You have four 1,000-ml beakers filled with four different clear solutions: 0.1 M

Question

You have four 1,000-ml beakers filled with four different clear solutions: 0.1 M NaH_2PO_4 0.1 M Na_2HPO_4 0.1 M phosphate buffer, pH 7.2 Distilled water Oops! You forgot to label them and they all look alike. You get the congo red and thymolph-thalein from the lab and test a sample of each solution, labeling the beakers randomly as A, B, C, and D. You use congo red when HC1 is added to the sample, and thymolphthalein when NaOH is added. You get the following results. What is the identity of solutions A, B, C, and D? A B C D

Explanation / Answer

Answer:

A: 0.1M NaH2PO4

because congo red have transition range pH (3.0 - 5.0). Higher pH have red color. before addition it show red color in higher pH range due to acidic nature of sodium dihydrogen phosphate. After adding HCl color not change.

But in case of thymolphthalein solution colourless in at lower pH range. after addition of NaOH it show blue colour due to high pH transition range.

B. 0.1 M Na2HPO4

C. 0.1 M phosphate buffer

because after addition of HCl OR NaOH no color change is observed. buffer have property to resist change in pH after adding acid/base.

D. distilled water.

Note : see the transition range pH of indicatior. check the color change observed by indicator at low or high pH in transition range. this will give clear idea about solution.

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