1. Questions

Chem exam

I'm tyring to study for my chem exam and I cant figure out how to do this question:
The solubility of MgCl2 is 54.25 g per 100ml of water at 25 degrees C. A student prepares a 2.50 mol/L solution of MgCl2 and states it is a saturated solution. Useing calculations determine if the student is correct.

Any help would be apperciated


Hint: Convert 2.50 mols/L to grams/L and compare with the stated solubility.
If you need help, show your work and we can help you through it.


Ok well I did that but I still have no idea how you tell wheter its saturated or not. I know that the solubility is 54.25g per 100 ml but I don't know what htats supposed to tell me once I find the rest. I did what you said and got 238.025 g/L....but still no idea.


So the student has prepared what he/she says is a 2.5 M solution of MgCl2. To do that means the student must have dissolved (or tried to dissolve) 238 g MgCl2 in a liter of solution. According to the problem, the solubility is 54.25 g/100 mL. (I will assume that is 54.25 g/100 mL solution and not 54.25 g in 100 mL water. It may make a difference if the comparison is close.) So 54.25 g/100 mL solution is equivalent to
54.25 g x 1,000 mL/100 mL = 542.5 MgCl2/L. Since the student dissolved only 238 to make a 2.5 M solution, do you think the solution is saturated? Or could the solution hold a little more? If it can hold more it isn't saturated, yet.


I don't get this part: I will assume that is 54.25 g/100 mL solution and not 54.25 g in 100 mL water. It may make a difference if the comparison is close.
The question says : 54.25 g in 100ml of water. I also don't understand the 54.25 g x 1,000 mL/100 mL = 542.5 MgCl2/L...where did that come from? isn't 1 liter 1000ml...so to chnage it it would be 54.25g/100ml x 1ml/1000L which would make it like 0.0005425g/L? so then it owuldnt be saturated?


You have two questions here. Let's deal with one at a time. For the time being, let's not worry about the 100 mL water vs 100 mL solution and concentrate on the other part.
If I dissolve 54.25 g/100 mL solution that is the same as 5.425 g/10 mL solution, 0.5425 g/1 mL solution, or 542.5 g/L of solution. Yes, I L is 1000 mL All I have done is to convert the solutility in g/100 mL solution to a liter. Since the solubility is listed as 100 mL solution (as least that is what we are assuming) then we must convert it to 1L so we can compare it to the 2.5 mols/L which is a L. We could have used the 238 g/L solution and divided that by 10 to obtain 23.8 g/100 mL solution and compared that to the original 54.25 g/100 mL solution. One problem you are having is that you are not converting the solubility properly. Let's clear that up before we go further.


Ok sorry my brians inside out today. I knew 1000ml is 1 liter but I was multpiplyinh them backwards. ok so it would be 542.5g/L...and the other one is 238g/l so this means its not saturated...right?


very good. Now do you want to go through the 100 mL water vs 100 mL solution? It probably isn't close enough to make a difference.


sure it might be needed


Molarlity is defined as mols/L of solution as opposed to mols in 1000 mL solvent (the latter is a molal solution and not a molar solution). Therefore, if the solubility is listed as 54.25 g/100 mL that USUALLY means 54.25 g in 100 mL solvent (usually water) which isn't the same as 54.25 g in 100 mL solution. When we prepare a 2.50 M solution of MgCl2, for example, we place 23.8 g MgCl2 in a 100 mL volumetric flask, dissolve the solid in a little water, then make the solution to the mark and we have exactly 100.0 mL of SOLUTION. Technically, then, we can't compare g/L solution with g/L solvent without knowing the density. Using the density, we can convert molal to molar and make a comparison in which we compare apples with apples and oranges with oranges. In this case, however, a 2.50 M solution contains 238 g/L solution and it can hold 542.5 g in a liter of water. The difference is so large that we can make that comparison with some confidence that the solution is not saturated. Think about this. When we place MgCl2 in the example above into the 100 mL volumetric flask, dissolve in a little water, then make to the mark giving us exactly 100.0 mL SOLUTION, would that be the same as filling the volumetric flask with 100 mL water (by filling to the mark), then dumping in 23.8 g MgCl2, and shaking until it was dissolved. Do you think the water level still would be on the mark? Of course it would not. We would have a larger volume than 100 mL now. Essentially that is the difference between a molar and a molal solution. I hope this helps but if you still have a point or two on you need clarification, just let me know. Good luck on your finals.