What is the pH of 1.00 L of the 0.100 M hydrofluoric acid/0.120 M fluoride ion buffer system from the previous problem after 0.020 mol of NaOH is added?
To find the pH of the buffer system after adding NaOH, we need to consider the reaction that occurs between the hydrofluoric acid (HF) and the sodium hydroxide (NaOH). The NaOH will react with the HF to form sodium fluoride (NaF) and water (H2O).
The balanced chemical equation for the reaction is as follows:
HF + NaOH → NaF + H2O
Since the buffer system consists of hydrofluoric acid (HF) and fluoride ion (F-), the addition of NaOH will result in the formation of more fluoride ion (F-), which will enhance the buffer capacity of the system.
To determine the pH after 0.020 mol of NaOH is added, we need to calculate the final concentrations of HF and F-. Let's go step by step:
1. Calculate the moles of HF initially present in the 1.00 L of the buffer system:
Moles of HF = Molarity of HF × Volume of solution
= 0.100 mol/L × 1.00 L
= 0.100 mol
2. Calculate the moles of F- initially present in the 1.00 L of the buffer system:
Moles of F- = Molarity of F- × Volume of solution
= 0.120 mol/L × 1.00 L
= 0.120 mol
3. Determine the limiting reactant:
Since NaOH is added, it will react with HF in a 1:1 molar ratio. The amount of HF remaining after the reaction will depend on the availability of NaOH. Therefore, the limiting reactant is either HF or NaOH, whichever is present in lesser moles.
In this case, we have 0.020 mol of NaOH added, which is less than the initial moles of HF (0.100 mol). Therefore, NaOH is the limiting reactant, and only 0.020 mol of HF will react.
4. Determine the remaining moles of HF:
Remaining moles of HF = Initial moles of HF - Moles of NaOH reacted
= 0.100 mol - 0.020 mol
= 0.080 mol
5. Calculate the final concentration of HF:
Final concentration of HF = Remaining moles of HF / Volume of solution
= 0.080 mol / 1.00 L
= 0.080 M
6. Calculate the final concentration of F-:
Since NaOH reacts with HF in a 1:1 molar ratio, for every 0.020 mol of NaOH reacted, the same amount of fluoride ion (F-) will be formed. Therefore, the final concentration of F- will be equal to the initial concentration of F- plus the concentration of NaOH added.
Final concentration of F- = Initial concentration of F- + Concentration of NaOH
= 0.120 M + (0.020 mol / 1.00 L)
= 0.120 M + 0.020 M
= 0.140 M
7. Calculate the pH of the buffer system using the Henderson-Hasselbalch equation:
pH = pKa + log ([A-] / [HA])
The pKa for the HF/F- buffer system needs to be provided in the previous problem or given in any available information.
If you have the pKa value, substitute it into the equation along with the final concentrations of F- and HF. Calculate the logarithm and add the pKa value to determine the pH of the buffer system after adding NaOH.