Calculate the pH at the equivalence point in titrating 0.110 M solution of sodium hydrogen chromate (NaHCrO4) with 8.0×10−2 M NaOH.
You need to know what volume is being titrated. I will call (concn of salt as y) and that will be mole of salt/total volume, whatever that is (total volume is volume of starting NaHCrO4 + volume of NaOH added).
.......CrO4^2- + HOH ==> HCrO4^- + OH^-
init....y...................0......0
change..-x..................x......x
equil.. y-x.................x.......x
Kb for CrO4^2- = (Kw/K2 for H2CrO4) = (HCrO4^-)(OH^-)/(CrO4^2-)
Substitute and solve for x = (OH^-) and convert to pH.
2.3
To calculate the pH at the equivalence point in this titration, we need to determine the nature of the acid and base involved, as well as the chemical reactions taking place. This will help us identify any possible acid-base reactions and determine the concentrations of the resulting species.
In this case, we have a solution of sodium hydrogen chromate (NaHCrO4) being titrated with sodium hydroxide (NaOH). Sodium hydrogen chromate is a weak acid, and sodium hydroxide is a strong base.
The chemical reaction that occurs is as follows:
NaHCrO4 + NaOH → Na2CrO4 + H2O
At the equivalence point, the moles of acid will be equal to the moles of base. This means that the concentration of sodium hydrogen chromate and sodium hydroxide will be equal. Thus, we can use the given concentrations to calculate the concentration of sodium hydroxide at the equivalence point.
Given:
Concentration of NaHCrO4 = 0.110 M
Concentration of NaOH = 8.0×10−2 M
Since the moles of acid equals the moles of base, we can set up an equation based on the balanced chemical equation to determine the concentration of sodium hydroxide at the equivalence point:
0.110 M NaHCrO4 = (8.0×10−2 M NaOH) × (1 mol NaHCrO4 / 1 mol NaOH)
Simplifying the equation, we get:
0.110 M NaHCrO4 = 0.080 M NaOH
Now that we have the concentration of sodium hydroxide at the equivalence point, we can calculate the concentration of hydroxide ions (OH-) using the dissociation of sodium hydroxide:
[OH-] = 0.080 M NaOH
Since sodium hydrogen chromate is a weak acid, we can ignore its dissociation. At the equivalence point, the concentration of hydroxide ions will be equal to the concentration of sodium hydroxide. Therefore, [OH-] = 0.080 M.
To calculate the pH at the equivalence point, we need to determine the concentration of hydronium ions (H3O+). Since we are dealing with a strong base (NaOH), the hydroxide ions will react with water to produce hydroxide and hydronium ions:
OH- + H2O ↔ H3O+ + OH-
At equilibrium, the concentration of hydroxide and hydronium ions will be equal, so [H3O+] = [OH-] = 0.080 M.
Now, we can calculate the pH using the formula:
pH = -log[H3O+]
pH = -log(0.080)
Using a calculator, we find:
pH ≈ 1.10
Therefore, at the equivalence point, the pH is approximately 1.10.