Common-Ion Effect and Buffers

EXPERIMENT 4: COMMON-ION EFFECT AND BUFFERS

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ANSWERS TO QUESTIONS:

1. ACCOUNT FOR THE DIFFERENCES IN COLOR OF SOLUTIONS 1 AND 2 AFTER ADDING METHYL ORANGE INDICATOR.

A red color is given off by a methyl orange indicator in an acidic solution (pH<3.1), whereas a yellow color is observed for a basic solution (pH>4.5). An intermediate between the two colors i.e. orange, can be observed at pH levels of 3.1-4.4. When methyl orange was added to solutions 1 (1a, 1b, 1c) and 2 (2a, 2b, 2c) containing 30 mL 0.10 M CH3COOH and 15 mL 0.20 M CH3COOH-15 mL 0.20 M NaCH3COO respectively, (with each solution divided into three 10-ml portions), color differences were observed. Solutions 1a and 1c’s colors were between red and salmon pink with Solution 1c being paler than 1a, while Solution 1b turned yellow, which tells us that Solution 1a was the most acidic, with Solution 1b being the most basic, and Solution 1c somewhere in between.

Meanwhile, adding methyl orange to Solution 2 (2a, 2b, 2c) turned all three solutions into a yellow color with not much difference. This indicated that all three solutions had a pH of more than 4.5 and were more basic than those of Solution 1.

2. ACCOUNT FOR THE DIFFERENCES IN COLOR OF SOLUTIONS 3 AND 4 AFTER ADDING PHENOLPHTHALEIN INDICATOR.

A phenolphthalein indicator is observed to be colorless in an acidic solution (pH<8.3) and becomes reddish in a basic solution (pH>10.0). An intermediate is observed at pH levels of 8.3 to 10.0, where a very light pink color can be observed. When phenolphthalein was added to Solutions 3 and 4, which contained 30 ml 0.10 M NH3 and 15 mL 0.20 M NH3 -15 mL 0.20 M NH4Cl respectively (with each solution divided into three 10-mL portions), color differences were observed. Solutions 3a, 3b, and 3c all gave off very similar dark pink-fuchsia colors (3b being the palest; 3b being the darkest), which tells us that Solution 3 had...