The rate of a chemical reaction is obtained by dividing the amount of a chemical consumed in a reaction by the time in which the chemical is consumed. The rate at which a chemical reaction occurs is dependent on many variables, two of which are the concentration of a chemical in solution and the temperature at which the reaction occurs. In order to determine the effect of concentration and temperature on reaction rates, a chemist studied the following reaction sequence.HCHO + Na2SO3 + H2O → NaO3SOCH3 + NaOHReaction 1NaOH+ NaHSO3 → Na2SO3 + H2OReaction 2While Reaction 1 occurs slowly, Reaction 2 happens instantaneously. Both of the reactions occur in the same solution and base (NaOH) is built up in solution until all the NaHSO3 is used. Phenolphthalein, which is also in the solution, will cause the solution to turn pink when a measurable amount of base is present.Experiment 1:A standard solution of formaldehyde (HCHO) in water was prepared. This solution contained 0.3 mole of HCHO per liter of solution (0.3 M HCHO) and a small amount of phenolphthalein. This solution was diluted further with varying amounts of water and mixed with 10.0 mL of a 0.01 M solution of both Na2SO3 and NaHSO3 in water. The time required for pink color to appear is shown in Table 1.The function of phenolphthalein in both experiments was to:
Correct answer: A. Signal when the NaOH was completely used up
A. Signal when the NaOH was completely used up
B. Signal when the HCHO was completely consumed
C. React with the HCHO solution
D. Alter the rate of reaction
Explanation
The correct answer is Option A: Signal when the NaOH was completely used up. Phenolphthalein was employed as an indicator to detect the presence of NaOH during the reactions. The appearance of the pink color signified the completion of the NaOH reaction. The other options are incorrect as phenolphthalein's role was not related to reacting with HCHO, causing reactions, or affecting reaction rates.
Last updated 3 September 2026

Question

The rate of a chemical reaction is obtained by dividing the amount of a chemical consumed in a reaction by the time in which the chemical is consumed. The rate at which a chemical reaction occurs is dependent on many variables, two of which are the concentration of a chemical in solution and the temperature at which the reaction occurs. In order to determine the effect of concentration and temperature on reaction rates, a chemist studied the following reaction sequence.HCHO + Na2SO3 + H2O → NaO3SOCH3 + NaOHReaction 1NaOH+ NaHSO3 → Na2SO3 + H2OReaction 2While Reaction 1 occurs slowly, Reaction 2 happens instantaneously. Both of the reactions occur in the same solution and base (NaOH) is built up in solution until all the NaHSO3 is used. Phenolphthalein, which is also in the solution, will cause the solution to turn pink when a measurable amount of base is present.Experiment 1:A standard solution of formaldehyde (HCHO) in water was prepared. This solution contained 0.3 mole of HCHO per liter of solution (0.3 M HCHO) and a small amount of phenolphthalein. This solution was diluted further with varying amounts of water and mixed with 10.0 mL of a 0.01 M solution of both Na2SO3 and NaHSO3 in water. The time required for pink color to appear is shown in Table 1.The function of phenolphthalein in both experiments was to:

Correct answer: A. Signal when the NaOH was completely used up

A. Signal when the NaOH was completely used up
B. Signal when the HCHO was completely consumed
C. React with the HCHO solution
D. Alter the rate of reaction

Explanation

The correct answer is Option A: Signal when the NaOH was completely used up. Phenolphthalein was employed as an indicator to detect the presence of NaOH during the reactions. The appearance of the pink color signified the completion of the NaOH reaction. The other options are incorrect as phenolphthalein's role was not related to reacting with HCHO, causing reactions, or affecting reaction rates.

Last updated 3 September 2026

TestUstad · Accepted Answer

Signal when the NaOH was completely used up. The correct answer is Option A: Signal when the NaOH was completely used up. Phenolphthalein was employed as an indicator to detect the presence of NaOH during the reactions. The appearance of the pink color signified the completion of the NaOH reaction. The other options are incorrect as phenolphthalein's role was not related to reacting with HCHO, causing reactions, or affecting reaction rates.

TestUstad · Answer

Signal when the HCHO was completely consumed

TestUstad · Answer

React with the HCHO solution

TestUstad · Answer

Alter the rate of reaction

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