Question

A substance contains 0.25 % iron by weight. The molecular mass of the substance is 89600. The number of iron atoms per molecule of the substance is:
A. 5
B. 4
C. 3
D. 2

Answer 1

The number of atoms in any substance (element) can be determined by mole concept. One mole is defined as the amount of substance that contains as many entities (atoms, molecules, or other particles) as these are in exactly 12 g of
${C^{12}}$ isotope.

Complete step by step answer:
Given, the substance contains = 0.25 % of iron.
The molecular mass of substance = 89600 g/mol
To find the number of iron atoms per molecule of the substance we have to first calculate the mass of iron.
Therefore, mass of iron in a substance = $\dfrac{{0.25}}{{100}} \times 89600$
= 224 g
But the atomic mass of iron Fe is = 56 g.
To find number of iron atoms per molecule of the substance,
Now, number of iron atom in a substance = $\dfrac{{Mass{\text{ of iron atom in a substance}}}}{{Atom{\text{ mass of iron atom}}}}$
By putting the values of mass of iron atom and atomic mass of iron atom, we will get.
Number of iron atom per molecule in a substance$= \dfrac{{224}}{{56}}$
= 4

Hence, the correct option is (B).

Note: All the calculations in chemistry that include number of atoms are based on mole concept. We must know the atomic mass or molecular mass of the substance to find out the mole of any particular substance.