[orgo814]

What is the pH at 25 celsius of a solution that results from mixing together equal volumes of a 0.05 M solution of ammonia and a 0.025 M solution of hydrochloric acid? The pKa of NH4+ is 9.25.

I know the general formula is pH = pKa + log ([Base]/[Acid]). The book starts the problem by saying mixing equal volumes of solutions dilutes them before and therefore [NH3] = 0.025 M and [HCl] = 0.0125 M in the combined solution. I don't understand how they went from 0.025 M HCl to 0.00125 M HCl. Please be detailed in your answer.

[Borek]

Mixing equal volumes...

Say you mixed 10 mL of 1M HCl and 10 mL of pure water. What is the concentration of HCl now?

http://www.chembuddy.com/?left=concentration&right=dilution-mixing

[orgo814]

I'm not sure. Whenever I've done dilution calculations it's always involved the same substance. I've never done it by "mixing"

[Borek]

There is no difference (unless mixed substances react, but we will deal with it later). You have some amount of substance in some initial volume, you add other solution so that volume changes, you can calculate new concentration knowing amount of substance has not changed, but volume did. All you need to use is a good, old [itex]C=\frac n V[/itex].

Can you do it now?