Final answer:
To calculate the pH of an acid-base reaction mixture, find the stoichiometric amount of the acid and base, use the Henderson-Hasselbalch equation with the calculated concentrations of the reactants and products, and the appropriate pKa or pKb values.
Step-by-step explanation:
To calculate the pH of a solution involving reactions between acids and bases, it is essential to understand the stoichiometric relationship between the reactants. In this particular question, we are asked to calculate the pH of a solution prepared by adding 45.0 mL of a 0.213 M HCl solution to 125.0 mL of a 0.150 M solution of ammonia. The relevant reaction is:
HCl + NH3 → NH4+ + Cl−
The total amount of ammonia in moles can be calculated as 0.150 M × 0.125 L, and the total amount of HCl in moles is 0.213 M × 0.045 L. Because HCl and NH3 will react in a 1:1 molar ratio, we can subtract the moles of HCl from the moles of NH3 to find the moles of NH4+ in solution. The remainder will be the unreacted ammonia.
The resulting concentrations of NH4+ and NH3 can be used in the Henderson-Hasselbalch equation:
pH = pKa + log([A−]/[HA])
where pKa is the negative logarithm of the acid dissociation constant (Ka) of the conjugate acid of ammonia, NH4+, which is related to the pKb of ammonia given by pKa = 14 - pKb. By substituting the calculated concentrations into the equation, we can determine the pH of the solution.