Calculate the pH of .100 of a buffer solution that is .25 M in HF and .50 M in NaF. What is the change in pH on addition of the following?
A. .002 mol of HNO3
B. >004 mol of KOH
I calculated the correct pH of the solution but am having trouble calculating A & B.
Thanks,
Ivy
To calculate the change in pH on addition of HNO3 and KOH, we need to consider the reaction that occurs between the added substance and the buffer components. Let's start with part A:
A. Adding HNO3:
In the presence of a buffer solution, the reaction between the added HNO3 and the HF component of the buffer is as follows:
HNO3 + HF ⇌ H2O + F- + NO3-
Note that HF is a weak acid and will not fully dissociate in water.
To calculate the change in pH, we need to determine the change in concentrations of the buffer components caused by adding 0.002 mol of HNO3. The change in HF concentration will be -0.002 mol (since it is consumed in the reaction).
1. Calculate the initial pH of the buffer:
To determine the initial pH of the buffer solution (.250 M HF and .500 M NaF), we need to use the Henderson-Hasselbalch equation:
pH = pKa + log (Base/Acid)
The pKa is the negative logarithm of the acid dissociation constant (Ka). For HF, the pKa is approximately 3.17, which you can find in a chemistry reference book or the internet.
2. Calculate the concentration of [Base]/[Acid] before addition:
Before adding HNO3, the ratio of [NaF]/[HF] can be calculated as:
[NaF]/[HF] = 0.500 M / 0.250 M = 2
Using this ratio and the Henderson-Hasselbalch equation, you can calculate the initial pH of the buffer solution.
3. Calculate the new concentration of [Base]/[Acid] after addition:
Since 0.002 mol of HF reacts with 0.002 mol of HNO3, the new concentration of HF will be 0.250 M - 0.002 M = 0.248 M. The concentration of F- will remain the same due to the excess NaF in the buffer.
4. Determine the new pH of the buffer solution after addition:
Using the Henderson-Hasselbalch equation with the new [F-]/[HF] ratio and the pKa value of HF, you can calculate the new pH of the buffer solution.
Repeat this process for part B, which involves adding KOH to the buffer solution. The reaction with KOH will involve the NaF component of the buffer and will result in the formation of NaOH. Remember to consider the change in concentrations and use the Henderson-Hasselbalch equation to calculate the new pH of the buffer solution.
By following these steps, you should be able to determine the change in pH of the buffer solution after adding HNO3 and KOH.
What's that 0.100? 0.100 L?
millimols HF = mL x M = 100 x 0.25 = 25.0
millimols NaF = 100 x 0.500 = 50,0
Add 0.002 = 2 millomils HNO3
.........F^- + H^+ ==> HF
I.......50.0...0......25.0
add.............2..........
C........-2....-2......+2
E........48.....0.....27
Then plug the E line values into the HH equation and solve for pH.
Addition of NaOH is done the same way but use the reverse chemical equation since this is a base added.
.........HF + OH^- ==> F^- + H2O