Question

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

Short answer

The formula of the compound is Al2S3.

Step-by-step solution

Determine the mass percent of the unknown element

Since the product contains 18.56% Al by mass, the mass percent of the unknown element can be found by subtracting the mass percent of Al from 100%: Mass percent of unknown element = 100% - 18.56% = 81.44%

Calculate the moles of aluminum and the unknown element

Assuming 100 grams of the compound, we can find the mass of Al and the unknown element in the compound. Then, we can use their molar masses to calculate moles. Mass of Al: 18.56 g (18.56% of 100 g) Moles of Al: \(\frac{18.56\,\text{g}}{26.98\,\text{g/mol}} = 0.688\,\text{mol}\) (molar mass of Al is 26.98 g/mol) Mass of unknown element: 81.44 g (81.44% of 100 g) We can denote the Group 6A element as X. In order to find the moles of X, we need more information about its molar mass. We can look at the possible Group 6A elements (O, S, Se, Te, Po) and determine which one would potentially form an ionic compound with Al and satisfy the required mass percentage.

Determine the molar ratio of aluminum and the unknown element

In order to obtain the empirical formula, we need to calculate the ratio of the moles of aluminum Al to the moles of the unknown element X. Let's consider Sulfur (S) as an example. Moles of S: \(\frac{81.44\,\text{g}}{32.07\,\text{g/mol}} = 2.537\,\text{mol}\) (molar mass of S is 32.07 g/mol) Now, we can find the ratio between the moles of Al and the moles of S: Molar ratio = \(\frac{0.688\,\text{mol Al}}{2.537\,\text{mol S}} = 0.271\)

Determine the empirical formula of the compound

An ionic compound consists of simple whole number ratios of atoms of each element. From our calculations, the molar ratio between Al and S (0.271) is approximately 1:3. Thus, the empirical formula of the compound is Al2S3. The formula of the compound is Al2S3.

Key concepts

Ionic Compounds

Ionic compounds are formed when atoms exchange electrons, creating a compound composed of positively and negatively charged ions. This typically occurs between metals, which lose electrons easily, and non-metals, which tend to gain them.
- Metals become cations (positively charged). - Non-metals become anions (negatively charged).
In the case of aluminum reacting with a Group 6A element, aluminum tends to lose three electrons to form Al³⁺ ions. The non-metal from Group 6A, being more electronegative, gains electrons to fulfill its need for a full outer electron shell, forming negatively charged ions.

Aluminum

Aluminum is a lightweight, silvery-white metal located in Group 13 of the periodic table. It is the most abundant metal in the Earth's crust and is highly reactive with oxygen, making it a good candidate for forming ionic compounds.
- It has the electron configuration [Ne] 3s² 3p¹. - Aluminum forms a +3 charge as it loses three electrons during bonding.
In the problem, aluminum reacts with a Group 6A element to form an ionic compound. Given its +3 oxidation state, aluminum forms stable ionic compounds by combining with elements that can balance its charge through their own oxidation states, like sulfur which can form a -2 charge.

Group 6A Elements

Group 6A elements in the periodic table include oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po). These elements are known for their tendency to form anions by gaining two electrons, resulting in a -2 charge.
- Oxygen, the most common, is often found in oxide compounds. - Sulfur belongs here and forms sulfides. - These elements are capable of forming compounds with metals, resulting in ionic compounds.
In the exercise, sulfur was considered. It can pair with aluminum to form Al₂S₃, as each sulfur ion requires two aluminum ions to balance the charge.

Molar Mass

Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). It is essential in chemistry for converting between mass and moles of a substance, allowing the determination of proportions in a compound.
- Formula: Molar Mass = Mass (g) / Moles (mol).
In this exercise, knowing the molar masses of aluminum (26.98 g/mol) and sulfur (32.07 g/mol) allows us to calculate the moles of each element present in a given mass.
This helps in determining the ratio of elements, which is crucial for finding the empirical formula.

Periodic Table

The periodic table is a systematic arrangement of chemical elements, organized based on atomic number, electron configurations, and recurring properties. It provides a valuable framework for understanding chemical behavior.
- Elements are arranged in periods (rows) and groups (columns). - Groups indicate elements with similar properties and valence electrons.
In the context of the problem, understanding the position of aluminum in Group 13 and Group 6A elements, such as sulfur, helps predict the type of compounds they form. Specifically, identifying potential elements for compound formation based on their periodic table group informs us about typical charges and combinations found in their ionic compounds.