In an experiment to determine the percentage of humus and water in a soil sample, the following results were…

Analyze the question: We need the percentage of humus in the soil sample. Humus is organic matter, which burns off when the dried soil is roasted (heated strongly). Water is lost earlier, during oven drying.

Concept:

• Loss in weight on oven drying = water
• Loss in weight on roasting = humus (organic matter)
• Percentage of humus = (mass of humus ÷ mass of original soil) × 100

Step-by-step solution:

1. Mass of original (fresh) soil = 101.5 − 80.5 = 21.0 g
2. Mass of water lost = 101.5 − 99.0 = 2.5 g
3. Mass of dry soil = 99.0 − 80.5 = 18.5 g
4. Mass of humus burnt off = 99.0 − 95.5 = 3.5 g

Percentage of humus:

• Based on the original soil sample (21.0 g): 3.5/21.0 × 100 = 16.7%
• Based on the dry soil (18.5 g): 3.5/18.5 × 100 = 18.9% (this is not an option)

The question asks for the percentage of humus "in the soil sample", so we use the original sample mass of 21.0 g. That gives 16.

(For comparison, water percentage = 2.5/21.0 × 100 = 11.9%.)

Answer: A. 16.7%

Exam tip: Remember the order: oven drying removes water, roasting removes humus. Always check which mass the question wants you to divide by. When it says "soil sample", use the original fresh soil mass (basin and soil minus basin alone).