# Chemical Forums

## Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: jokulsa on October 05, 2014, 11:00:02 AM

Post by: jokulsa on October 05, 2014, 11:00:02 AM
1. How many moles of HF (pKa = 3.14) must be added to 500mL of 0.30M sodium fluoride (NaF) to give a buffer of pH 3.5? Ignore volume changes due to addition of hydrofluoric acid.

2. What is the molarity of CH3COO- in a buffer with a final pH of 5.20 if the concentration of CH3COOH is 0.25M? (pKa of CH3COOH is 4.7)

thanks
Post by: Hunter2 on October 05, 2014, 11:15:33 AM
Do you know Henderson Hasselbalch equation
Post by: jokulsa on October 05, 2014, 11:19:56 AM
yeah I know it.
I think I managed to work out the second one, I don't really know where to start on the first..
Post by: Hunter2 on October 05, 2014, 11:22:17 AM
The first one is similar
develop HH-equation for it
Post by: jokulsa on October 05, 2014, 11:25:46 AM
Is the 500mL given in the first question just there to confuse you?

If so, would the solution be

3.50 = 3.14 + log(0.30/[acid])

and then finding [acid] mathematically?
Post by: Hunter2 on October 05, 2014, 11:27:48 AM
Yes, but the value is wrong what does 0,3 M means
Post by: jokulsa on October 05, 2014, 11:34:47 AM
Isn't this the [base]?
Post by: Hunter2 on October 05, 2014, 12:01:07 PM
M = molarity it has the unit mol/l, so how much mole you have in 500 ml
Post by: jokulsa on October 05, 2014, 12:11:26 PM
I believe you would have 0.15 moles in 500mL

Why do you need to know this for this question?
Post by: Arkcon on October 05, 2014, 12:14:06 PM
Write out the Henderson-Haselbach equation for us.  Then write it again, with the correct values in the correct places.  We'll look and see where you might be getting it wrong.  That's how this forum works: http://www.chemicalforums.com/index.php?topic=65859.0
Post by: Hunter2 on October 05, 2014, 12:23:35 PM
I believe you would have 0.15 moles in 500mL

Why do you need to know this for this question?

You have HH equation you have pH, pka and the amount of the moles of salt, now develop the equation please.
Post by: jokulsa on October 05, 2014, 10:36:21 PM
the HH equation is

pH = pKa + log([base]/[acid])

using the values I have,

3.50 = 3.14 + log([base]/[acid])

I'm sorry but I still don't understand where and how the moles of NaF come into it.
Post by: Hunter2 on October 06, 2014, 12:41:45 AM
What is the base and what is the acid here.
Post by: jokulsa on October 06, 2014, 02:25:10 AM
HF is the acid and NaF is the base
Post by: Hunter2 on October 06, 2014, 03:40:25 AM
Yes correct. So now add the values into the HH equation.
Post by: jokulsa on October 06, 2014, 04:00:54 AM
ok so is it

pH = pKa + log([base]/[acid])

3.50 = 3.14 + log([0.15]/[acid])

...and then finding [acid]?
Post by: Hunter2 on October 06, 2014, 04:14:07 AM
Yes correct. Solve the equation to Acid.
Post by: jokulsa on October 06, 2014, 04:50:34 AM
I solved it and got 0.065M HF.

I used mol=M×vol to find the moles of acid
= 0.0325

Is this correct?

However, I still don't understand why I had to calculate the moles of the base, when the [base] of 0.30M was given in the question.
Would you be able to explain this please?
thanks