Question

The ph of the 50mM buffer is 6.54 before addition of 1M Naoh dropwisely. The ph of the 50mM buffer is 6.85 after the addition of 1M of Naoh.

a) For the 50 mM buffer you prepared, how well did it buffer against the addition of acid/base?

b) If you had added 1 mL of 1 M NaOH, would you still have a valid buffer? What would you expect the pH to be? Include your calculations

Answer 1

We assume that initially 1 L of this buffer is present with salt:acid ratio equal to 1:1

Change in pH = 6.85-6.54 = 0.31

Moles of OH- added = Molarity*Volume = 1*0.001 = 0.001

Using the equation:

pH = pKa + log(moles of salt/moles of acid) = pKa + log(0.05/0.05)

After addition of 0.001 moles of base, it becomes:

pH' = pKa + log(moles of salt/moles of acid) = pKa + log(0.05+0.001/0.05-0.001) = 0.0173

So,

dpH = 0.0173

Thus, pH increase by 0.0173

Yes it will still be a valid buffer, but its buffer capacity will be reduced than it previously was.

Hope this helps !