A buffer solution is 1.40 M in NH3 and 1.00 M in NH4Cl. If 0.100 moles of NaOH are added to 1.00 L of the buffer, what is its pH?
...........NH4^+ + OH^- ==>NH3 + H2O
initial....1.0......0......1.4.....
add.................0.1.............
change....-0.1.....-0.1.....+0.1
equil.....0.9......0........1.5
Substitute into pH = pKa + log (base/acid) and solve for pH.
To determine the pH of a buffer solution after adding a strong base, you need to consider the reaction between the base and the acidic component of the buffer. In this case, the acidic component is NH4Cl, and the base is NaOH.
The reaction between NH4Cl and NaOH can be represented as follows:
NH4Cl (aq) + NaOH (aq) → NH3 (aq) + H2O (l) + NaCl (aq)
Given that the solution is 1.40 M in NH3 and 1.00 M in NH4Cl, we know the initial concentration of NH4Cl is 1.00 M. When 0.100 moles of NaOH are added, they react with an equal amount of NH4Cl, causing its concentration to decrease.
First, calculate the final concentration of NH4Cl. Since 0.100 moles of NH4Cl react with an equal amount of NaOH, the final concentration of NH4Cl can be calculated as:
Concentration of NH4Cl = initial concentration - moles of NH4Cl reacted
= 1.00 M - 0.100 moles
= 0.900 M
Next, calculate the final concentration of NH3. Since NH3 is formed as a product of the reaction, its concentration increases by the same number of moles that reacted, which is 0.100 moles. Therefore:
Concentration of NH3 = initial concentration + moles of NH3 formed
= 1.40 M + 0.100 moles
= 1.50 M
Now, we can use the Henderson-Hasselbalch equation to determine the pH of the buffer solution. The Henderson-Hasselbalch equation is given as:
pH = pKa + log ([A-]/[HA])
In this case, NH4Cl is the acidic component (HA), and NH3 is the conjugate base (A-). To calculate pKa, we need to know the pKa value for the NH4Cl/NH3 system. The pKa for this system is approximately 9.25.
Using the Henderson-Hasselbalch equation:
pH = 9.25 + log ([NH3]/[NH4Cl])
= 9.25 + log (1.50/0.900)
Calculating this expression will give you the pH of the buffer solution after adding NaOH.