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In an experiment, sulfuric acid reacted with different volumes of sodium thiosulfate in water. A yellow precipitate was formed during the reaction. A cross drawn at the base of each flask became gradually invisible due to the formation of this yellow precipitate. The time taken for the cross to become invisible was recorded. A partial record of the experiment is shown.

Experimental Record
\begin{tabular}{|l|l|l|l|l|}
\hline Flask & \begin{tabular}{c}
Volume of \\
[tex]$H_2SO_4$[/tex]
\end{tabular} & \begin{tabular}{c}
Volume of \\
Sodium Thiosulfate
\end{tabular} & \begin{tabular}{c}
Volume of \\
Water
\end{tabular} & Time \\
\hline 1 & 5 mL & 50 mL & 0 mL & 19 seconds \\
\hline 2 & 5 mL & 40 mL & 10 mL & \\
\hline 3 & 5 mL & 30 mL & 20 mL & \\
\hline 4 & 5 mL & 20 mL & 30 mL & \\
\hline
\end{tabular}

Based on factors that affect the rates of chemical reactions, which of the following would describe the trend expected in the table?

A. As volume of sulfuric acid increases, time decreases.
B. As volume of sulfuric acid remains the same, time increases.
C. As volume of sodium thiosulfate increases, time decreases.
D. As volume of sodium thiosulfate remains the same, time increases.



Answer :

GinnyAnswer
Let's analyze the given experimental setup and the factors affecting the rate of reaction to answer this question.

### Table Summary:
| Flask | Volume of H₂SO₄ | Volume of Sodium Thiosulfate | Volume of Water | Time |
|-------|------------------|-----------------------------|-----------------|--------------|
| 1 | 5 mL | 50 mL | 0 mL | 19 seconds |
| 2 | 5 mL | 40 mL | 10 mL | ? |
| 3 | 5 mL | 30 mL | 20 mL | ? |
| 4 | 5 mL | 20 mL | 30 mL | ? |

### Key Observations:
1. Volume of H₂SO₄: This is kept constant at 5 mL in each flask.
2. Volume of Sodium Thiosulfate: This decreases from 50 mL in Flask 1 to 20 mL in Flask 4.
3. Volume of Water: This increases from 0 mL in Flask 1 to 30 mL in Flask 4.
4. Time for Cross to Become Invisible: Given for Flask 1 (19 seconds) and unknown for other flasks.

### Trend Analysis:
- From Flask 1 to Flask 4, as the volume of sodium thiosulfate decreases (while keeping the volume of H₂SO₄ constant), we would expect the rate at which the cross becomes invisible to decrease.
- The concentration of sodium thiosulfate decreases from Flask 1 to Flask 4 because a larger volume of water is added, diluting the sodium thiosulfate solution.
- Rate of Reaction: Generally, for reactions involving a solid precipitate:
- Higher concentration of reactant (sodium thiosulfate) leads to a faster reaction.
- Lower concentration of reactant (sodium thiosulfate) leads to a slower reaction.

### Conclusion:
- Looking at the options provided:
- The volume of sulfuric acid increases, time decreases: Not applicable since H₂SO₄ volume is constant.
- The volume of sulfuric acid remains the same, time increases: Observing the trend, time should indeed increase as concentration of sodium thiosulfate decreases.
- As volume of sodium thiosulfate increases, time decreases: This matches our observed trend. More sodium thiosulfate means a faster reaction because of higher concentration, leading to less time for the cross to become invisible.
- The volume of sodium thiosulfate remains the same, time increases: Not applicable because the volume changes.

### Correct Answer:
As the volume of sodium thiosulfate increases, time decreases.

This conclusion is aligned with the trend that a higher concentration of sodium thiosulfate results in a faster reaction, making the precipitate form more quickly, which in turn makes the cross disappear faster.

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