Choose the molecular formula of an iron oxide in which the mass percentages of iron and oxygen are 69.9% and 30.1%, respectively, and the molecular mass is 160.
(A) FeO
(B) Fe3O4
(C) Fe2O3
(D) FeO2

To determine the molecular formula of the iron oxide with the given mass percent composition, we can follow these steps:

Calculate the moles of iron (Fe) and oxygen (O) in the compound based on the given mass percentages.

Determine the simplest whole number ratio of moles of iron to moles of oxygen.

Use the ratio to write the molecular formula.

Given:

Mass percent of iron (Fe) = 69.9%
Mass percent of oxygen (O) = 30.1%
Molecular mass = 160 g/mol
Step 1: Calculate the moles of iron and oxygen.

Moles of Fe = (Mass percent of Fe / Atomic mass of Fe) / 100
Moles of Fe = (69.9 / 55.85) / 100 = 0.125 moles

Moles of O = (Mass percent of O / Atomic mass of O) / 100
Moles of O = (30.1 / 16) / 100 = 0.188125 moles

Step 2: Determine the simplest whole number ratio.

Divide the moles of Fe and O by the smaller value (0.125 moles in this case):
Moles of Fe ratio = 0.125 moles / 0.125 moles = 1
Moles of O ratio = 0.188125 moles / 0.125 moles = 1.505
The ratio of moles of Fe to moles of O is approximately 1:1.505.

Step 3: Write the molecular formula using the ratio.
The molecular formula of the iron oxide is Fe₁O₁. However, we usually don't write the subscript 1, so the molecular formula is just FeO.

So, the correct answer is (A) FeO.