You and a partner are asked to complete a lab entitled “Carbonates of Group 2 metal” that is scheduled to extend over two lab periods. The first lab, which is to be completed by your partner, is devoted to carrying out compositional analysis and determining the identity of the Group 2 metal (M). In the second lab, you are to determine the melting point of this compound. Upon going to the lab, you find two unlabeled vials containing white powder. You also find the following notes in your partner’s notebook—Compound 1: 40.04% M, 12.00% C, and 47.96% O (by mass); Compound 2: 69.59% M, 6.09% C, and 24.32% O (by mass). (a) What is the empirical formula for Compound 1 and the identity of M? (b) What is the empirical formula for Compound 2 and the identity of M? Upon determining the melting points of these two compounds, you find that both compounds do not melt up to the maximum temperature of your apparatus; instead, the compounds decompose and liberate a colorless gas. (c) What is the identity of the colorless gas?
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Step 1: To find the empirical formula for Compound 1, start by assuming you have 100 grams of the compound. This means you have 40.04 grams of M, 12.00 grams of C, and 47.96 grams of O.
Step 2: Convert the masses of each element in Compound 1 to moles by dividing by their respective atomic masses (M: unknown, C: 12.01 g/mol, O: 16.00 g/mol).
Step 3: Determine the mole ratio of the elements in Compound 1 by dividing each element's mole value by the smallest number of moles calculated in Step 2. This will give you the subscripts for the empirical formula.
Step 4: Repeat Steps 1-3 for Compound 2 using its composition: 69.59% M, 6.09% C, and 24.32% O. Convert these percentages to grams, then to moles, and find the mole ratio to determine the empirical formula.
Step 5: The colorless gas released upon decomposition of both compounds is likely carbon dioxide (CO₂), as carbonates typically decompose to release CO₂ gas.
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Empirical Formula
The empirical formula represents the simplest whole-number ratio of elements in a compound. To determine it, the mass percentages of each element are converted to moles, and then the mole ratios are simplified. This concept is crucial for identifying the composition of the compounds in the lab, as it provides insight into the elemental makeup of the substances being analyzed.
Group 2 metals, also known as alkaline earth metals, include elements like magnesium, calcium, and barium. These metals typically form ionic compounds with carbonates, which can be analyzed for their properties. Understanding the characteristics of these metals is essential for identifying the unknown metal (M) in the compounds, as they exhibit specific trends in reactivity and compound formation.
Decomposition reactions occur when a compound breaks down into simpler substances, often upon heating. In this context, the observation that both compounds decompose and release a colorless gas indicates a chemical change. Recognizing the type of gas produced, such as carbon dioxide from carbonates, is vital for understanding the behavior of the compounds during the melting point determination.
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