Calculating pH of solutions?

Buffer Solutions and Calculating pH?

• So i need to calculate the pH of a solution thats 50mL of 0.4M propanoic acid and 50mL of 0.10M KOH. (pKa of propanoic acid is 4.87) I have gotten as far as KOH + CH3CH2COOH = CH3CH2COO-K+ + H2O where do I go from here to calculate pH?

• Answer:

  Hello, You do have the equation right - good start! You have to find the number of moles of KOH and propanoic acid so you can find out after they react, how much is left of each. After calculations, you find out that you have 0.0050 mol of KOH ready to react with 0.020 mol of propanoic acid. Since KOH is therefore the limiting reactant, there is none left, and after the reaction, you have 0.015 mol propanoic acid and 0.0050 mol propanoate. To find molarity, you place both values over 0.1 L. You get 0.15 M propanoic acid and 0.050 M propanoate. Using the Henderson-Hasselbach equation (below), you can find the pH. pH = pKa + log[A-/HA] pH = 4.87 + log[0.05/0.15] pH = 4.4 <<