Question

A substance contains {\text{0}}{\text{.25 % }} iron by weight. The molecular mass of the substance is 89600. The number of iron atoms per molecule of the substance are:
A.5
B.4
C.3
D.2

Answer 1

The solution of this problem requires the knowledge of the mole concept, where one mole of a substance is defined as $6.023 \times {10^{23}}$ particles of that substance. This is equal to the molecular weight if the substance is a compound and equal to the atomic weight if it is an element.

Complete step by step answer:
The molecular weight of the given substance = 89600,
Now,{\text{0}}{\text{.25 % }} of that substance = $\dfrac{{{\text{0}}{\text{.25}}}}{{100}}{\text{ }} \times 89600$ = 224
So the weight of iron in the substance = 224.
Now, the weight of one atom of iron = $55.845 \approx 56$ atomic mass units.
Therefore, $56$ atomic mass units of iron = 1 atom of iron.
So, 224 atomic mass units of iron = $\dfrac{1}{{56}} \times 224 = 4$ atoms of iron.
Hence the mass of four atoms of iron is equal to 224 atomic mass units and it is the number of atoms of iron present in one molecule of the substance.

Thus, the correct answer is option B.

Note:
The atomic mass is the average mass of the atoms of an element measured in atomic mass units or Daltons. The atomic mass is the weighted average of all the isotopes of that element in which the mass of each isotope of that element is multiplied by the abundance of that particular isotope.
One atomic mass unit is a unit of mass, widely used in physics and chemistry as $\dfrac{1}{{12}}$ the weight of a C-12 isotope in its ground state.