In part 2 of this experiment, you used the indicator methyl orange. Why was this

Question

In part 2 of this experiment, you used the indicator methyl orange. Why was this indicator chosen instead of phenolphthalein? Why is phenolphthalein not an appropriate indicator for this equilibrium? CH_3COOH(aq) + H_2O(l) CH_3COO^- (aq) + H_3O^+ (aq) Since no species in this reaction is colored, an auxiliary reagent is needed to help detect any shift in the equilibrium You will use an indicator, methyl orange, for this purpose. In strongly acidic solutions (high H_3O^+ concentrations), methyl orange is red A decrease in H_3O^+ concentration will cause a color change from red to yellow, with an intermediate color of orange. Pour 10-15 drops of 01 M acetic acid, Ch_3COOH, into a test tube and add 1 or 2 drops of methyl orange. Place a few crystals of sodium acetate, CH_3COONa, in the solution and shake gently to dissolve them. Explain your observations. Do not do experimentally, but reason out what change you would see if instead of sodium acetate, you added a few drops of 1 M NaOH. Explain.

Explanation / Answer

The added salt, sodium acetate, has an CH3COO- ion common with acetic acid, a weak acid that dissociates in water to give acetate ion. Adding acetate ion produces a change in the colour of the indicator. As the hydrogen ion concentration became smaller when the sodium acetate was added. Therefore the colour of the indicator will change. The effect on the dissociation of acetic acid that is produced by adding sodium acetate is called the common ion effect..

2) In this reaction, the base is strong and the acid is weak. For the weak acid-strong base titration, the pH will change rapidly at the very beginning. As the H+ concentration would rapidly decrease, so the colour would rapidly change to yellow.

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