What is pH if 0.02M NaOH is added to buffer solution, consisting 0.4M aqueous NH3 & 0.3M NH4Cl(Kb=0.000018)
To determine the pH of a buffer solution after adding a strong base, we need to consider the equilibrium reactions that occur and calculate the resulting concentrations of the species involved.
In this case, we have a buffer system consisting of aqueous NH3 (weak base) and NH4Cl (salt of the weak acid NH4+). The reaction involved is:
NH4+ + OH- ⇌ NH3 + H2O
The dissociation of NH4Cl is negligible compared to the reaction given above. Therefore, we primarily focus on the reaction between NH4+ and OH-.
To start, let's calculate the initial concentrations of NH4+ and NH3 in the buffer solution:
[NH4+]initial = 0.3 M
[NH3]initial = 0.4 M
Next, determine the change in concentration of NH4+ and NH3 after the reaction with NaOH. Since 1 mole of NH4+ reacts with 1 mole of OH-, they will be neutralized in a 1:1 ratio.
Let's assume x moles of NH4+ react with x moles of OH-. Therefore, the change in concentration will be:
[NH4+]change = -x
[NH3]change = x
After the reaction, the concentrations of NH4+ and NH3 will be:
[NH4+]final = ([NH4+]initial + [NH4+]change) = (0.3 - x) M
[NH3]final = ([NH3]initial + [NH3]change) = (0.4 + x) M
Since NH4+ and NH3 form a conjugate acid-base pair, we can apply the Henderson-Hasselbalch equation to determine the pH of the buffer solution:
pH = pKa + log([A-]/[HA])
However, in this case, we have Kb instead of pKa (since we are dealing with a base).
Remember that pKw + pKb = 14 (at 25 degrees Celsius).
Given that Kb = 0.000018, we can calculate pKb:
pKb = -log10(Kb) = -log10(0.000018) ≈ 4.74
Now, we need to determine the ratio of [NH3]final to [NH4+]final in the Henderson-Hasselbalch equation. Since NH4+ and NH3 react in a 1:1 ratio, this ratio is simply:
[NH3]final / [NH4+]final = (0.4 + x) / (0.3 - x)
Finally, we can substitute the values into the Henderson-Hasselbalch equation:
pH = pKb + log([NH3]final / [NH4+]final)
= 4.74 + log((0.4 + x) / (0.3 - x))
Now, if we know the value of x (the extent of reaction), we can substitute it into the equation to find the pH of the buffer solution after adding 0.02 M NaOH. However, without knowing the initial volume of the buffer solution, we cannot directly determine the value of x. To fully solve this problem, we need additional information provided, such as the volume of the buffer solution or the moles of NaOH added.