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Chemical Formulas and Compounds Using Chemical Formulas

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Objective Calculate the formula mass or molar mass of any given compound Use molar mass to convert between mass in grams and amount in moles of a chemical compound Calculate the number of molecules, formula units, or ions in a given molar amount of a chemical compound Calculate the percentage composition of a given chemical compound

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Using Chemical Formulas Chemical Formulas A chemical formula indicates the elements present in a compound the relative number of atoms or ions of each element present in a compound Chemical formulas also allow chemists to calculate a number of other characteristic values for a compound: formula mass molar mass percentage composition

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Using Chemical Formulas Formula Masses The formula mass of any molecule, formula unit, or ion is the sum of the average atomic masses of all atoms represented in its formula formula mass of water, H 2 O average atomic mass of H: 1.01 amu average atomic mass of O: 16.00 amu average mass of H 2 O molecule = 18.02 amu

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Using Chemical Formulas Formula Masses The mass of a water molecule can be referred to as a molecular mass The mass of one formula unit of an ionic compound, such as NaCl, is not a molecular mass The mass of any unit represented by a chemical formula (H 2 O, NaCl) can be referred to as the formula mass

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Using Chemical Formulas Formula Masses Sample Problem Find the formula mass of potassium chlorate, KClO 3

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Using Chemical Formulas Formula Masses Sample Problem Solution The mass of a formula unit of KClO 3 is found by adding the masses of one K atom, one Cl atom, and three O atoms formula mass of KClO 3 = 122.55 amu

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Using Chemical Formulas Molar Masses The molar mass of a substance is equal to the mass in grams of one mole, or approximately 6.022 × 10 23 particles, of the substance The molar mass of a compound is calculated by adding the masses of the elements present in a mole of the molecules or formula units that make up the compound

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Using Chemical Formulas Molar Masses One mole of water molecules contains exactly two moles of H atoms and one mole of O atoms. The molar mass of water is calculated as follows A compound’s molar mass is numerically equal to its formula mass molar mass of H 2 O molecule = 18.02 g/mol

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Using Chemical Formulas Formula Masses Sample Problem What is the molar mass of barium nitrate, Ba(NO 3 ) 2

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Formula Masses Sample Problem Solution One mole of barium nitrate, contains one mole of Ba, two moles of N (1 × 2), and six moles of O (3 × 2) Using Chemical Formulas molar mass of Ba(NO 3 ) 2 = 261.35 g/mol

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Using Chemical Formulas Molar Mass Conversions

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Using Chemical Formulas Molar Mass Conversions Sample Problem What is the mass in grams of 2.50 mol of oxygen gas (O 2 )

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Molar Mass Conversions Sample Problem Solution Given: 2.50 mol O 2 Unknown: mass of O 2 in grams Using Chemical Formulas moles O 2 grams O 2 amount of O 2 (mol) × molar mass of O 2 (g/mol) = mass of O 2 (g)

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Using Chemical Formulas Molar Mass Conversions Sample Problem Ibuprofen, C 13 H 18 O 2, is the active ingredient in many nonprescription pain relievers. Its molar mass is 206.31 g/mol If the tablets in a bottle contain a total of 33g of ibuprofen, how many moles of ibuprofen are in the bottle? How many molecules of ibuprofen are in the bottle? What is the total mass in grams of carbon in 33g of ibuprofen?

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Molar Mass Conversions Sample Problem Solution Given: 33g of C 13 H 18 O 2, molar mass 206.31 g/mol Unknown: moles C 13 H 18 O 2 molecules C 13 H 18 O 2 total mass of C Using Chemical Formulas

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Molar Mass Conversions Sample Problem Solution grams moles moles molecules moles C 13 H 18 O 2 moles C grams C

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Using Chemical Formulas Molar Mass Conversions Sample Problem Solution

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Using Chemical Formulas Percentage Composition It is often useful to know the percentage by mass of a particular element in a chemical compound To find the mass percentage of an element in a compound, the following equation can be used The mass percentage of an element in a compound is the same regardless of the sample’s size

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Using Chemical Formulas Percentage Composition The percentage of an element in a compound can be calculated by determining how many grams of the element are present in one mole of the compound The percentage by mass of each element in a compound is known as the percentage composition of the compound

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Using Chemical Formulas Percentage Composition

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Using Chemical Formulas Molar Mass Conversions Sample Problem Find the percentage composition of copper(I) sulfide, Cu 2 S

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Molar Mass Conversions Sample Problem Solution Given: formula, Cu 2 S Unknown: percentage composition of Cu 2 S Using Chemical Formulas formula molar mass mass percentage of each element

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Molar Mass Conversions Sample Problem Solution Using Chemical Formulas Molar mass of Cu 2 S = 159.2 g

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Using Chemical Formulas Molar Mass Conversions Sample Problem As some salts crystallize from a water solution, they bind water molecules in their crystal structure. Sodium carbonate forms such a hydrate, in which 10 water molecules are present for every formula unit of sodium carbonate. Find the mass percentage of water in sodium carbonate decahydrate, Na 2 CO 3 10H 2 O, which has a molar mass of 286.19 g/mol.

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