[Compaq]

I've calculated the pH in this buffer: 0,15 moles NH4Cl and 0,25 moles NH3 in water. Total volume = 200mL

then there's b)

Calculate the pH in 100mL of the buffer after adding 10mL 1M HCl

I could do this task easily if it weren't for the 100mL thing. What am I supposed to do? Divide V with two? Very confusing.

I hope you understand what I mean here, I'm Norwegian so... Ohh, and the buffer here is a strong acid and salt in water.ftp://

[Borek]

It is all about concentrations of NH₃ and NH₄⁺. To calculate concentration you need number of moles and volume, everything else is a simple stoichiometry.

[Compaq]

I understand that, but I'm supposed to find the pH in only half of the total volume. Should I divide the moles by 2? I've tried many things, but never gotten the correct answer, which is 9,39... I keep getting 9,42.. argh.

These kinds of tasks are important considering possible exams.

[Borek]

So where is the problem - you are told you have a buffer with given concentrations. You are told to take 100 mL of the solution of given concentrations. Initial volume doesn't matter, concentration is the intensive property.

Show what you did (both parts) and list pKa value that you have used.

[Compaq]

I figuered it out. All I had to do was to divide the initial moles by 2, and then follow the basic instructions. It's obvious now, I should have seen it.

One thing I find kind of fun is to just keep adding strong acid until the buffer explodes (or whatever you call it in English).