Calculating pH of Buffer Solution with HCl Addition

  • Thread starter pari786
  • Start date
  • Tags
    Buffer Ph
In summary, after adding 15 ml of 0.1M HCl to a solution of 30 ml of 0.1M sodium acetate, the pH of the solution increased to 5.
  • #1
pari786
29
0

Homework Statement



A buffer solution was formed by mixing 30ml of 0.1M sodium acetate and 15ml of 0.1M HCl
a) Calculate the pH of this buffer solution.
b) Calculate the pH of this buffer solution after the addition of 15ml of 0.1M HCl.


Homework Equations



pH = pKa + log[salt]/[acid]

3. The Attempt at a Solution



for part a) I substituted the value in the above equation as pKa= 4.75 for acetic acud. [salt]=0.1 and [acid]=0.1

and got pH as 4.75


Is it right ? and I'm stuck with part B) can anyone please help me ... please soon!
 
Physics news on Phys.org
  • #2
EDIT: this here is misleading because the resulting concentrations ARE equal: BIG HINT for part 'a' -------- the starting 'molarities' of each solution were the same, but the volumes used from each were NOT the same. Now which reactant is in excess?

30 ml of 0.1M sodium acetate, how many moles?
15 ml of 0.1M HCl, how many moles?
...In what final volume?
 
Last edited:
  • #3
I am trying to say that my response was misleading because the resulting concentrations WILL be equal -- I'm still rechecking my computation.
 
  • #4
The formality of salt = 0.1/3
Formality of the weak acid = 0.1/3

Therefore, pH=pKa

You could try more precise arithmetic if you account for hydrolysis of the
salt and of the weak acid. The formality of the formed acid will be 0.0333...
and will also be the formality of the remaining salt. In this case, both
formalities F being equal, you can set up the equilibrium constant expression

K=(H)*(F+H)/(F-H)

You obtain the quadratic equation:
H^2 +(F+K)*H - KF =0

From that quadratic equation, find solution for H,
the hydronium ion concentration molarity; then find pH.

[ If the formalities of the salt, Fs, and the weak acid, Fa, were not
the same, then your equilibrium expression equation would be
K=(H)*(Fs + H)/(Fa - H) ]
 
Last edited:
  • #5
First of all - START WITH THE REACTION EQUATION.

What happens when you add strong acid (HCl) to the solution of weak acid salt?
 
  • #6
Sorry , I misunderstood the part b) Calculate the pH of this buffer solution after the addition of 15ml of 0.1M HCl.

I didn't realize that it was 5 ml of 0.1M HCl (instead).

I realized after doing the calculations for three hours. Sorry about any kind of inconvience.
 

FAQ: Calculating pH of Buffer Solution with HCl Addition

What is a buffer solution and why is it important in pH calculations?

A buffer solution is a solution that resists changes in pH when small amounts of acid or base are added. It is important in pH calculations because it allows for more accurate and stable measurements of pH, especially in biological and chemical systems.

How do you calculate the pH of a buffer solution before and after adding HCl?

To calculate the pH of a buffer solution before adding HCl, you can use the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]), where pKa is the acid dissociation constant of the weak acid in the buffer, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the weak acid. After adding HCl, you can use the same equation, but with the new concentrations of [A-] and [HA] to calculate the new pH.

What is the role of the acid dissociation constant (pKa) in pH calculations for buffer solutions?

The acid dissociation constant, or pKa, is a measure of the strength of an acid. In pH calculations for buffer solutions, the pKa is used in the Henderson-Hasselbalch equation to determine the pH of the buffer solution. It is an important factor in determining the buffer's ability to resist changes in pH when an acid or base is added.

Can the pH of a buffer solution change significantly with the addition of a strong acid like HCl?

No, the pH of a buffer solution will not change significantly with the addition of a strong acid like HCl. This is because the buffer solution is able to resist changes in pH due to the presence of a weak acid and its conjugate base. However, if a large amount of strong acid is added, the buffer may eventually become overwhelmed and the pH will start to change more significantly.

How can you determine the concentration of a weak acid or its conjugate base in a buffer solution?

The concentration of a weak acid or its conjugate base in a buffer solution can be determined using the Henderson-Hasselbalch equation. By rearranging the equation, you can solve for the concentration of either the acid or base. Additionally, you can use titration techniques or spectroscopic methods to determine the concentrations of the acid and base in the buffer solution.

Similar threads

Replies
1
Views
2K
Replies
23
Views
2K
Replies
7
Views
3K
Replies
7
Views
3K
Replies
16
Views
2K
Back
Top