# How Do You Calculate NaCH3COO Concentration and Predict pH at Equivalence Point?

• cvc121
In summary, the student attempted to solve a problem involving the titration of a weak base with a strong acid and found that their calculation was inaccurate.
cvc121

## Homework Statement

a) Determine the concentration of NaCH3COO. The volume of HCl used to reach the equivalence point is 25.05 mL, the concentration of HCl is 0.10 M, and the volume of NaCH3COO solution used is 10.0 mL.

b) Using the initial concentrations of NaCH3COO and HCl, and a reliable literature value for the pKb for NaCH3COO or PKa for CH3COOH, predict the theoretical pH of the equivalence point.

## The Attempt at a Solution

Here is my reasoning and calculations:

a)

The equivalence point is the point in the titration when the number of moles of a standard solution (titrant) is equal to the number of moles of a solution of unknown concentration (analyte).

Therefore, at the equivalence point:

ntitrant = nanalyte

nHCl = nNaCH3COO

moles of HCl = (concentration)(volume)

moles of HCl = (0.10 M)(0.02505 L)

moles of HCl = 2.5 x 10-3 mol

moles of HCl = moles of NaCH3COO = 2.5 x 10-3 molConcentration of NaCH3COO = moles / total volume

Concentration of NaCH3COO = 2.5 x 10-3 mol / (0.01 L + 0.02505 L)

Concentration of NaCH3COO = 0.07 M

b)

A reliable literature value for the pKa for acetic acid is 4.756.

pKa = -log Ka

Ka = 10-pKa

Ka = 10-4.756

Ka = 1.754 x 10-5

At the equivalence point of the titration of a weak base with a strong acid, the general reaction H3O+ (aq) + B (aq) ---> BH+ (aq) + H2O (l) has gone approximately to completion. Therefore, approximately all sodium acetate (NaCH3COO) will be converted to its conjugate acid, acetic acid (CH3COOH), in a 1:1 molar ratio.

Ka = [CH3COO-][H3O+] / [CH3COOH]
1.754 x 10-5 = x2 / (0.07 M – x)

Ka is very small. Therefore, the x value in the denominator is negligible and can be omitted.

1.754 x 10-5 = x2 / 0.07 M

x2 = (1.754 x 10-5 )(0.07 M)

x2 = 1.228 x 10-6 M

x = 1.108 x 10-3 M

[H3O+] = x = 1.108 x 10-3 M

Validity check:
(1.108 x 10-3 M / 0.07 M )(100%) = 1.58%

Theoretical pH = -log[H3O+]
Theoretical pH = -log[1.108 x 10-3]
Theoretical pH = 2.96

Could anyone verify my reasoning and calculations? Thanks. All help is very much appreciated.

There is no CH3COONa at the endpoint so your calculation of concentration (using total volume at the endpoint) doesn't make much sense. Question most likely asks for the initial CH3COONa concentration.

b looks reasonalby accurate.

Question is a bit idiotic to be honest. Good luck detecting the end point in this titration (not your fault though).

Thank you for your response. Assuming that the question is asking for the initial concentration of CH3COONa, does the question provide enough information to solve for this? Would I simply use the volume of CH3COONa used instead of the total volume?

Concentration of NaCH3COO = 2.5 x 10-3 mol / 0.01 L = 0.25 M?

Looks OK.

I concur with BoreK that the question does not make a lot of sense. Are you sure you have copied it and any background out exactly? One titrates an acid with a base or vice versa;, one does not titrate a salt which is what sodium acetate is.

Your calculation in #1 is just giving you the pH of 0.07 M acetic acid.

Maybe that was not the question but until we know exactly what it was it is not really worth further thought

## 1. What is an acid-base titration?

An acid-base titration is a laboratory technique used to determine the concentration of an acid or base solution. It involves adding a known concentration of one solution (called the titrant) to a known volume of the other solution (called the analyte) until a chemical reaction between the two is complete.

## 2. Why is an acid-base titration important?

An acid-base titration is important because it allows us to accurately measure the concentration of a solution. This information is useful in many scientific fields, including chemistry, biology, and environmental science.

## 3. How do you calculate the concentration of an unknown solution using titration data?

To calculate the concentration of an unknown solution, you need to know the volume and concentration of the titrant, as well as the volume and concentration of the analyte. Using the equation M1V1 = M2V2, where M1 is the concentration of the titrant, V1 is the volume of the titrant used, M2 is the concentration of the analyte, and V2 is the volume of the analyte, you can solve for the unknown concentration.

## 4. What is an indicator and how is it used in an acid-base titration?

An indicator is a substance that changes color in response to a change in pH. In an acid-base titration, an indicator is added to the analyte solution to signal when the reaction is complete. The color change indicates when the solution has reached its equivalence point, where the moles of acid and base are equal.

## 5. What are some sources of error in acid-base titration experiments?

Some sources of error in acid-base titration experiments include inaccurate measurements of volume or concentration, improper calibration of equipment, and human error in identifying the endpoint of the titration. Temperature changes and the presence of impurities in the solutions can also affect the accuracy of the results.

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