You prepare a 0.100 M solution of an unknown weak acid HA and measure the pH at

Question

You prepare a 0.100 M solution of an unknown weak acid HA and measure the pH at 25°C. The pH of your solution is unique. Every student in the class may have a different pH value. Take the date of your birthday, and divide by 100. Add 3.50 to determine your ph For example, if your birthday is on the 23rd of some month, the pH of your solution is 3.73. 1. Based on your unique pH, determine the equilibrium concentration of [H,O'lE in your solution. 2. Determine the equilibrium concentrations of IA-lE and [HAJE in your solution. 3. Calculate the acid ionization constant K, for this new acid HA. -3? You then titrate 50.0 mL of this solution of HA (aq) with a 0.200 M solution of the strong base, NaOH (aq. 4. Calculate the volume of NaOH that must be added to reach equivalence 5. Calculate the pH of your solution at the equivalence point. 6. Calculate the the pH of the solution after adding a total of 30.0 mL of NaOH(aq).

Explanation / Answer

1)

pH = 3.73

pH = -log [H3O+]

[H3O+] = 10^-3.73

[H3O+] = 1.86 x 10^-4 M

2)

[A-] = 1.86 x 10^-4 M

[HA] = 0.0998 M

3)

Ka = x^2 / 0.1 - x

Ka = (1.86 x 10^-4)^2 / 0.1 - 1.86 x 10^-4

Ka = 3.47 x 10^-7

4)

50 x 0.1 = 0.200 x V

V = 25 mL

volume of NaOH added = 25 mL

5)

At equivalence point : salt remains.

salt concentration = 5 / 50 + 25 = 0.067 M

pH = 7 + 1/2 (pKa + log C)

     = 7 + 1/2 (6.46 + log 0.067)

pH = 9.64

6)

pH = 12.10

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