Prepare a buffer by direct addition. Consider how best to prepare one liter of a
ID: 949042 • Letter: P
Question
Prepare a buffer by direct addition.
Consider how best to prepare one liter of a buffer solution with pH = 3.62 using one of the weak acid/conjugate base systems shown here.
HC2O4-
C2O42-
6.4 x 10-5
4.19
H2PO4-
HPO42-
6.2 x 10-8
7.21
HCO3-
CO32-
4.8 x 10-11
10.32
How many grams of the sodium salt of the weak acid must be combined with how many grams of the sodium salt of its conjugate base, to produce 1.00 L of a buffer that is 1.00 M in the weak base?
grams sodium salt of weak acid =
grams sodium salt of conjugate base =
HC2O4-
C2O42-
6.4 x 10-5
4.19
H2PO4-
HPO42-
6.2 x 10-8
7.21
HCO3-
CO32-
4.8 x 10-11
10.32
Explanation / Answer
best combination : HC2O4- and C2O4-2 . its pKa value close to pH . value it is only suitable to prepare desired buffer
HC2O4Na + NaOH ----------------------> Na2C2O4 + H2O
acid base
buffer moles = 1 x 1 = 1
acid + base = 1 -------------------->1
pH = pKa + log [base /acid]
3.62 = 4.19 + log [base / acid]
0.269 = base /acid
base = 0.269 acid ----------------->2
from 1 and 2
acid moles = 0.788
base moles = 0.220
sodium salt of weak acid mass (NaHC2O4-) = moles x molar mass
= 0.788 x 112
= 88.3 g
sodium salt of conjugate base (Na2C2O4) = moles x molar mass
= 0.220 x 134
= 29.5 g