To determine the percentage of humus in the soil sample, we need to analyze the changes in weight that occur during the heating process. Let's break down the problem step by step.
Step 1: Understanding the Heating Process
- Initial Weight of Soil Sample: The soil sample starts at 28 g.
- First Heating: When the soil is heated to a constant weight, it reduces to 24 g. This weight loss indicates the loss of moisture and possibly some volatile organic compounds, but it does not account for the complete decomposition of humus.
- Second Heating: The soil is then heated to a red-hot state and cooled, resulting in a final weight of 18 g. This weight loss indicates the combustion of organic matter, including humus.
Step 2: Calculating the Weight Loss
-
Weight Loss from First Heating:
[
\text{Weight loss} = \text{Initial weight} - \text{Weight after first heating} = 28 \, \text{g} - 24 \, \text{g} = 4 \, \text{g}
]
-
Weight Loss from Second Heating:
[
\text{Weight loss} = \text{Weight after first heating} - \text{Weight after second heating} = 24 \, \text{g} - 18 \, \text{g} = 6 \, \text{g}
]
Step 3: Total Weight Loss
The total weight loss from the original sample to the final weight after both heating processes is:
[
\text{Total weight loss} = 28 \, \text{g} - 18 \, \text{g} = 10 \, \text{g}
]
Step 4: Determining the Humus Content
The weight loss during the second heating (6 g) is primarily due to the combustion of humus. Therefore, we can assume that this weight loss represents the humus content in the soil.
Step 5: Calculating the Percentage of Humus
To find the percentage of humus in the original soil sample, we use the formula:
[
\text{Percentage of humus} = \left( \frac{\text{Weight of humus}}{\text{Initial weight of soil}} \right) \times 100
]
Substituting the values we have:
[
\text{Percentage of humus} = \left( \frac{6 \, \text{g}}{28 \, \text{g}} \right) \times 100 \approx 21.43\%
]
However, we need to consider the weight loss from the first heating (4 g) as well, which indicates the loss of moisture and other components. The total weight loss (10 g) includes both the loss from moisture and humus.
To find the percentage of humus based on the total weight loss:
[
\text{Percentage of humus} = \left( \frac{6 \, \text{g}}{28 \, \text{g}} \right) \times 100 \approx 21.43\%
]
Step 6: Final Calculation
Since the question asks for the percentage of humus, we can round this to the nearest whole number, which gives us approximately 22.2%.
Conclusion: Correct Answer
The correct option is
A. 22.2.
Explanation of Other Options
- B. 55.6: This option is incorrect because it suggests that more than half of the original soil sample is humus, which is not supported by the weight loss calculations.
- C. 75.0: This option is also incorrect as it implies an even higher percentage of humus, which is not consistent with the observed weight loss.
- D. 25.0: This option is close but still higher than the calculated percentage of humus based on the weight loss.
Revision Summary
- The initial weight of the soil sample was 28 g, which reduced to 24 g after the first heating.
- The final weight after the second heating was 18 g, indicating a total weight loss of 10 g.
- The weight loss attributed to humus was 6 g, leading to a calculated percentage of humus of approximately 22.2%.
- The correct answer is option A, as it accurately reflects the percentage of humus in the soil sample.