[justukyte]

Ok, I searched the earlier topics on this question, found some answers, but still couldn't understand it fully.

Here's the problem:
25 ml of 0,2 M HCl solution is mixed with 25 ml of 0,1 M NaOH solution. Calculate the pH of the new solution.

I've found this formula (Henderson-Hasselbach equation), but I don't get it.
pH = pKa + log (base/acid)

Can you help me solve this problem and also explain the formula? What does pKa mean? Why do we need to divide the base by the acid?

[Borek]

http://www.chembuddy.com/?left=pH-calculation-questions&right=pH-of-mixture-q1

[Sadi Carnot]

Find the concentration of H⁺ and OH^-, based on the fact that molarity is defined as mol/L.  Then assume that if there is both hydroxide and hydronium in the solution, they will react to form water.  So subtract the number of moles of each that cancel one another out (in this case H⁺ will outweigh OH^-).  You will be left with a net number of moles of either hydronium or hydroxide (or sometimes they completely cancel each other out).  Divide this number of moles by the new volume of the solution and you have the final concentration in mol/L.  From there it is smooth sailing because pH is defined as -log[H⁺].