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 93

Chapter 3, Problem 93

An unknown liquid is composed of 34.31% C, 5.28% H, and 60.41% I. The molecular weight found by mass spectrometry is 210.06. What is the molecular formula of the compound?

Verified step by step guidance

Calculate the moles of each element in 100 grams of the compound: For carbon (C), use the formula: moles of C = (34.31 g) / (12.01 g/mol). For hydrogen (H), use: moles of H = (5.28 g) / (1.008 g/mol). For iodine (I), use: moles of I = (60.41 g) / (126.90 g/mol).

Determine the mole ratio of the elements by dividing each element's moles by the smallest number of moles calculated in the previous step.

Round the mole ratios to the nearest whole number to find the empirical formula. If necessary, multiply all ratios by the same factor to get whole numbers.

Calculate the empirical formula mass by adding the atomic masses of the elements in the empirical formula.

Determine the molecular formula by dividing the given molecular weight (210.06 g/mol) by the empirical formula mass. Multiply the subscripts in the empirical formula by this factor to get the molecular formula.

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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 the elements in a compound. To determine it, the percentage composition of each element is converted to moles, and then the ratios are simplified. This formula provides a foundational understanding of the compound's composition before deriving the molecular formula.

Molecular Weight

Molecular weight, or molar mass, is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). It is calculated by summing the atomic weights of all atoms in the molecular formula. In this question, the molecular weight obtained from mass spectrometry (210.06 g/mol) is crucial for determining the molecular formula from the empirical formula.

Molecular Formula

The molecular formula indicates the actual number of atoms of each element in a molecule of a compound. It can be derived from the empirical formula by multiplying the subscripts by a whole number, which is determined by dividing the molecular weight by the empirical formula weight. This formula provides a complete picture of the compound's composition.

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