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Chapter 19: Molecules and Compounds Section 19.3 Comparing Molecules

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COMPARING DIFFERENT MOLECULES How does the mass of different molecules compare?How does the mass of different molecules compare?

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Do you think that a molecule of water has the same mass as a molecule of calcium carbonate?

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Recall atomic mass units (amu)… Atoms are assigned a relative mass based on carbon as the standard.Atoms are assigned a relative mass based on carbon as the standard. Known as atomic mass unitKnown as atomic mass unit 6 C Carbon Carbon 2424

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Chemical formula gives 3 pieces of info: –types / numbers of atoms. –if polyatomic ions are present. –allows calculation of mass of 1 molecule of a compound relative to mass of other compounds.

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Formula Mass Way to compare masses of molecules of different compounds.Way to compare masses of molecules of different compounds. Calculate by adding up atomic masses of all atoms in a compound.Calculate by adding up atomic masses of all atoms in a compound.

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Example: Figuring Formula Mass H 2 O means 2H and 1OH 2 O means 2H and 1O 2 (1.01 amu) = (16.00 amu) = (1.01 amu) = (16.00 amu) = Formula mass = amu of H 2 OFormula mass = amu of H 2 O

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An amu is very small, so to be usable in measurements, we equate the number value of the formula mass in amu to an equal amount in grams.

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Avogadros Number The formula mass in grams of any element or compound contains 6.02 x atoms or molecules.The formula mass in grams of any element or compound contains 6.02 x atoms or molecules. Known as Avogadros # or a mole of the substance.Known as Avogadros # or a mole of the substance.

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Calculate the formula mass of calcium carbonate. 1.Write formula

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2. List number of atoms and atomic mass of each: CaCO 3 1 Ca = 1(40.08) = Ca = 1(40.08) = C = 1(12.01) = C = 1(12.01) = O = 3(16.00) = O = 3(16.00) = 48.00

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3. Add up values to calculate formula mass amu for CaCO amu for CaCO 3

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So, how do we use this value? If you measure out grams of CaCO 3, you have 6.02 x molecules of CaCO 3.If you measure out grams of CaCO 3, you have 6.02 x molecules of CaCO 3. Likewise, g of H 2 O contains 6.02 x molecules of water.Likewise, g of H 2 O contains 6.02 x molecules of water.

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Hydrates (BaCl 2 2H 2 O) Some molecules contain precise numbers of H 2 O molecules chemically bonded to their ions.Some molecules contain precise numbers of H 2 O molecules chemically bonded to their ions. Called hydrates.Called hydrates. Can remove H 2 O by heating.Can remove H 2 O by heating.

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When H 2 O is gone, the compound is known as anhydrous (BaCl 2 ). To calculate formula mass, simply add the mass of the attached H 2 O molecules to that of the anhydrous mass.To calculate formula mass, simply add the mass of the attached H 2 O molecules to that of the anhydrous mass.

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Example: BaCl 2 2H 2 O 1 Ba = 1(137.30) = Ba = 1(137.30) = Cl = 2(35.45) = Cl = 2(35.45) = H = 4(1.01) = H = 4(1.01) = O = 2(16.00) = O = 2(16.00) = Formula mass = amu Formula mass = amu

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