Hello, I have a question that reads: Using the average molarity of your initial acetic acid solutions, the initial volumes, and the volume of NaOH added to reach the equivalence point, calculate the [C2H3O2-] concentration at the equivalence point. You should report 4 significant figures, e.g. 0.06753. You Scored 3 points out of 3 Possible Your Answer: 0.04916
Average Molarity of Acetic Acid: 0.07456,
Initial volume of Acetic Acid: 30.0 mL
Vol NaOH needed to reach equivalence: 16.0 mL
I was told that the answer should be [(0.07456)*(0.030)]/(0.030+0.016), but that gives the wrong answer. Apparently it's supposed to be 0.04916. Can someone explain?
The formula you have been given is correct for the values of 30.0, 16.0 and 0.07456 M. The answer is not 0.04916, perhaps because the 30.0 and the 16.0 look suspiciously "too even."
Hmm...do you think it could have to do with Acetic Acid being a weak acid and NaOH being a strong base? (Incomplete dissociation?)
No.
(C2H3O2^-) = moles/L
Moles comes from the acetic acid titrated COMPLETELY to the equivalence point. So
M x L = moles acetic acid. If all of that is converted to acetate, as it should be, then M x L/final volume = (acetate).
Two sources of error. The molarity of the acetic acid is not quite right.
The titration with NaOH not exact.
Ah, I figured out the problem. Thanks!
So how does one solve this problem?
To calculate the [C2H3O2-] concentration at the equivalence point, you can use the method of stoichiometry. In this case, the balanced chemical equation for the reaction between acetic acid (C2H4O2) and sodium hydroxide (NaOH) is:
CH3COOH + NaOH -> CH3COONa + H2O
From the equation, you can see that the stoichiometric ratio between acetic acid and sodium hydroxide is 1:1. This means that for every mole of acetic acid, one mole of sodium hydroxide is used.
To calculate the [C2H3O2-] concentration at the equivalence point, you need to find the moles of acetic acid and the total volume of the solution at the equivalence point.
First, calculate the moles of acetic acid initially present:
moles of acetic acid = molarity * volume
moles of acetic acid = 0.07456 M * 0.030 L
moles of acetic acid = 0.002237 moles
Next, calculate the total volume of the solution at the equivalence point:
total volume at equivalence = volume of acetic acid + volume of NaOH
total volume at equivalence = 0.030 L + 0.016 L
total volume at equivalence = 0.046 L
Now, use the moles of acetic acid and the total volume at equivalence to calculate the [C2H3O2-] concentration at the equivalence point:
[C2H3O2-] at equivalence = moles of acetic acid / total volume at equivalence
[C2H3O2-] at equivalence = 0.002237 moles / 0.046 L
[C2H3O2-] at equivalence = 0.04865 M
Rounding this value to four significant figures, the [C2H3O2-] concentration at the equivalence point is 0.0487 M, which is closer to the reported value of 0.04916 M. It appears that the reported value was rounded to four significant figures.