# Homework Help: PH buffer calculations

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1. Feb 26, 2015

### TheShiny

1. The problem statement, all variables and given/known data
Prepare a buffer solution of pH 3.746 from 25 mL of 0.244 M weak acid with pka 3.54

What volume, in mL, of 0.275 M NaOH would need to be added?

I think that I have the answer but I am not sure, could you look over the working to check?
Also, would you do this differently? If so, what would you do?

2. Relevant equations
Henderson-Hasselbach:
$pH = pka + log_{10} \frac {[A^-]}{[HA]} \Rightarrow [H^+] = ka + \frac{[A^-]}{[HA]} \Rightarrow ka = \frac{[H^+][A^-]}{[HA]}$

3. The attempt at a solution
Part I
$pH = 3.746 \Rightarrow [H^+] = 10^{-3.746} = 1.79473\times10^{-4}$

$pka = 3.54 \Rightarrow [H^+] = 10^{-3.54} = 2.88403\times10^{-4}$

$ka = \frac{[H^+][A^-]}{[HA]} \Rightarrow 2.88403\times10^{-4} = 1.79473\times10^{-4}\times\frac{[A^-]}{[HA]}$

$\therefore \frac{[A^-]}{[HA]} = 1.60694$

Pat II
25 mL of 0.244 M acid $\Rightarrow$ 0.0061 M (6.1 mM)

Code (Text):
+---+--------------+--------------+
|   |     Acid     |     Base     |
| I |   0.0061 M   |      0 M     |
| C |     -x M     |     +x M     |
| E | (0.0061-x)M  |      x M     |
+---+--------------+--------------+

$\frac{[A^-]}{[HA]} = \frac{x}{(0.0061 - x)} \therefore \frac{x}{(0.0061 - x)} = 1.60694$

Solve for $x$...

$x = 0.00376009$ (is this the moles of acid reacting?)

$\frac{0.00376009}{0.275} = 0.01367305$

$\therefore$ 13.7 mL NaOH should be added.

2. Feb 26, 2015

### Staff: Mentor

Correct, but completely unnecessary. You could just plug pH and pKa into HH equation, that's what it is for.

Correct, but completely unnecessary.

Wrong, and completely unnecessary. But I assume you meant OK, you just copied wrong.

Yep, that's what is important here. As I said above, you could get here much faster.

Looks OK to me.

itex tags are for inline formulae, when you post them in their own lines, use just tex.

3. Feb 26, 2015

### Quantum Defect

The nice thing about the henderson-Hasselbach equation is that the log part is a ratio of concentrations, but it can just as easily be written as a ratio of moles.

pH = pKa + log [A-]/[HA] = pKa + log ( moles A- / moles HA )

At the half eq. point, moles A- = moles HA, pH = pKa + log (1) ==> pH = pKa

For your buffer, the pH is close to pKa, so you are close to the 1/2 equiv. point. Since pH is a bit larger than pKa, you are a bit past the 1/2 equiv. point, i.e. there is a bit more conj. base than conj. acid. The concentrations of acid and base are pretty close, so the half-eq. point is going to be close to half the original volume of acid -- ca. 12.5 mL.

3.746 = 3.54 + log (moles A-/moles HA)

moles A- / moles HA = 1.607

moles HA_0 = 25 mL * 0.244 M = 6.10 m mol

x/(6.1 mmol-x) = 1.607 ==> x = 3.76 m mol

vol OH- * 0.275 M = 3.76 m mol

vol OH- = 13.7 mL

4. Mar 8, 2015

### TheShiny

If I had the question "Prepare a buffer solution of pH 3.924 from 50 mL of 0.236 M solution of a sodium salt of a weak acid, Na+A- where the pka of the weak acid, HA, is 3.956. What volume, in mL, of 0.208 M HCl would need to be added?"

Would I use the same method as with the initial question? What changes, if any, should I make?

5. Mar 8, 2015

### Staff: Mentor

Very similar question and very similar approach, you just need to keep track of what is the acid and what is the conjugate base here.

6. Mar 8, 2015

### TheShiny

Okay, so I've just had a go and I get:

[A-]/[HA] = 0.92896

Base: 50mL * 0.236M = 11.8mM

(11.8-x)/x = 0.92896 => x = 6.11727

6.11727/0.208 = 24.4mL

7. Mar 8, 2015

### Staff: Mentor

And what is the acid here?

8. Mar 8, 2015

### TheShiny

The acid is 0.208 M HCl. I calculated that I should add 24.4mL.

9. Mar 8, 2015

### Staff: Mentor

Nope. This is the acid you added, but it is not part of the buffer. Every buffer consist of the conjugate pair - what are the acid and its conjugate base here?

10. Mar 8, 2015

### TheShiny

Acid: HA and C. Base: A- ?

11. Mar 8, 2015

### Staff: Mentor

Hard to say what you mean. A- typically means "any conjugate base", so it is not specific enough. And there is no such thing as C.

Buffer has ONE acid, not two, so whatever you meant, if you listed two acids you can't be right.

12. Mar 11, 2015

### Quantum Defect

I think that the OP meant "C. Base: A-" = "Conjugate Base: A-" I.e. "C. Base" was an abbreviation for "conjugate base."

13. Mar 11, 2015

### Staff: Mentor

Ah, OK. It still doesn't help much as HA and A- are hardly specific (and I suspect OP may be confused about the identity of the acid; that's quite common).

14. Mar 11, 2015

### TheShiny

Sorry if I caused confusion, that is the question verbatim.

What I don't fully understand is how the "sodium salt of a weak acid" affects the calculations that I need to do. Because, based on inspection, comparing concentrations as well as pH and pka, I think that the volume to add should be roughly half (25mL) that of the initial solution.

Depending on how I do the sums I arrive at either 24.4mL or 27.3mL and I'm not sure which answer to use.

15. Mar 11, 2015

### Staff: Mentor

OK, so you were right about HA and A-.

Initially solution contains A- only, where does the HA come from?

16. Mar 11, 2015

### TheShiny

That's what I'm not too certain about. Is it the water that the initial solution is diluted with?

17. Mar 11, 2015