MP Board · Class 9 · Science · Improvement in Food ResourcesSolve the following quantitative problem and answer the theoretical question: Part A: A farmer owns a field of area $5\text{ hectares}$. The recommended dose of Nitrogen for his crop is $120\text{ kg/ha}$. He decides to supply Nitrogen using Urea, $\text{CO(NH}2)2$. Calculate the molecular mass of Urea and the percentage of Nitrogen in Urea. (Given atomic masses: $\text{N} = 14\text{ u}$, $\text{C} = 12\text{ u}$, $\text{O} = 16\text{ u}$, $\text{H} = 1\text{ u}$) Calculate the total mass of Urea required for the entire $5\text{-hectare}$ field. Part B: Briefly state two biotic and two abiotic factors responsible for crop losses during storage of harvested grains.

Part A: Calculations

1. Molecular Mass and % Nitrogen in Urea $\text{CO(NH}_2)_2$:

• Chemical formula of Urea: $\text{CH}_4\text{N}_2\text{O}$

• Molar mass calculation: $$\text{Mass of C} = 1 \times 12 = 12\text{ u}$$ $$\text{Mass of O} = 1 \times 16 = 16\text{ u}$$ $$\text{Mass of N} = 2 \times 14 = 28\text{ u}$$ $$\text{Mass of H} = 4 \times 1 = 4\text{ u}$$ $$\text{Total Molecular Mass of Urea} = 12 + 16 + 28 + 4 = 60\text{ u}$$

• Percentage of Nitrogen in Urea: $$\text{Percentage of N} = \left( \frac{\text{Mass of N}}{\text{Total Molecular Mass}} \right) \times 100$$ $$\text{Percentage of N} = \left( \frac{28}{60} \right) \times 100 = 46.67%$$

2. Total Urea Required:

• Area of field $= 5\text{ hectares}$
• Nitrogen required per hectare $= 120\text{ kg}$
• Total Nitrogen required $= 5 \times 120 = 600\text{ kg}$
• Since Urea contains $46.67%$ Nitrogen: $$\text{Mass of Urea required} = \frac{\text{Total Nitrogen required}}{\text{Fraction of N in Urea}}$$ $$\text{Mass of Urea required} = \frac{600}{0.4667} = 1285.71\text{ kg}$$

Final Answer: Mass of Urea required $= 1285.71\text{ kg}$ (approx $1.286\text{ tonnes}$).

Part B: Factors Responsible for Storage Losses

• Biotic Factors:
  1. Insects and Pests: Directly feed on and contaminate stored grains.
  2. Fungi and Bacteria: Cause discoloration, rotting, and production of toxic substances under moist conditions.
• Abiotic Factors:
  1. Inappropriate Moisture: Excess moisture content in grains accelerates fungal growth and microbial decay.
  2. Unfavourable Temperature: High storage temperatures increase metabolic activities of micro-organisms, degrading seed quality.