Kaplan Chemistry problem I don't get

[toothfairy05]

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Hey Everyone, I don't really understand the solution to the following problem. Anyone out there can explain it?

So here goes:
What is the molarity of K2SO4 in an aqueous solution created by adding 112g of solid KOH to 500 mL of 0.5M H2SO4 (aq).

The Answer is: 0.50 M

Thank you!

 

[gigawatt]

This is along the lines of a limiting reagent problem...it's pretty straightforward actually.

In the acid/base reaction K^+ and SO4^2- are spectator ions. 500 mL of a 0.5 M solution has 0.25 mol of SO4^2-, so if we get at least 0.5 mol of K^+ added then we can say we have 0.25 mol of K2SO4. We actually get 2 mol of K^+, so the sulfate is what is limiting.

Hence, we have 0.25 mol potassium sulfate in 500 mL = 0.5 M. (Actually, the volume would go up because of the added KOH and the molarity would be a little less than this, but just go with it.)

 

[wantVCUdental]

Hey Everyone, I don't really understand the solution to the following problem. Anyone out there can explain it?

So here goes:
What is the molarity of K2SO4 in an aqueous solution created by adding 112g of solid KOH to 500 mL of 0.5M H2SO4 (aq).

The Answer is: 0.50 M

Thank you!

This is similar to a limiting reagent question. KOH is a strong base, H2SO4 is a strong acid, both of them have 100% dissociation.

K2SO4= 2K + 1SO4

SO4 can only come from the 0.5M soln, that means you only have 0.5M SO4 and that is the max amt of K2SO4 you can make.

Let's take a look at the KOH, molar mass of KOH is 56g/mol, 112g in 0.5L=4M

Putting all your eggs in the basket, you have:

K2SO4=2K + 1SO4 ====> K2SO4= 2M of 2K + 0.5M of SO4
Therefore you can only make 0.5M of K2SO4 soln, hope that helps.

 

[wantVCUdental]

This is along the lines of a limiting reagent problem...it's pretty straightforward actually.

In the acid/base reaction K^+ and SO4^2- are spectator ions. 500 mL of a 0.5 M solution has 0.25 mol of SO4^2-, so if we get at least 0.5 mol of K^+ added then we can say we have 0.25 mol of K2SO4. We actually get 2 mol of K^+, so the sulfate is what is limiting.

Hence, we have 0.25 mol potassium sulfate in 500 mL = 0.5 M. (Actually, the volume would go up because of the added KOH and the molarity would be a little less than this, but just go with it.)

Being a chemist, you would know to use a volumetric flask and that the 500mL is a final volume 😉.

Okay fine, i am just being a dick because you answered before me. j/k

 

[gigawatt]

Being a chemist, you would know to use a volumetric flask and that the 500mL is a final volume 😉.

Okay fine, i am just being a dick because you answered before me. j/k

Haha, no worries. But actually if you use a volumetric flask and 500 mL is your final volume, then you would have less than 500 mL of 0.5 M sulfuric acid to begin with...hence less than 0.25 mol sulfate...and the molarity is still less than 0.5 M.

Bam! 👍 I was actually going to reply jinx! because our answers were so similar.

 

[toothfairy05]

Thanks so much everyone!😀

It makes sense to me!!! I'm so worried about Chemistry-- It's my weakest subject. I haven't been able to break an 18 on the Kaplan full-lengths and my exam is in less than 3 weeks 🙁

 

[gigawatt]

Thanks so much everyone!😀

It makes sense to me!!! I'm so worried about Chemistry-- It's my weakest subject. I haven't been able to break an 18 on the Kaplan full-lengths and my exam is in less than 3 weeks 🙁

Gen Chem calculations are kind of like QR calculations in the sense that there's only so many different types of problems that get recycled with different numbers. Go through the back-of-the-chapter problems in a textbook for the chapters you're weak on. Some people get a concept-heavy GC and others get a calculation-heavy GC, so be prepared for both.

 

[toothfairy05]

Gen Chem calculations are kind of like QR calculations in the sense that there's only so many different types of problems that get recycled with different numbers. Go through the back-of-the-chapter problems in a textbook for the chapters you're weak on. Some people get a concept-heavy GC and others get a calculation-heavy GC, so be prepared for both.

Thanks for your advice! I went through the destroyer chem problems once. I think I'm going to re-do them. Hopefully, things will start to sink in then. I actually don't have a Chemistry textbook...I sold it after I finished my Gen Chem class. I've been using the Schaum's College Chemistry Outline when I'm confused about something, or I just google it.

 

[gigawatt]

Thanks for your advice! I went through the destroyer chem problems once. I think I'm going to re-do them. Hopefully, things will start to sink in then. I actually don't have a Chemistry textbook...I sold it after I finished my Gen Chem class. I've been using the Schaum's College Chemistry Outline when I'm confused about something, or I just google it.

That works! Wikipedia is a life-saver, too. Good luck and throw a question or two our way if you need to. 👍