OFFICIAL 2007 DAT Question Discussion Thread!!!!

[jdoc04]

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Ok there are a lot of single question threads starting up so I want to organize things a bit. All DAT questions post in this thread.

Good Luck to all and Lets ROCK THE DAT!!!!!!!!

 

[jdoc04]

General Chemistry Question to get things started....

The pH of a weak monoprotic solution is 4.5. What is the molarity of the acid solution with a Ka of 4x10^-6???

 

[IsRaEl21]

since the PH of a monoprotic acid is 4.5 than the H+ concentration will be around 5.5x10^-5. At Equilb. the Concentration of an acid H+ and A- equal to each other therefore. HA + H2O= A- + H3o+ Produ/React= Ka

(H30+)(A-)/ HA= Ka
So we have Ka= 4x10^-6 = (5.5x10^-5)(5.5x10^-5)/ HA. Than we have to solve for HA which comes out to be 7.56x10-4 M. I hope I helped you. My advice is to understand the concept and don't look for the mathematical errors. If you know the concept of H30+ = A- than you will be able to solve this probleam really fast.

 

[Shunwei]

since the PH of a monoprotic acid is 4.5 than the H+ concentration will be around 5.5x10^-5. At Equilb. the Concentration of an acid H+ and A- equal to each other therefore. HA + H2O= A- + H3o+ Produ/React= Ka

(H30+)(A-)/ HA= Ka
So we have Ka= 4x10^-6 = (5.5x10^-5)(5.5x10^-5)/ HA. Than we have to solve for HA which comes out to be 7.56x10-4 M. I hope I helped you. My advice is to understand the concept and don't look for the mathematical errors. If you know the concept of H30+ = A- than you will be able to solve this probleam really fast.

It should be mentioned in this case that one is making a crucial assumption that the [H+] dissociation will be very small compared to the concentration of the acid. Usually, that would be the case. Here, it is kind of on the borderline, with only one order of magnitude difference.

 

[IsRaEl21]

You should really pay attention when they mention a weak acid or strong one. Strong will dissociate almost 100%. Since we know that it's a weak monoprotic acid we should assume that H+ is equal to A-.

 

[jdoc04]

since the PH of a monoprotic acid is 4.5 than the H+ concentration will be around 5.5x10^-5. At Equilb. the Concentration of an acid H+ and A- equal to each other therefore. HA + H2O= A- + H3o+ Produ/React= Ka

(H30+)(A-)/ HA= Ka
So we have Ka= 4x10^-6 = (5.5x10^-5)(5.5x10^-5)/ HA. Than we have to solve for HA which comes out to be 7.56x10-4 M. I hope I helped you. My advice is to understand the concept and don't look for the mathematical errors. If you know the concept of H30+ = A- than you will be able to solve this probleam really fast.

Thanks!!! I over looked that they are equal to one another. pH= -log [H] then [H]=1x10^-pH [H] [X]/HX =Ka = (1x10^-4.5)(1x10^-4.5)/HX=4x10^-6 so HX=[(1x10^-4.5)^2]/4x10^-6 👍👍

 

[IsRaEl21]

You can look at that in to more details in achiever or the destroyer. I think they are very helpfull materials which will explain all of this aspects into more details. If you have the destroyer you should read the explanations which will help you understand the material much more in depth.