A 1.50 L buffer solution is 0.250 mol L-1 in HF and 0.250 mol L-1 in NaF. Calculate the pH of the solution after the addition of 0.0500 moles of solid NaOH. Assume no volume change upon the addition of the base. The Ka for HF is 3.5 × 10-4.
Thanks in advance!
millimols HF = mL x M = approx 375
mmols NaF = 1500 x 0.25 = 375
mmols NaOH added = 50
........HF + OH^- ==> H2O + F^-
I......375...0........0.....375
add..........50................
C.....-50...-50.......50.....50
E......325...0...............425
Substitute the E line into the HH equation and solve for pH.
By the way the pH of the initial solution is just pH = pKa since the acid and base are equal.
To calculate the pH of the buffer solution after the addition of NaOH, you need to consider the reaction between HF and NaOH. NaOH is a strong base, and HF is a weak acid. The reaction between the two can be represented as follows:
HF + NaOH → NaF + H2O
Here, NaF is a salt that will be formed as a result of the reaction. It is a strong electrolyte and will dissociate completely in water.
To calculate the pH of the solution after the addition of NaOH, you need to consider the Henderson-Hasselbalch equation, which is given by:
pH = pKa + log ([A-]/[HA])
In this equation, pKa is the negative logarithm of the acid dissociation constant, [A-] is the concentration of the conjugate base (in this case, NaF), and [HA] is the concentration of the weak acid (in this case, HF).
Given that the solution is a buffer, the concentration of NaF and HF will remain constant upon addition of NaOH. Therefore, we can use the initial concentrations of the buffer components to calculate the pH.
1. Calculate the initial concentrations of HF and NaF:
HF: 0.250 mol L-1
NaF: 0.250 mol L-1
2. Calculate the concentrations of HF and NaF after the reaction with NaOH:
Since the reaction between HF and NaOH is 1:1, 0.0500 moles of HF will react with 0.0500 moles of NaOH. This means that the concentration of NaF will increase by 0.0500 mol L-1, and the concentration of HF will decrease by the same amount.
Final concentration of HF = 0.250 mol L-1 - 0.0500 mol L-1 = 0.200 mol L-1
Final concentration of NaF = 0.250 mol L-1 + 0.0500 mol L-1 = 0.300 mol L-1
3. Calculate the pH using the Henderson-Hasselbalch equation:
First, calculate pKa using the Ka value for HF.
pKa = -log(Ka) = -log(3.5 × 10-4) = 3.46
Next, substitute the values into the Henderson-Hasselbalch equation.
pH = 3.46 + log(0.300 mol L-1 / 0.200 mol L-1)
Simplify the equation:
pH = 3.46 + log(1.5)
Finally, calculate the value of the logarithm and add it to pKa to find the pH.
pH = 3.46 + 0.1761
pH ≈ 3.6361
Therefore, the pH of the solution after the addition of NaOH is approximately 3.6361.