Preparing Buffer Solutions: Calculating the Volume of Weak Base Needed Use the t
ID: 551892 • Letter: P
Question
Preparing Buffer Solutions: Calculating the Volume of Weak Base Needed Use the table of K values given with this problem to choose the best weak base to start from for making a buffer that holds the ph of the solution at 4.70. Make your selection so tha you maximize the capacity of the buffer Assume that you want to make 600 ml of a buffer and you have already added enough of the conjugate acid salt to make the final buffer 0.78 M in this salt. How many mL of a 4.8 M solution of the weak base you selected must be added to the volumetric flask so that when filed to the mark, the pH of the buffer will equal 4.70 NH3 (Ammonia) CsHsNH2 (Aniline) (CH3CH2)2NH (Diethyl amine) C$HsN (Pyridine) CH3CH2NH2 (Ethyl amine) (CH3)3N (Trimethyl amine) (CH3)2NH (Dimethyl amine) 1.76× 10-5 3.9 x 10-10 6.9 × 10-4 1.7×10-9 5.6 X 10-4 6.4 X 10-5 5.4 × 10.4 mlExplanation / Answer
if pH = 4.7
nearest data
10^-4.7 = 2*10^-5
kb= (10^-14)/(2*10^-5) = 5*10^-10
from the list, choose aniline, with 3.9*10^-10 data
so
B + H2O <-> BBh+ + OH-
pOH = pKb + log(BH+/B)
pOH = 14-pH = 14-4.70 = 9.3
pKb = -log(3.9*10^-10) = 9.40
9.3 = 9.4 + log(BH+/B)
if BH = 0.78
9.3 = 9.4 + log(0.78/B)
B = 0.78 / (10^(9.3-9.4)) = 0.9819
M1*V1 = M2*V2
4.8*V1 = 0.9819*600
V1 = 0.9819*600/4.8
V1 = 122.73 mL of base required