, osmosis would go on as usual.
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