pH question help!!!

[4563728]

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The pka of a weak acid is 5.4. If the ratio of the concentration of the weak acid to the conjugate base is 10:1, what is the pH of the solution?

I tried using the BR equation for determining the pH of a weak acid: 1/2(pKa)-1/2*log[HA). But this equation did not get me the right answer. The solution said the way to solve it was using henderson hasselbach but why doesn't BR's equation work in this case?

Thanks!!

PS. the answer is 4.4

 

[brood910]

You are not even supposed to use that equation.

pH = pka + log(A-/HA)

pH = 5.4 +log0.1 = 5.4 - 1 = 4.4

 

[Cmdr_Shepard]

Yeah, it's a buffer so you have to use the David Hasselhoff equation.

 

[4563728]

Then when do we use the BR equation?

 

[brood910]

When you are using TBR, you shouldnt just memorize equations. You must understand them and know when to use them..

You use pH = 1/2pka - 1/2log[HA] only when [HA] is greater than Ka and when pKa is between 2 and 12.

In your question, there's a base present. You cant just ignore base and use this equation to find pH.