Calculate the pH of the solution formed when:
a) 20cm^3 of 0.10moldm-3 HCl is added to 30cm^3 of 0.04moldm-3 NaOH
1. Write a balanced equation for the reaction between HCl and NaOH
2. Calculate the moles of each one
3. Figure out which moles are in excess
4. Convert the excess moles back to concentration (using the NEW volume of 50 cm3)
5. If HCl is in excess - the molar ratio of HCl to H+ is 1:1, so find pH by simply substituting HCl concentration in the pH = -log[H+] equation
6. If NaOH is in excess, use the Kw expression to find H+ and then find pH. OH- conc = NaOH conc (1:1 ratio)
a) 0.10 mol dm^3 NH4Cl, pKa (NH4+)= 9.25
1. Convert pKa back to Ka (=10-pKa)
2. Use Ka = [H+]2/[HA] expression
3. [HA] = 0.10
4. Rearrange to find [H+]2, then square root to find [H+]
5. Use this value to find pH