[dvmaz]

To check the electrode of a pH meter used to measure the pH of wine, 10 mL of pH 6.88 buffer is diluted with 90 mL of distilled water. If the pH meter is correct the new reading should be only about 0.10 pH units higher. Verify that this is correct by calculating the actual pH of the diluted solution?

Does anyone understand this question?
I am failing to understand it

[argulor]

What do you try to find when calculating pH?

[dvmaz]

What do you try to find when calculating pH?

pH is a way of expressing the amount of hydrogen ions and is given by: pH=-log[H⁺]

Therefore you try and find the concentration of H⁺ ions.

My question is how do you find the concentration of H⁺ in this situation?

[argulor]

So, if pH is the -log[H⁺] then [H⁺]=10^-pH then get it into moles and find the new concentration.

[BluePill]

Hmm, I find this a little problematic. I think it's not a direct dilution equation because it is a buffer.

Henderson-Hasselbach equation, I guess? pH = pKa + log [(conjugate base)/(acid)]

Therefore, pH is still the same. I dunno. It's just a guess. Experts takeover. haha.

[Borek]

See discussion here:

http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch

Question can be not solved. The way I see it, whoever asked it, blindly assumed that dilution doesn't change pH of the buffer. This is only an approximation, one that depends on the buffer concentration (infinitely diluted buffer has obviously pH 7.0, not matter what you started with). But the buffer type nor concentration are not given, so there is no way of giving the right answer. And the answer expected - that pH doesn't change - is in general incorrect, as shown by the table on the page linked to.

[BluePill]

But the question is just 10x dilution. In the table in the link, 10x dilution doesn't affect the pH buffer. As I've remembered from our analytical chem class, you will have to consider the autodissociation of water if the concentration is 10^-4. So I think we can assume that it won't change. But then again, I'm not that sure.

[Borek]

But the question is just 10x dilution. In the table in the link, 10x dilution doesn't affect the pH buffer.

Take a look at the table again - it depends on the initial concentration. If you start with 10^-4M, pH change after tenfold dilution is 0.5 unit. If you don't know initial concentration you can't say "pH won't change", even if in most cases it doesn't.

[BluePill]

Yep. I got your point. The question posted is difficult to answer because we don't know the composition and the initial concentrations. (BTW, if you look at the table at 10^-7 M, it's close to the pH of the question, which proves Borek was right). I rest my case.