Analysis shows that FeO has a non-stoichiometric composition with formula Fe_0.95O. Give reason.

The non-stoichiometry occurs because of the instability in the FeO molecule as undergoing the oxidation of Fe²⁺ to Fe³⁺ effectively replacing a small portion of Fe²⁺ with two thirds their number of Fe³⁺.
Thus for every three "missing" Fe²⁺ ions, the crystal contains two Fe³⁺ ions to balance the charge. This change in composition violets the law of constant composition as the composition of the compound usually varies in a continuous manner over a narrow range. Thus FeO becomes a non-stoichiometric compound, and the formula is written as Fe_(1-x)O, where x is a small number representing the deviation from the "ideal" formula.
Generally, it is written as Fe_0.95O or found in the form of FeO.Fe₂O₃ represented as Fe₃O₄.