Determining K for a Weak Acid Name: Section Date Pre-Lab Questions 1. A student

Question

Determining K for a Weak Acid Name: Section Date Pre-Lab Questions 1. A student performing this experiment spilled a small amount of 0.500 M NaOH on the Describe the procedures to be followed to clean up this spil. bench top. 2. A student is trying to identify an unknown acid. The label on the acid bottle says 0.15 M but the identity of the acid on the label has been obscured. The student prepares a solution by taking 50.0 mL of the acid, adding 20.0 mL of 0.05 M NaOH, and diluting the mixture to a final volume of 150.0 mL. The pH of the solution was 4.28. Calculate the K, for this unknown acid. Show all work. 3. The student in Question 2 forgot to calibrate the pH meter before performing the analysis. The pH reading was 0.12 pH units higher than the actual pH. (a) What was the real K, for the unknown acid? (b) Calculate the percent error that resulted from the uncalibrated pH meter. 91

Explanation / Answer

1. Cleaning of spilled NaOH. Wash with saturated aqueous NH4Cl solution to neutralize NaOH. Wash with lots of water.

2. pH = -log[H3O+] = 4.28

[H3O+] = 5.25 x 10^-5 M

[acid] = [HA] = 0.05 M x 20 ml/50 ml = 0.02 M

[A-] = [H3O+]

Ka = [H3O+][A-]/[HA]

     = (5.25 x 10^-5)^2/0.02

     = 1.4 x 10^-7

3. student forgot to calibrate pH meter, read 0.12 higher than actual value,

(a) real Ka

pH = -log[H3O+] = 4.16

[H3O+] = [A-] = 6.92 x 10^-5 M

Ka = (6.92 x 10^-5)^2/0.02

     = 2.40 x 10^-7

(b) percent error = (2.40 x 10^-7 - 1.40 x 10^-7) x 100/2.40 x 10^-7 = 41.67%

Related Questions

Navigate

• Browse (All)
• Browse D
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.