1. What is the pH of a 1.0 x 10-3 M solution of boric acid, H3BO3? The Ka value for boric acid is 6 x 10-10.
(choices: 3, 7, between 7 and 12, between 3 and 7)
2. You want to use a 0.500 M HNO3 solution to titrate an unknown concentration of KOH solution. Which indicator would be the best for this titration?
(choices: Methyl violet, Methyl red, Alizarin yellow)
................H3BO3.....................H^+...........H2BO3^-
I..............0.001..........................0................0
C................-x.............................x..................x
E.............0.001-x........................x..................x
Ka = (H^+)(H2BO3^-)/(H3BO3)
Substitute and solve for x = (H^+) and convert to pH
For the titration, the pH at the equivalence point is 7.0 or very close to that. Alizarin yellow is 10 and above, methyl violet is very low. would use methyl red. It's average is about 5 the the pH titration curve is so steep at that point in the titration that the error will be minimal.
1. Well, the pH of the boric acid solution can be calculated using the Ka value. However, instead of calculating it, I could perform a magic trick to come up with the answer. *Poof* Abracadabra! The pH of the solution is between 3 and 7. Ta-da!
2. Ah, the wonderful world of titrations! Let me consult my hilarious chemistry handbook... Ah, here it is! The best indicator for this titration would be Methyl red, because it goes really well with the tangy taste of nitric acid. It's a match made in chemical heaven!
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.
1. To find the pH of a solution of boric acid, we need to use the equilibrium constant expression, also known as the acid dissociation constant (Ka). The Ka value for boric acid is given as 6 x 10^-10.
The general expression for the acid dissociation of boric acid is:
H3BO3 ⇌ H+ + H2BO3^-
The Ka expression is:
Ka = [H+][H2BO3^-]/[H3BO3]
Given a concentration of 1.0 x 10^-3 M for H3BO3, we can assume that the concentration of H+ is negligible compared to the initial concentration of H3BO3. Therefore, we can substitute [H3BO3] as the initial concentration into the Ka expression without considering the change in concentration due to the dissociation.
Ka = [H+][H2BO3^-]/[H3BO3]
6 x 10^-10 = x * x / (1.0 x 10^-3)
By solving this quadratic equation for x (the concentration of H+ ions), we can determine the concentration of H+ ions in the solution and find the pH.
2. To choose the best indicator for a titration between a 0.500 M HNO3 (nitric acid) solution and an unknown concentration of KOH (potassium hydroxide) solution, we need to consider the pH range in which the indicator changes color.
Methyl violet and methyl red are commonly used acid-base indicators. Alizarin yellow is less commonly used for titrations and is more appropriate for complexometric titrations involving metal ions.
Methyl violet changes color in the pH range of 0.0 to 1.6, which is more suitable for strong acids and not optimal for this titration with HNO3.
Methyl red changes color in the pH range of 4.4 to 6.2, which is more appropriate for this titration since it falls within a pH range that is slightly acidic.
Therefore, the best choice of indicator for this titration would be Methyl red.