Question

Read these instructions thoroughly. Following them carefully is part of this graded assignment. Show your work when there are calculations, write units where appropriate, and use the correct significant figures. Before attempting these problems: 1. Go back to your lecture notes and redo the 2 problems we did in class that are identical to the problems below. 2. Go to the textbook examples and check your learning problems on this material and redo them. They are also identical to the problems below. 3. AFTER you have done these things, sign your name here: Do the work on a separate piece of paper and staple it to this handout (required). Make sure your work is clear and easy to follow. Write out all relevant balanced chemical equations for each problem as we did in class. All chemical equations require balancing, correct charges, and phase designators. 15.00 mL of 0.350 hypochlorous acid is titrated with 0.175 M NaOH. Calculate the pH: 1) before adding any NaOH solution. 2) after adding 27.00 mL of the NaOH solution. 3) at the equivalence point. After you finish these 3 problems, go back to the beginning and make sure you understand every step you took Then go back to your lecture notes, the textbook, and the in class exercises on this material and make sure you understand those problems too and how they are the same as these problems.

Answer 1

1) Before adding of any NaOH, the concentration of hypochlorous acid is 0.350 M

Therefore,

[H⁺] = 0.350 N ( Hypochlorous acid is a mono basic acid, and hence it's Normality is equal to Molarity)

pH = -log [H⁺]

pH = -log 0.350

pH = 0.4559.

2) After adding the 27 mL of 0.175 M NaOH Solution, the pH of hypochlorous acid:

Step 1: Calculate the number of Milli equivalents of both NaOH and HOCl.

Number of Milli equivalents of NaOH = 0.175 N X 27 mL.

= 4.725

Number of milli equivalents of HOCl = 0.350 N X 15 mL

= 5.25

Step 2 : Calculation of the concentration of acid remaining in the solution.

Number of Milli equivalents of the acid left = Number of Milli equivalents of the acid - number of Milli equivalents of the base

Number of Milli equivalents of acid left = 5.25 - 4.725

Number of Milli equivalents of acid left = 0.525

Concentration of hypochlorous acid left = 0.525/42 mL (Total volume of solution).

Concentration of hypochlorous acid left = 0.0125 N

Step 3: Calculation of pH

pH = -log [H⁺]

pH = -log 0.0125N

pH = 1.90

3) pH at equivalent point is 7, because of complete neutralisation.