Question

If the reactant Magnesium reacts with Reactant Hydrochloric acid to form the product Magnesium chloride + Hydrogen, then what will be the atomic mass of both the product?

(a) 90

(b) 97

(c) 80

(d) 87

This question was posed to me during a job interview.

This question is from Law of Conservation of Mass topic in chapter Material Balance of Bioprocess Engineering

Answer 1

Correct option is (b) 97

Easiest explanation: Mg (Magnesium) + 2 HCl (Hydrochloric acid) = MgCl2 (Magnesium chloride) + H2 (Hydrogen),

Atomic masses: Mg = 24, H = 1, Cl = 35.5

one atom of Mg, 2 atoms of H and 2 atoms of Cl on both sides of the equation,

Law of conservation of mass balance: 24 + 2 × (1+ 35.5) = 24+ (2 × 35.5) + (2×1) = 97 (Both equal 97)

Note the subscript 2 after the Cl in magnesium chloride or the 2 after the H in the hydrogen molecule, means two atoms of that element.

The 2 before the HCl doubles the number of hydrochloric acid molecules.