Take into account correction factors for phosphate buffers, predict the pH when 0.50ml of 1M HCl is added to 40mL of .10M buffer at expected pH 6 and pH 6.5. The buffer was made with Na2HPO4 and NaH2PO4. pKa is 7.2.
When I did this before, I got a lower pH for 6.5 than I did for 6. Which this doesn't make sense.
To predict the pH after adding HCl to a phosphate buffer, we need to consider the correction factors and the equilibrium reaction that occurs between the acid and the base in the buffer system.
The phosphate buffer system consists of the weak acid, H2PO4- (derived from Na2HPO4), and its conjugate base, HPO4^2- (derived from NaH2PO4). In this case, the pKa of the phosphate buffer system is given to be 7.2.
To calculate the pH after adding HCl, we need to consider two scenarios: when the expected pH is 6 and when the expected pH is 6.5.
Let's start with the scenario where the expected pH is 6.
1. First, calculate the initial amounts of the weak acid (H2PO4-) and its conjugate base (HPO4^2-) in the buffer solution.
- The initial concentration of the weak acid is 0.10 M × (40 mL / 1000 mL) = 0.004 M (mol/L).
- The initial concentration of the conjugate base is also 0.10 M × (40 mL / 1000 mL) = 0.004 M (mol/L).
2. Next, determine the change in moles of HCl added to the buffer solution.
- The moles of HCl added can be calculated by multiplying the concentration of HCl by the volume added: 1 M × 0.50 mL = 0.0005 moles.
3. Calculate the new concentrations of the weak acid and the conjugate base after the addition of HCl.
- The concentration of the weak acid is 0.004 M - 0.0005 moles / (40 mL / 1000 mL) = 0.004 - 0.0005(40/1000) M.
- The concentration of the conjugate base is 0.004 M - 0.0005 moles / (40 mL / 1000 mL) = 0.004 - 0.0005(40/1000) M.
4. Use the Henderson-Hasselbalch equation to calculate the pH.
- pH = pKa + log ([conjugate base] / [weak acid])
- pH = 7.2 + log ( [0.004 - 0.0005(40/1000)] / [0.004-0.0005(40/1000)] )
Now, let's move to the scenario where the expected pH is 6.5 and repeat the steps above.
After performing the calculations, it is possible that you obtained a lower pH for expected pH 6.5 than for expected pH 6. This is because as the initial pH of the buffer increases, it becomes more difficult to lower the pH due to the buffering capacity of the solution.
If you obtain results that do not follow this pattern, double-check your calculations and make sure that you have correctly accounted for all the correction factors and balanced the equations.