Question

What is change in pH upon addition of 1.0x10^-3mol strong acid to 1.00L of a buffer mader of 26.8mL of 0.200M HCl added to 50.0mL of 0.200M Tris (pKa=8.3), diluted to a total volume of 0.200mL?

Buffer Capacity • Ability of a buffer to resist change in pH upon addition of acid or base. Example 4.12 in Tinoco What is change in pH upon addition of 1.0x10- 3 mol strong acid to 1.00 L of a buffer made of 26.8 mL of 0.200 M HCl added to 50.0 mL of 0.200 M Tris (pK = 8.3), diluted to a total volume of 200.0 mL?

Answer 1

Moles of Tsis base - Molanity & volume in l. = 0.200 M X 50 = 0.0lo mol Molus of He added = 0.200 M X 26. 8L lood = 0.00536 mol As Imol of the reacts with Imol of Tris to form mol of conjugate acid of Tris So, moles of Tris-HT -10.00536 mol moles of Teis = 0.0/0 - 0.00536 = 10.00464 moll Total volume of buffer zou me on 0.2L (Tris-H+] = 0.00536 mol = 10.0268 m 0.2 L {teis] = 0.00464 mol & 10.0232M 0. 21 Now, lool of above buffer is taken, So, moles of Tris He a 0.0268 M IL 2 0.0268 mol moles of Teis = 0.0232 M XIL 0.0232 mol Moles of strong acid added 1.0x10 3 mol zo oo mol Strong acid will react with Teis and will form Teis Ht.

After addition of strong acide moles of Tris - 0.0232 mol-0.001 mol z 0.0222 mol moles of Tris H t = 0.0268 mol to.ool mol 2 0.0278 mol 0.0222 mol 2 0.0222 M IL Teis Ht2 0.0278 mol 20.0278M IL 10 pka of Teis-Ht a 8.3 Using Henderson Hasselbalch equation - pH2 pkat eg [teis ] [Teis H+7 PH= 8.3 + log (0.0222 ) (0.0278 M) [pH = 8.20 pH before addition of stong acid, - pH = 8.3 + log (0.0232M) p (0.02687 = 8:237 -8.24] Change in pHz 8.24-820 - 10.04