Ch.3 - Mass Relationships in Chemical Reactions

McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook

McMurry 8th Edition

Ch.3 - Mass Relationships in Chemical Reactions

Problem 112

Chapter 3, Problem 112

A pulverized rock sample believed to be pure calcium carbonate, CaCO3, is subjected to chemical analysis and found to contain 51.3% Ca, 7.7% C, and 41.0% O by mass. Why can't this rock sample be pure CaCO3?

Verified step by step guidance

Step 1: Understand the problem. The problem is asking why the rock sample can't be pure calcium carbonate (CaCO3) even though it contains the elements calcium (Ca), carbon (C), and oxygen (O).

Step 2: Recall the definition of a pure substance. A pure substance is a substance that has a fixed composition and distinct properties. In the case of calcium carbonate, the ratio of the masses of Ca, C, and O should be constant and equal to their ratio in the molecular formula of CaCO3.

Step 3: Calculate the percentage composition of Ca, C, and O in a pure CaCO3 sample. The molar mass of CaCO3 is the sum of the molar masses of Ca, C, and O, which are approximately 40.08 g/mol, 12.01 g/mol, and 16.00 g/mol respectively. The percentage composition is then calculated by dividing the molar mass of each element by the molar mass of CaCO3 and multiplying by 100%.

Step 4: Compare the calculated percentage composition of a pure CaCO3 sample with the given percentage composition of the rock sample. If the percentages do not match, then the rock sample cannot be pure CaCO3.

Step 5: Conclude that the rock sample is not pure CaCO3 if the percentage compositions do not match. This is because a pure substance should have a fixed composition, and any deviation from this composition indicates the presence of other substances.

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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. In the case of calcium carbonate (CaCO3), the empirical formula indicates a specific ratio of calcium, carbon, and oxygen. To determine if the rock sample is pure CaCO3, one must compare the mass percentages of the elements to the expected ratios in the empirical formula.

Mass Percent Composition

Mass percent composition is the percentage by mass of each element in a compound. For CaCO3, the theoretical mass percentages are approximately 40% Ca, 12% C, and 48% O. The given mass percentages of the rock sample (51.3% Ca, 7.7% C, and 41.0% O) do not match these theoretical values, indicating that the sample cannot be pure CaCO3.

Stoichiometry

Stoichiometry involves the calculation of reactants and products in chemical reactions based on the conservation of mass. It allows chemists to predict the amounts of substances consumed and produced in a reaction. In this context, stoichiometry helps to analyze the discrepancies in the mass percentages of the elements in the rock sample compared to those expected for pure CaCO3, leading to the conclusion that the sample contains other substances.

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