When aluminum metal is heated with an element from Group 6A of the periodic table, an ionic compound forms. When the experiment is performed with an unknown Group 6 A element, the product is \(18.56 \%\) Al by mass. What is the formula of the compound?

Since the compound is \(18.56 \%\) Al by mass, the unknown element must compose the remaining mass, which is \(100\% - 18.56\% = 81.44 \%\).

Let's assume that we have a \(100 \thinspace g\) sample of this ionic compound. In this case, the mass of aluminum and the unknown element can be calculated by taking the respective percentages of the total mass: - Mass of Al = \((18.56 \%) \times 100 \thinspace g = 18.56 \thinspace g\) - Mass of unknown element = \((81.44 \%) \times 100 \thinspace g = 81.44 \thinspace g\)

Divide the mass of each element by their respective molar mass to find the moles of each element. The molar mass of aluminum is approximately \(26.98 \thinspace g/mol\), and let the molar mass of the unknown element be X. - Moles of Al = \(\frac{18.56 \thinspace g}{26.98 \thinspace g/mol}\) - Moles of unknown element = \(\frac{81.44 \thinspace g}{X \thinspace g/mol}\)

To find the empirical formula, we need to find the lowest whole number ratio of the moles of aluminum to the moles of the unknown element. Let's call this ratio n. n = \(\frac{\frac{18.56 \thinspace g}{26.98 \thinspace g/mol}}{\frac{81.44 \thinspace g}{X \thinspace g/mol}}\)

The elements in Group 6A are oxygen (O, atomic mass = 16), sulfur (S, atomic mass = 32), selenium (Se, atomic mass = 79), tellurium (Te, atomic mass = 128), and polonium (Po, atomic mass = 209). The moles ratio (n) calculated in the previous step will be different for each of these elements based on their atomic masses. However, only one of these elements will yield a whole number ratio with the moles of aluminum. Plug in the atomic mass of each element in the value of X and calculate the value of n. The element that results in a whole number ratio is the unknown element. Once you have identified the unknown element, use the whole number ratio as the subscript for each element to represent the empirical formula of the ionic compound.