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Chapter 7: Chemical Formulas and Chemical Compounds

Chapter 7: Chemical Formulas and Chemical Compounds. Section 3: Using Chemical Formulas. Overview. We will calculate the formula mass or molar mass of any given substance. We will work on molar conversions for molecules.

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Chapter 7: Chemical Formulas and Chemical Compounds

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  1. Chapter 7: Chemical Formulas and Chemical Compounds Section 3: Using Chemical Formulas

  2. Overview • We will calculate the formula mass or molar mass of any given substance. • We will work on molar conversions for molecules. • We will calculate the number of molecules or formula units in a given molar amount. • We will calculate the percent composition of a chemical compound.

  3. Formula Mass • The sum of all average atomic masses (molar masses) of all the atoms represented in the chemical formula. • Recall the average atomic mass of H = 1.01g, which is the same as the molar mass of H which equals 1.01g/mol. • So what is the formula mass of H2O2? • 2 H x 1.01 g = 2.02 g • 2 O x 16.00 g = 32.00 g • Total H + Total O = 34.02 g

  4. Molar Mass • It is found the same way. They are numerically equal to each other. • So what is the molar mass of H2O2? • 2 H x 1.01 g/mol = 2.02 g/mol • 2 O x 16.00 g/mol = 32.00 g/mol • Total H + Total O = 34.02 g/mol

  5. Molar Conversions • Recall the value of g/mol can be used to convert the number of moles of a substance to moles or grams. • Also remember that to find the number of molecules in a substance you will need to how many moles of the substance and take that times Avogadro’s Number. • 1 mol = 6.022x1023 = molar mass = 22.4 L (STP)

  6. Percent Composition • The percent by mass of each element in a compound. • So what is the percent composition of H2O2? • First find the formula mass. • 2 H x 1.01 g = 2.02 g • 2 O x 16.00 g = 32.00 g • Total H + Total O = 34.02 g • Then divide the total mass of each element by the formula mass and take it times 100. • H = 5.94% O = 94.06%

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