If you have 200 ml of a 0.1M solution of acetic acid (pka=4.75),
1. How many ml of a solution of 1.O M NaOH would you need to adjust the pH to 5.2?
"Chemistry - DrBob222, Wednesday, June 29, 2011 at 8:41pm
200 mL x 0.1M = 20 mmoles.
1.
............HAc + OH^- ==> Ac^- H2O
initial.....20.0...0........0
added..............x...........
change.......-x....-x........+x
equil......20-x.....0.........+x
Substitute into the Henderson-Hasselbalch equation and solve for x which will be mmoles NaOH. You can take it from there."
So I have substituted for hernderson hasselbalch
5.2= 4.75 + {log [x]/[20 mmoles]
1.75= log [x]/20mmoles
10^1.75= [x]/20
56. 234 = x/20
1,124.68 mmoles of NaOH.
Could this be right? Now to convert this to ml to answer the question, would I divided by 1.0M? So I would need 1.124 L ?
No, you didn't substitute correctly. The 20 you have should be 20-x
5.2 = 4.75 + log(x/20-x) and solve for x.
You last step shown should be
56.37 = x/(20-x)
Also, I don't know where you obtained 1.75.
x should be approximately 15 mmoles and at 1 M NaOH that would be approximately 15 mL.
I dont' know what I was thinking with the 1.75. So the 20-x indicates the amount of acid left over after the base reacts with it. Thanks a lot Dr. Bob, I really appreciate it.
That's right. You want to substitute the EQUILIBRIUM numbers and not the STARTING numbers.
To find the number of milliliters (ml) of a 1.0 M NaOH solution needed to adjust the pH to 5.2, you have correctly calculated that you need 1,124.68 millimoles (mmol) of NaOH.
To convert this to milliliters, you can follow these steps:
Step 1: Convert millimoles (mmol) to moles (mol). Divide the number of millimoles (1,124.68 mmol) by 1,000 to get the number of moles (mol).
1,124.68 mmol / 1,000 = 1.12468 mol
Step 2: Use the molarity (M) and volume (V) relationship, which is given by the formula:
Molarity (M) = moles (mol) / volume (L)
Rearranging the formula, volume (L) = moles (mol) / molarity (M).
To convert the volume from liters to milliliters, you multiply by 1,000.
So, in this case, to convert mol to ml, divide the number of moles (1.12468 mol) by the molarity (1.0 M) and then multiply by 1,000:
(1.12468 mol / 1.0 M) * 1,000 = 1,124.68 ml
Therefore, you would need approximately 1,124.68 milliliters (ml) of a 1.0 M NaOH solution to adjust the pH to 5.2.