Question

2. A one liter of solution phosphate buffer is made so that it is 1.00M in NaH2PO4 and 1.50M in Na2HPO4. If the pka of NaH2PO4 is 7.21 assuming the volume negligible change upon addition of acid or base calculate?

a. the Ph of buffer?

b. the resulting pH after addition of 0.25 mol of NaOH?

c. the amount of NaOH that would have to original buffer to bring pH upto 8.0

Answer 1

You have the followong equilibrium

The acid on the left side and the conjugate base on the right side.

Using the henderson hasselbach equation:

a)You can calculate the pH of the buffer:

b)The pH after addition of 25mol of NaOH

after adding 0.25 of NaOH the concentrations are varying as follows:

The original moles of the conjugate base is going to increase the same amount as NaOH added

, since the volume added is negligible this yields:

For the acid the concentration is drecreasing the same amount as base added:

and assuming the volume is negligible

Using the henderson hasselbach equation:

c)to bring the pH to 8 you need to calculate the ratio BASE/ACID

You need to add X moles of NaOH to change the pH

and as in b) if you add NaOH, you are raising the concentration of the base and lowering the concentration of acid:

for the base:

Initially you have

after addition of X moles:

for the acid initially:

after adding X moles of base:

Usinge the ration above:

, solving for X yields

<------moles of NaOH need to change the pH to 8