Fortification of food with iron is done using FeSO₄ · 7 H₂ O. The mass in grams of the FeSO₄. 7 H₂ O required to achieve 12 ppm of iron in 150 kg of wheat is ______ (Nearest integer)
[Given: Molar mass of Fe, S and and O respectively are 56, 32 and 16 g mol⁻¹ ]

Solution

Chemistry ma’am · 3 steps

First find how much iron 12 ppm means, then scale it up to the whole hydrated salt that carries it.

1. Mass of iron. 12 ppm means 12 g of iron in every 10⁶ g of wheat. The wheat is 150 kg = 1.5 × 10⁵ g, so iron needed = 12/10⁶ × 1.5 × 10⁵ = 1.8 g.
2. Molar mass of salt. Each formula unit of FeSO₄.7H₂O has one Fe atom. Taking H as 1, its molar mass is 56 + 32 + 64 + 7 × 18 = 278 g mol⁻¹, and 56 g of that is iron.

   mₛₐₗₜ = 1.8 g × 278/56 = 8.94 g

3. So. About 8.94 g of FeSO₄.7H₂O is needed, which rounds to 9.

The common slip: 5. Leaving out the seven waters and using FeSO₄ (152) gives 1.8 × 152/56 ≈ 4.9, so 5, not 9.

The idea to keep. When ppm is set for one element, find that element's mass first. Then multiply by (molar mass of the compound ÷ mass of that element in one formula unit), water of crystallisation included.

Answer from the NTA final answer key. Solution by PrepWiser.