1. A buffer solution contains 0.458 M Na HCO 3 and 0.328 M K 2 CO 3 . Determine
ID: 965059 • Letter: 1
Question
1. A buffer solution contains 0.458 M NaHCO3 and 0.328 M K2CO3. Determine the pH change when 0.083 mol HNO3 is added to 1.00 L of the buffer.
pH change =
2. Determine the pH change when 0.083 mol HCl is added to 1.00 L of a buffer solution that is 0.402 M in CH3COOH and 0.340 M in CH3COO-.
pH after addition pH before addition = pH change =
***If you could please explain how you got each step. I've attempted these problems several times, but still can't seem to get the correct numbers. I don't understand what I'm doing incorrectly.***
Explanation / Answer
I am allowed to asnwer only 1 question at a time
Before adding HNO3:
moles of NaHCO3 = 0.458 mol
moles of K2CO3 = 0.328 mol
since volume of solution is 1 L
pH = pKa + log {[K2CO3]/[NaHCO3]}
= 10.32 + log (0.328/0.458)
= 10.32 - 0.344
=10.175
number of moles of HNO3 added = 0.083 mol
CO32- + H+ ---> HCO3-
so, number of moles of HCO3-/NaHCO3 will increase by 0.083 mol
number of moles of CO32- / K2CO3 will decrease by 0.083 mol
final
[NaHCO3] = 0.458 + 0.083 = 0.541 M
[K2CO3] = 0.328-0.083 = 0.245 M
SInce volume is 1L, number of moles = concentration
pKa of HCO3- = 10.32
USE:
pH = pKa + log {[K2CO3]/[NaHCO3]}
= 10.32 + log (0.245/0.541)
= 10.32 - 0.344
=9.976
Answer: 9.976 - 10.175 = -0.199 = -0.2
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