Topscore CChem Question #61???????????

[GoBlue24]

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So this is a osmotic pressure question (attached as a pic). The equation is osmotic pressure = iMRT. The topscore explanation says that M (which is MOLARITY) is 4 mols / 1 Liter Water.

Can anyone clarify why molarity (M) is not 4 mols/ (1L water + .234L NaCl) since you have to take into account the .234kg of NaCl contributing to the overall solution, right? I got the right answer by doing 4mols/1 L H2O = 4 M and because no other answer works but now that I'm looking at it.... the logic clearly does not make sense since Molarity is Moles of Solute/ Total Volume of Solution, and the .234 grams of NaCl should contribute ALOT to the total volume of solution. Can anyone help me out? I don't want to fumble this type of question on the actual exam.

 

[ktran17]

according to chad, when calculating osmotic pressure, the M for molarity is the difference between the concentrated side and the pure side. However, since we aren't given anymore information, we have to assume the other side of the permeable membrane is pure liquid, therefore the M is just the concentration of the solution.

I really don't think .234 g of NaCl should contribute much to the overall volume of the solution. It is a solid not a gas @ stp.. I think that's the big concept you're supposed to get out of this problem.

Just to give you an idea, .234 g of NaCl is like a small pinch of table salt lol.. Try to think back to labs to reason out your answers

 

[mario0923]

So this is a osmotic pressure question (attached as a pic). The equation is osmotic pressure = iMRT. The topscore explanation says that M (which is MOLARITY) is 4 mols / 1 Liter Water.

Can anyone clarify why molarity (M) is not 4 mols/ (1L water + .234L NaCl) since you have to take into account the .234kg of NaCl contributing to the overall solution, right? I got the right answer by doing 4mols/1 L H2O = 4 M and because no other answer works but now that I'm looking at it.... the logic clearly does not make sense since Molarity is Moles of Solute/ Total Volume of Solution, and the .234 grams of NaCl should contribute ALOT to the total volume of solution. Can anyone help me out? I don't want to fumble this type of question on the actual exam.

I am not 100% sure but adding 0.234L of NaCl is wrong since 234g is not 0.234L. 1kg of water is 1L of water because the density of water is so but NaCl might have a different density. if you want to add the volume of NaCl you should know the density of NaCl, and usually if the question wants you to put the NaCl volume they give you the density.