I having a hard time trying to find the correct pH value for number 9 and 10 whe
ID: 492101 • Letter: I
Question
I having a hard time trying to find the correct pH value for number 9 and 10 where number 9's pH is 3.260 and number 10's pH value is 3.590. However the pH value is supposed to be going down in number 10 and not upward when adding HCl. I would appreciate any insight into the answers that I acquired for number 9 and 10 that might have made them incorrect.
problem 9: Calculate the pH of a buffer made with 50.0ml of 0.200 M HA and 150.0ml of 0.100 M NaA, if the Ka is 2.75x10^-4.
problem 10: if 2.00ml of 1.00 M HCl is added to the buffer above, what is the resulting pH?
Explanation / Answer
9)
Ka = 2.75 x 10^-4
pKa = -log (2.75 x 10^-4 )
= 3.56
millimoles of HA = 50 x 0.2 = 10
millimoles of NaA = 150 x 0.1 = 15
pH = pKa + log [NaA / HA]
= 3.56 + log [15 / 10]
= 3.74
pH = 3.74
10)
millimoles of HCl added = 2 x 1 = 2
pH = 3.56 + log [15 - 2 / 10 +2]
= 3.60
pH = 3.60