Calculate the pH at the equivalence point in titrating 0.100 M solutions of each of the following with 0.080 M NaOH.
chlorous acid (HClO2)
benzoic acid (C6H5COOH)
NaOH + HClO2 ==> NaClO2 + H2O so the pH at the equivalence point is determined by the hydrolysis of NaClO2 (actually the hydrolysis of the ClO2^-. The (ClO2^-) = 0.0444M. If you don't know how to do this I can show you.
.........ClO2^- + HOH ==>HClO2 + OH^-
I........0.0444..........0........0
C.........-x..............x........x
E.......0.0444-x..........x........x
Kb for ClO2^- = (Kw/Ka for HClO2) = (x)(x)/(0.0444-x)
Solve for x = (OH^-) and convert to pH.
i am confused how you got the .0444?
To calculate the pH at the equivalence point in titrating a weak acid with a strong base, we need to find the concentration of the conjugate base at the equivalence point and use the relevant equilibrium expression.
1. Chlorous acid (HClO2):
HClO2 is a weak acid that dissociates in water according to the following equilibrium:
HClO2(aq) ⇌ H+(aq) + ClO2-(aq)
At the equivalence point, all the weak acid will react with the strong base, resulting in the formation of the conjugate base ClO2-. Since the titration is stoichiometric, the concentration of ClO2- at the equivalence point will be equal to the concentration of NaOH used.
Given:
Initial concentration of HClO2 = 0.100 M
Concentration of NaOH used = 0.080 M
Therefore, at the equivalence point:
[HClO2] = 0.100 M - 0 (reacted completely)
[ClO2-] = [NaOH] = 0.080 M
Now, let's calculate the pOH of the solution at the equivalence point using the concentration of ClO2-:
pOH = -log[ClO2-] = -log(0.080) ≈ 1.097
To find the pH at the equivalence point, we can use the fact that pH + pOH = 14:
pH = 14 - pOH = 14 - 1.097 ≈ 12.903
Therefore, the pH at the equivalence point of a titration between HClO2 and NaOH is approximately 12.903.
2. Benzoic acid (C6H5COOH):
C6H5COOH is a weak acid that dissociates in water according to the following equilibrium:
C6H5COOH(aq) ⇌ H+(aq) + C6H5COO-(aq)
At the equivalence point, all the weak acid will react with the strong base, resulting in the formation of the conjugate base C6H5COO-. Again, since the titration is stoichiometric, the concentration of C6H5COO- at the equivalence point will be equal to the concentration of NaOH used.
Given:
Initial concentration of C6H5COOH = 0.100 M
Concentration of NaOH used = 0.080 M
Therefore, at the equivalence point:
[C6H5COOH] = 0.100 M - 0 (reacted completely)
[C6H5COO-] = [NaOH] = 0.080 M
Now, let's calculate the pOH of the solution at the equivalence point using the concentration of C6H5COO-:
pOH = -log[C6H5COO-] = -log(0.080) ≈ 1.097
To find the pH at the equivalence point, we can use the fact that pH + pOH = 14:
pH = 14 - pOH = 14 - 1.097 ≈ 12.903
Therefore, the pH at the equivalence point of a titration between benzoic acid and NaOH is approximately 12.903.
To calculate the pH at the equivalence point in titrations, we need to determine the nature of the acids in question.
1. Chlorous acid (HClO2):
Chlorous acid is a weak acid, which means that it does not completely dissociate into ions in water. We can start by writing the balanced chemical equation for the reaction between HClO2 and NaOH:
HClO2 + NaOH → NaClO2 + H2O
At the equivalence point, the moles of NaOH added will be equal to the moles of HClO2 initially present. This implies that the reaction has gone to completion and all the HClO2 has been neutralized. Hence, only NaClO2 and water will be left at the equivalence point.
Since NaClO2 is a salt, it will completely dissociate into ions in water, whereas water itself is neutral. Therefore, the pH at the equivalence point will depend on the dissociation of water, which is pH 7.
So, the pH at the equivalence point for chlorous acid titrated with NaOH is 7.
2. Benzoic acid (C6H5COOH):
Benzoic acid is also a weak acid. Its reaction with NaOH can be represented by the following balanced chemical equation:
C6H5COOH + NaOH → C6H5COONa + H2O
Once again, at the equivalence point, all the benzoic acid will be neutralized, leaving only the sodium benzoate salt (C6H5COONa) and water.
Similar to the previous case, the pH at the equivalence point will depend on the dissociation of sodium benzoate in water and the dissociation of water itself. Sodium benzoate is a salt and will completely dissociate, but it's the conjugate base of a weak acid (benzoic acid), making it slightly basic.
Therefore, the pH at the equivalence point for benzoic acid titrated with NaOH will be slightly above 7, due to the contribution of the slightly basic sodium benzoate.
It's important to note that the pH at the equivalence point in weak acid titrations may vary based on factors such as the initial concentration and the acid dissociation constant (Ka) of the weak acid.