The equation to calculate the pH of a buffer solution is:
pH = pKa - log [HA]/[A-]
The most important thing here is to recognize what our buffer system is. This is going to be an NH4+/NH3 buffer, where the acid component is NH4+ (HA), and the base component or the "anion" is NH3. Only NH4+ can be HA, i.e. an acid (we're looking for a weak acid), because NH4+ can lose a proton and become NH3. The pKa of a conjugate acid formed from a base (in this case NH4+ formed from NH3 can be calculated from the ...
Full solution using the equation to calculate the pH of a buffer solution is pH = pKa - log [HA]/[A-]. 400 words.