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7.3 USING CHEMICAL FORMULAS APRIL 27, 2015

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7.3 USING CHEMICAL FORMULAS Formula Masses: Sum of all the average atomic masses of all atoms represented in its formula. Rounded to two decimal places. Units in Atomic Mass Units (amu) Example: Water Molecule H 2 O Complete the following sample problem for one molecule of Phosphoric Acid.

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7.3 USING CHEMICAL FORMULAS Molar Mass: The molar mass of a substance is equal to the mass in grams of one mole, or approx. 6.022x10^23 particles of the substance. Round to two decimal places. Units: g/mol Example: The molar mass of pure Calcium, Ca, is 40.08g/mol because one mole of Ca has a mass of 40.08g. The molar mass of a compound is calculated by summing the masses of the elements present in a mole of the molecules or formula units that make up the compound. Often, a compound’s molar mass is equal to its formula mass. Example: One mole of H2O. Sample Problem: Ba(NO 3 ) 2

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7.3 USING CHEMICAL FORMULAS Molar Mass as a Conversion Factor: The molar mass of a compound can be used as a conversion factor to relate an amount in moles to a mass in grams for a given substance. To convert a known amount of a compound in moles to a mass in grams, multiply the amount in moles by the molar mass: Amount in moles x molar mass (g/mol) = mass in grams Ex: 2.50 mol of O 2 to grams Sample Problem: 0.0596mol of SO 2 to grams

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7.3 USING CHEMICAL FORMULAS Sample Problem: Ibuprofen, C13H18O2, is the active ingredient in many nonprescription pain relievers. Its molar mass is 206.31g/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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7.3 USING CHEMICAL FORMULAS Percentage Composition: It is often useful to know the percentage by mass of a particular element in a chemical compound. To calculate the mass percentage of an element in a compound, divide the mass of the element in a sample of the compound by the total mass of the sample, then multiply this value by 100. (Mass of element in sample/mass of sample compound) x 100 = % element in compound The percentage by mass of each element in a compound is known as the percentage composition. Example: Find the percentage composition of copper (I) sulfide, Cu2S.

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