A compound contains two elements \(^{*} X^{\prime}\) and ' \(Y^{\prime}\) in the ratio of \(50 \%\) each. Atomic mass of \(^{\prime} X^{*}\) is 20 and \({ }^{\prime} Y\) is \(40 .\) What can be its simplest formula? (a) \(X Y\) (b) \(X_{2} Y\) (c) \(X Y_{2}\) (d) \(X_{2} Y_{3}\)

Molecular formula calculation is a process that gives us the actual number of atoms of each element present in a molecule. It is vital for understanding the composition of compounds and for conducting various chemical calculations. To begin with, one must know the atomic masses of the constituent elements, as well as the percentage composition of the compound.

When working out the molecular formula, you start by converting these percentages to moles, because moles provide a way to easily compare amounts of substances. By using the atomic mass of the element, you divide the given percentage by the atomic mass to find the amount of moles. In the example given, both elements, X and Y, form 50% of the compound. Upon dividing each percentage by their respective atomic masses, the moles for each element are determined. This step forms the basis to move forward to the next crucial stage - finding the simplest mole ratio.

Let's remember, the molecular formula of a compound may sometimes be a multiple of its simplest formula (empirical formula). The simplest formula reflects the smallest whole number ratio of atoms in the compound, which is what the provided exercise aims to find. In some cases, to find the molecular formula, additional information, such as the molar mass of the compound, might be needed to compare with the mass of the empirical formula.

The mole ratio is key in determining the simplest formula of a compound. It's the proportion of the moles of one element to the moles of another element in a compound. After finding the number of moles of each element, as described earlier, calculating this ratio allows for the determination of the empirical formula.

To achieve a whole number ratio, which is necessary for a proper formula, the moles of each element are divided by the smallest amount of moles found. This step is essential because chemical formulas must have whole number subscripts. In our example with elements X and Y, we divide the moles of X and Y by 1.25 moles—the smallest value obtained—to get a simple ratio of 2:1. This ratio translates directly into the subscripts in the chemical formula, leading to the empirical formula of X₂Y.

The ability to find this mole ratio is a fundamental skill in chemistry, as it directly impacts the ability to write chemical formulas, balance chemical equations, and calculate reactants and products in chemical reactions.

Percentage composition is an expression of the relative mass each element contributes to the total mass of a compound. It plays a crucial role in the determination of empirical and molecular formulas. To find the percentage composition, one would typically divide the mass of each element by the total mass of the compound and then multiply by 100 to get a percentage.

In our textbook example, where each element represents 50% of the compound's mass, this information serves as a starting point for calculating the moles of each element. These percentage values are indicative of how much of each element is present in a 100-gram sample of the compound. In scenarios where the percentage composition is not provided outright, it can be deduced from experimental data or molecular formulas.

Understanding percentage composition aids students in recognizing how elements combine in fixed ratios to form compounds. This concept is the bridge between the quantitative mass data of a compound and the qualitative mole ratios that determine the compound's simplest chemical formula, ultimately connecting the empirical data to theoretical chemical knowledge.