- #1
erik-the-red
- 88
- 1
1) In an experiment to determine the molecular weight and the ionization constant for ascorbic acid (vitamin C), a student dissolved 1.3717 grams of the acid in water to make 50.00 milliliters of solution. The entire solution was titrated with a 0.2211-molar NaOH solution. The pH was monitored throughout the titration. The equivalence point was reached when 35.23 milliliters of the base had been added. Under the conditions of this experiment, ascorbic acid acts as a monoprotic acid that can be represented as HA.
(a) From the information above, calculate the molecular weight of ascorbic acid.
(b) When 20.00 milliliters of NaOH had been added during the titration, the pH of the solution was 4.23. Calculate the acid ionization constant for ascorbic acid.
(c) Calculate the equilibrium constant for the reaction of the ascorbate ion, A¯, with water.
(d) Calculate the pH of the solution at the equivalence point of the titration.
This was an AP Chemistry question in 1989.
I had no problems doing the first part. At the equivalence point, the number of moles of acid is equal the number of moles of base. I used this relationship to find out the number of moles, and grams / moles = molecular weight.
I have problems with part b, which consequently affects the correct answer to part c and d.
Because the pH is 4.23, I know the hydrogen ion concentration must be 10^{-4.23}. I know that the equilibrium expression is K_a = \frac{(A^{-})(H^{+})}{HA}. So, I know the numerator is (10^{-4.23})^2. But, what is the denominator?
(a) From the information above, calculate the molecular weight of ascorbic acid.
(b) When 20.00 milliliters of NaOH had been added during the titration, the pH of the solution was 4.23. Calculate the acid ionization constant for ascorbic acid.
(c) Calculate the equilibrium constant for the reaction of the ascorbate ion, A¯, with water.
(d) Calculate the pH of the solution at the equivalence point of the titration.
This was an AP Chemistry question in 1989.
I had no problems doing the first part. At the equivalence point, the number of moles of acid is equal the number of moles of base. I used this relationship to find out the number of moles, and grams / moles = molecular weight.
I have problems with part b, which consequently affects the correct answer to part c and d.
Because the pH is 4.23, I know the hydrogen ion concentration must be 10^{-4.23}. I know that the equilibrium expression is K_a = \frac{(A^{-})(H^{+})}{HA}. So, I know the numerator is (10^{-4.23})^2. But, what is the denominator?
