Okay let me write it this way.
I mixed 30 mL of 0.1 M Acetic Acid and 15 mL of 0.1 M Sodium Hydroxide and measured the pH. Then I added 5 mL of 0.1 M NaOH to this buffer and measured the pH.
It's asking me:
Show the calculations for the pH of the buffer before and after the addition of HCL?
Thank You
I posted my response at the original post along with a note why I took so long to answer.
Ok. Thank you so much Dr. Bob
To calculate the pH of a buffer solution, you will need to consider the concentrations of the acid and its conjugate base (formed by the reaction between the acid and a base), as well as the dissociation constant (Ka) of the acid. In this case, acetic acid (CH3COOH) acts as the acid, and its conjugate base is acetate (CH3COO-). The dissociation constant for acetic acid is 1.8 x 10^-5.
1. Before the addition of NaOH:
To find the concentration of acetic acid and acetate in the buffer solution, we'll use the following expressions:
Acetic Acid:
moles of acetic acid = volume (L) x concentration (M)
= 0.030 L x 0.1 M = 0.003 mol
Acetate:
moles of acetate = volume (L) x concentration (M)
= 0.015 L x 0.1 M = 0.0015 mol
Since the buffer solution is made by mixing equal volumes of acetic acid and sodium hydroxide, the initial concentrations of both acetic acid and acetate will be equal, and the total volume will be the sum of the volumes of the two solutions:
total volume = 0.030 L + 0.015 L = 0.045 L
To calculate the initial concentration of the acid/acetate in the buffer, divide the number of moles by the total volume:
Initial concentration of acetic acid and acetate = moles / total volume
= 0.003 mol / 0.045 L = 0.0667 M
Next, calculate the ratio of the concentration of the base (acetate) to the concentration of the acid (acetic acid):
Ratio = [Acetate] / [Acid]
= 0.0667 M / 0.0667 M = 1
Since the ratio is 1, the logarithm of the ratio is 0, which means the pH is equal to the pKa of acetic acid:
pH = pKa = -log(Ka) = -log(1.8 x 10^-5) = 4.74
2. After the addition of NaOH:
Adding NaOH to the buffer will result in the neutralization of some acetic acid. The new concentration of acetic acid and acetate in the buffer solution needs to be recalculated.
Acetic Acid:
moles of acetic acid remaining = initial moles of acetic acid - moles of NaOH added
= 0.003 mol - 0.005 L x 0.1 M = 0.0025 mol
Acetate:
moles of acetate = initial moles of acetate + moles of NaOH added
= 0.0015 mol + 0.005 L x 0.1 M = 0.002 mol
The new total volume of the buffer solution remains the same:
total volume = 0.045 L
Recalculate the concentrations of acetic acid and acetate in the buffer:
New concentration of acetic acid = moles of acetic acid remaining / total volume
= 0.0025 mol / 0.045 L = 0.0556 M
New concentration of acetate = moles of acetate / total volume
= 0.002 mol / 0.045 L = 0.0444 M
The new ratio of acetate to acetic acid is:
New ratio = [Acetate] / [Acid]
= 0.0444 M / 0.0556 M = 0.80
Find the pH using the Henderson-Hasselbalch equation:
pH = pKa + log([Acetate] / [Acid])
= 4.74 + log(0.80) ≈ 4.78
So, the pH of the buffer solution after the addition of NaOH is approximately 4.78.