General Chemistry question

[BrownieDDD]

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I found this in my kaplan course for the dat:

If 11g of CaCl2 is added to 100mL of water, what's the molarity?

The answer is: cannot be determined
Kaplan's reason: 100mL is of the solvent, not the solution. Had the question been if enough water is added to 11g CaCl2 to make 100mL of solution, then molarity could be found and the answer would have been 1.0M.

My problem with the question: So 11g added to 100mL solvent will surely make the volume greater than 100mL. I need the total V of sol'n to solve the problem. But there is a conversion 1000g=1L. Can't I use this to find the V of the solute and add that to the volume of solvent to get the V of sol'n? Then I can find molarity?

 

[Felixor90]

Yes you're right. 1g =1mL. So it would be moles of CaCl2 over 111mL of solution. Now, if they didn't give you the molar weight of CaCl2 or gave you access to a periodic table (on the rest) then you can't solve it. This is assuming they don't expect you to memorize the molar weight of the entire thing.... :|

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[sacapuntas]

Yes you're right. 1g =1mL. So it would be moles of CaCl2 over 111mL of solution. Now, if they didn't give you the molar weight of CaCl2 or gave you access to a periodic table (on the rest) then you can't solve it. This is assuming they don't expect you to memorize the molar weight of the entire thing.... :|

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1g = 1 ml is only true for water. That doesn't apply to CaCl2 as far as I know. The real volume probably isn't much more than 100 mL, probably closer to 100 than 111 mL would be my guess. But yeah, like the answer key said, you can't solve this problem without more info.

 

[BrownieDDD]

thanks. yeah you're right, it only applies to water so additional info is needed.

 

[Dotoday]

However, assuming that they do give you a periodic table with mass numbers, you can easily solve this problem and the volume will be 100 ml or 0.1 L, not 111 ml. I think the problem they presented is not with the volume, you just cannot determine Molarity here without knowing the Molar mass of CaCl2.

 

[sacapuntas]

However, assuming that they do give you a periodic table with mass numbers, you can easily solve this problem and the volume will be 100 ml or 0.1 L, not 111 ml. I think the problem they presented is not with the volume, you just cannot determine Molarity here without knowing the Molar mass of CaCl2.

I don't think you are right.

Molarity is (moles solvent / Volume Solution). You don't get the volume of solution, so you can't solve Molarity. You cannot assume it to be 100 ml, as that is the volume of solvent.

You will have a periodic table with mass numbers.