1. To find the pH of a solution of boric acid, we need to determine the concentration of H+ ions in the solution.
Boric acid, H3BO3, is a weak acid, so it will partially ionize in solution. The balanced equation for the ionization of boric acid is:
H3BO3 + H2O ⇌ H2BO3- + H3O+
The Ka value for boric acid is given as 6 x 10^-10. The Ka expression is:
Ka = [H2BO3-][H3O+]/[H3BO3]
Since the initial concentration of H3BO3 is 1.0 x 10^-3 M, and the initial concentrations of both H2BO3- and H3O+ are 0 M, we can assume that x is the concentration of both H2BO3- and H3O+ formed. Thus, at equilibrium, the concentration of H2BO3- and H3O+ is x.
Substituting the values into the Ka expression and solving for x:
6 x 10^-10 = x^2 / (1.0 x 10^-3)
x^2 = 6 x 10^-13
x = √(6 x 10^-13) = 2.4 x 10^-7
Since x represents the concentration of H3O+, which is equal to the concentration of H+, the pH is given by the equation:
pH = -log[H+]
Substituting the value of x into the pH equation:
pH = -log(2.4 x 10^-7) ≈ 6.62
Therefore, the pH of the 1.0 x 10^-3 M solution of boric acid is approximately 6.62.
2. In order to choose the best indicator for the titration of HNO3 with KOH, we need to consider the pH range in which the indicator changes color, which should match the pH range of the equivalence point of the titration.
HNO3 is a strong acid, and KOH is a strong base, so the pH at the equivalence point will be close to 7 (neutral). Therefore, the best indicator to use for this titration would be one that changes color near pH 7.
Among the choices given, methyl red is the best indicator for this titration, as it changes color in the pH range of 4.4 to 6.2, which includes the pH of the equivalence point, pH 7. Methyl violet and alizarin yellow have pH ranges that do not include pH 7.
Therefore, the best indicator to use for this titration is methyl red.