pls help i need to do this in less than 2 hours :(( A buffer is made up of 250 mL each of 0.43 M KH2PO4 and 0.27 M K2HPO4.(Ka=6.2 x 10-8) Assuming that volumes are additive, calculate the pH of the buffer after addition of 100 mL of 0.8 M NaOH to the buffer solution

Question

pls help i need to do this in less than 2 hours :(( A buffer is made up of 250 mL each of 0.43 M KH2PO4 and 0.27 M K2HPO4.(Ka=6.2 x 10-8) Assuming that volumes are additive, calculate the pH of the buffer after addition of 100 mL of 0.8 M NaOH to the buffer solution

Answer 1

To determine the pH of the buffer solution after the addition of NaOH, we need to determine how the components of the buffer react with the base and calculate the resulting concentrations of the acid and conjugate base.

1. First, let's calculate the moles of KH2PO4 and K2HPO4 in the original buffer solution.
Moles of KH2PO4 = concentration (M) × volume (L) = 0.43 M × 0.25 L = 0.1075 moles
Moles of K2HPO4 = concentration (M) × volume (L) = 0.27 M × 0.25 L = 0.0675 moles

2. When NaOH is added to the buffer solution, it will react with KH2PO4 to form water and K2HPO4. The moles of KH2PO4 will decrease while the moles of K2HPO4 will increase.
The balanced equation for the reaction is:
KH2PO4 + NaOH → H2O + K2HPO4

3. Next, calculate the new moles of KH2PO4 and K2HPO4 after the reaction.
Since the moles of NaOH is given as 0.8 M × 0.1 L = 0.08 moles, according to the balanced equation, it reacts with an equal number of moles of KH2PO4.
Moles of KH2PO4 after reaction = initial moles - moles reacted = 0.1075 - 0.08 = 0.0275 moles
Moles of K2HPO4 after reaction = initial moles + moles formed = 0.0675 + 0.08 = 0.1475 moles

4. Now, we need to calculate the concentrations of KH2PO4 and K2HPO4 after the reaction.
Concentration of KH2PO4 = moles / total volume = 0.0275 moles / 0.35 L = 0.0786 M
Concentration of K2HPO4 = moles / total volume = 0.1475 moles / 0.35 L = 0.4214 M

5. Since the concentrations of both the acid and conjugate base have changed, we can calculate the pH using the Henderson-Hasselbalch equation.
pH = pKa + log([conjugate base] / [acid])

The pKa for the phosphate buffer system can be calculated from the Ka value: pKa = -log(Ka) = -log(6.2 x 10^-8) = 7.21

pH = 7.21 + log(0.4214 M / 0.0786 M)

6. Calculate the ratio of concentrations and substitute in the equation:
pH = 7.21 + log(5.35)

7. Evaluate the logarithm and find the final pH:
pH = 7.21 + 0.73 = 7.94

Therefore, the pH of the buffer solution after the addition of 100 mL of 0.8 M NaOH is 7.94.