Chapter 29: Problem 7

A student says "If I know the empirical formula for a compound and I also know the molar mass of the compound, then I can determine the molecular formula of the compound." Explain why this student is correct, and why simply knowing the empirical formula is not sufficient to determine the molecular formula.

Short Answer

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The student is correct because, while the empirical formula gives the simplest ratio of atoms of each element in a compound, it doesn't provide information about the actual number of atoms in each molecule. By knowing the molar mass of the compound, one can find the ratio in which the empirical formula needs to be multiplied to get the molecular formula, thus making it possible to determine the molecular formula.

Step by step solution

Understand the Difference Between Empirical And Molecular Formulas

The empirical formula represents the simplest whole number ratio of the different types of atoms in a compound. However, this doesn't indicate the actual number of atoms in each molecule of the compound, which is what the molecular formula shows.

Examine the Relationship Between Empirical Formula, Molar Mass, and Molecular Formula

If the molar mass of the compound and its empirical formula are known, the molecular formula can be determined. This is done by first determining the molar mass of the empirical formula. Then, divide the given molar mass of the compound by the molar mass of the empirical formula. This results in a multiplication factor that, when applied to the empirical formula, yields the molecular formula.

Understand Why Knowing Only the Empirical Formula is Insufficient

Knowing only the empirical formula isn't enough to determine the molecular formula. This is because there might be more than one type of molecules with different molecular formulas but with the same empirical formula. In other words, molecules differing by a whole number factor will have the same empirical formula. Hence, more information such as the molar mass is needed to get the molecular formula.

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Short Answer

Step by step solution

Understand the Difference Between Empirical And Molecular Formulas

Examine the Relationship Between Empirical Formula, Molar Mass, and Molecular Formula

Understand Why Knowing Only the Empirical Formula is Insufficient

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