Calculate the pH and concentration of H3O+ of a 0.285M oxalic acid solution.
.......H2C2O4(aq) ==> H^+ + HC2O4^-
I.......285...........0......0
C.......-x............x......x
E....0.285-x..........x......x
Substitute the E line into the Ka1 expression for H2C2O4 and solve for (H^+), then pH = -log(H^+).
I assume that the (H^+) is largely due to that contributed by Ka1 and none by Ka2. Actually, some will come from Ka2 but only about 1 in 1000 molecules that dissociate so the error will not be that large by neglecting that contribution from Ka2.
To calculate the pH and concentration of H3O+ of a solution of oxalic acid, it is important to know the dissociation constant of the acid. The dissociation constant for oxalic acid (H2C2O4) is a polyprotic acid and has two dissociation steps:
Step 1: H2C2O4 ⇌ H+ + HC2O4-
Step 2: HC2O4- ⇌ H+ + C2O4^2-
Given that oxalic acid is a weak acid, we can assume that the dissociation of HC2O4- is negligible compared to the dissociation of H2C2O4. Therefore, we will focus on Step 1.
The dissociation constant for Step 1 is denoted as Ka1 and can be found in a reference table. For oxalic acid, Ka1 is approximately 5.9 x 10^-2.
Now, let's proceed with the calculations:
Step 1: Determine the initial concentration of H2C2O4 (oxalic acid):
Given: 0.285 M
Step 2: Determine the concentration of H+:
The concentration of H+ equals the concentration of H2C2O4 that has dissociated. Since the dissociation is assumed to be negligible, the concentration of H+ is approximately equal to the initial concentration of H2C2O4.
[ H+] = 0.285 M
Step 3: Calculate the concentration of H3O+:
In water, H+ ions combine with water molecules to form hydronium ions (H3O+), so the concentration of H3O+ is the same as the concentration of H+.
[ H3O+] = 0.285 M
Step 4: Calculate the pH:
The pH is calculated using the formula: pH = -log10 [H3O+]
pH = -log10 (0.285) ≈ 0.546
Therefore, the pH of the 0.285 M oxalic acid solution is approximately 0.546, and the concentration of H3O+ is also 0.285 M.