Textbook Question
What is the molecular formula of each of the following compounds? (a) empirical formula CH, molar mass 5 78.0 g/mol (b) empirical formula NH2Cl, molar mass = 51.5 g/mol
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Ch.3 - Chemical Reactions and Reaction Stoichiometry
Chapter 3, Problem 53a
Determine the empirical and molecular formulas of each of the following substances: (a) Styrene, a compound used to make Styrofoam® cups and insulation, contains 92.3% C and 7.7% H by mass and has a molar mass of 104 g/mol.
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Determine the mass of each element in 100 g of styrene. Since the compound is composed of 92.3% carbon and 7.7% hydrogen, assume you have 100 g of styrene, which means you have 92.3 g of carbon and 7.7 g of hydrogen.
Convert the mass of each element to moles by using the atomic masses (C = 12.01 g/mol, H = 1.008 g/mol). Calculate the moles of carbon by dividing 92.3 g by 12.01 g/mol and the moles of hydrogen by dividing 7.7 g by 1.008 g/mol.
Determine the simplest whole number ratio of the moles of carbon to hydrogen. Divide the number of moles of each element by the smallest number of moles calculated in the previous step.
Write the empirical formula by using the whole number mole ratios as subscripts for each element. If any subscript is not a whole number, multiply all subscripts by the smallest number that converts all subscripts into whole numbers.
Calculate the molecular formula using the empirical formula and the given molar mass of the compound. Divide the molar mass of the compound (104 g/mol) by the molar mass of the empirical formula to find the number of empirical formula units per molecule. Multiply the subscripts in the empirical formula by this number 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 mass percentages of each element are converted to moles, and then the mole ratios are simplified to the smallest integers. For styrene, this involves calculating the moles of carbon and hydrogen based on their mass percentages.
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02:26
Empirical vs Molecular Formula
Molecular Formula
The molecular formula indicates the actual number of atoms of each element in a molecule of the compound. It can be derived from the empirical formula by comparing the molar mass of the compound to the molar mass of the empirical formula. In the case of styrene, knowing the molar mass (104 g/mol) allows for the determination of how many times the empirical formula fits into the molecular formula.
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02:08Determining Molecular Formulas
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 a molecular formula. For styrene, the molar mass is crucial for determining the molecular formula from the empirical formula, as it provides the necessary information to scale the empirical formula to the actual molecular composition.
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02:11Molar Mass Concept
Related Practice
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What is the molecular formula of each of the following compounds? (a) empirical formula HCO2, molar mass = 90.0 g/mol
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What is the molecular formula of each of the following compounds? (b) empirical formula C2H4O, molar mass = 88.0 g/mol
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Determine the empirical and molecular formulas of each of the following substances: (b) Caffeine, a stimulant found in coffee, contains 49.5% C, 5.15% H, 28.9% N, and 16.5% O by mass and has a molar mass of 195 g/mol.
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Determine the empirical and molecular formulas of each of the following substances: (c) Monosodium glutamate (MSG), a flavor enhancer in certain foods, contains 35.51% C, 4.77% H, 37.85% O, 8.29% N, and 13.60% Na, and has a molar mass of 169 g/mol.
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Determine the empirical and molecular formulas of each of the following substances: (a) Ibuprofen, a headache remedy, contains 75.69% C, 8.80% H, and 15.51% O by mass and has a molar mass of 206 g/mol.