Video summaries: Buffers solution pH calculation

Hello!

Deriving the Henderson-Hasselbach equation we’ve got that pH=pKa+log (-A)/(HA); here we have a conjugate acid-base pair, HA the acid and A the base.

Our problem says “What is the pH of a buffer solution that is 0.24 M NH3 and 0.20 M NH4Cl?” here HA would be NH4Cl and A NH3 (ammonia). First, we need to find the pKa. Knowing that the Ka for NH4Cl=5.6×10 to the -10 we put this into the next equation that it’s pKa=-log(ka); now the maths are like this pKa= -log (5.6×10′-10)= 9.25. Secondly, having the value of pKa= 9.25 we plug that into our Henderson-Hasselbach equation, so pH=9.25+log(0.24M)/(0.20M)= 9.33, being 9.33 the pH of our buffer solution.