Ch.3 - Molecules and Compounds

Tro - Chemistry: A Molecular Approach 6th Edition

Tro6th EditionChemistry: A Molecular ApproachISBN: 9780137832217Not the one you use?Change textbook

Tro 6th Edition

Ch.3 - Molecules and Compounds

Problem 128

Chapter 3, Problem 128

Fructose is a common sugar found in fruit. Elemental analysis of fructose gives the following mass percent composition: C 40.00%, H 6.72%, O 53.28%. The molar mass of fructose is 180.16 g/mol. Find the molecular formula of fructose.

Verified step by step guidance

insert step 1: Assume you have 100 grams of fructose. This allows you to directly convert the mass percentages into grams: 40.00 g of C, 6.72 g of H, and 53.28 g of O.

insert step 2: Convert the masses of each element to moles by dividing by their respective atomic masses: C (12.01 g/mol), H (1.008 g/mol), and O (16.00 g/mol).

insert step 3: Determine the simplest whole number ratio of moles of each element by dividing each by the smallest number of moles calculated in the previous step.

insert step 4: Use the simplest whole number ratio to write the empirical formula of fructose.

insert step 5: Calculate the empirical formula mass and divide the given molar mass of fructose (180.16 g/mol) by the empirical formula mass to find the ratio. Multiply the subscripts in the empirical formula by this ratio to find 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 the empirical formula from mass percent composition, the mass percentages are converted to moles by dividing by the atomic masses of the elements. The resulting mole ratios are then simplified to the smallest whole numbers, providing a foundational understanding of the compound's composition.

Molar Mass

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 masses of all atoms in the molecular formula. Knowing the molar mass is crucial for converting between grams and moles, and it helps in determining the molecular formula when combined with the empirical formula.

Molecular Formula

The molecular formula indicates the actual number of each type of atom 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 molar mass of the compound by the molar mass of the empirical formula. This formula provides more detailed information about the compound's structure and composition.

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Epsom salts is a hydrated ionic compound with the following formula: MgSO₄ • x H₂O. A 4.93-g sample of Epsom salts is heated to drive off the water of hydration. The mass of the sample after complete dehydration is 2.41 g. Find the number of waters of hydration (x) in Epsom salts.

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Estrone, which contains only carbon, hydrogen, and oxygen, is a female sexual hormone in the urine of pregnant women. Combustion analysis of a 1.893-g sample of estrone produces 5.545 g of CO₂ and 1.388 g H₂O. The molar mass of estrone is 270.36 g/mol. Find its molecular formula.

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