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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Superskillzz on April 11, 2011, 07:30:21 PM

Title: Help with buffer problem?
Post by: Superskillzz on April 11, 2011, 07:30:21 PM

I'm having trouble figuring out this buffer solution problem...

A buffer is prepared by combining 250 mL of 0.3 M NaOH and 250 mL of a 0.8 M weak acid, HA. If the pH of the buffer is 8.44, what is the Ka of the acid?

Title: Re: Help with buffer problem?
Post by: rabolisk on April 11, 2011, 07:36:34 PM

What exactly are you having trouble with?

Title: Re: Help with buffer problem?
Post by: Superskillzz on April 11, 2011, 07:47:31 PM

Ok, well I'm guessing I have to use the Henderson-Hasselblach equation because of the information I'm given and I have to solve for pKa from the equation. So the equation is going to be...

8.44 = pKa + log([conjugate base]/[acid])

I'm having trouble figuring out what values should be used for the conjugate base and acid concentrations. I know you use the NaOH and HA somehow, but that's where I'm stuck.

Title: Re: Help with buffer problem?
Post by: rabolisk on April 11, 2011, 07:57:24 PM

What happens when you add NaOH to HA?

Title: Re: Help with buffer problem?
Post by: Superskillzz on April 11, 2011, 08:01:46 PM

Well, an acid plus a base gives a salt and water...so in this case, it would be the salt NaA and water, right?

Title: Re: Help with buffer problem?
Post by: AWK on April 12, 2011, 02:36:13 AM

Now do stoichiometry of this reaction.