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## CHAPTER 7

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**CHAPTER 7**Chemical Calculations 7.1-7.7**The Mole**• A number of atoms, ions, or molecules that is large enough to see and handle. • A mole = number of things • Just like a dozen = 12 things • One mole = 6.022 x 1023 things • Avogadro’s number = 6.022 x 1023 • Symbol for Avogadro’s number is NA.**The Mole**• How do we know when we have a mole? • count it out • weigh it out • Molar mass - mass in grams numerically equal to the atomic weight of the element in grams. • H has an atomic weight of 1.00794 g • 1.00794 g of H atoms = 6.022 x 1023 H atoms • Mg has an atomic weight of 24.3050 g • 24.3050 g of Mg atoms = 6.022 x 1023 Mg atoms**The Mole**• Example 2-1: Calculate the mass of a single Mg atom in grams to 3 significant figures.**The Mole**• Example 2-1: Calculate the mass of a single Mg atom in grams to 3 significant figures.**The Mole**• Example 2-1: Calculate the mass of a single Mg atom in grams to 3 significant figures.**The Mole**• Example 2-1: Calculate the mass of a single Mg atom, in grams, to 3 significant figures.**The Mole**• Example 2-2: Calculate the number of atoms in one-millionth of a gram of Mg to 3 significant figures.**The Mole**• Example 2-2: Calculate the number of atoms in one-millionth of a gram of Mg to 3 significant figures.**The Mole**• Example 2-2: Calculate the number of atoms in one-millionth of a gram of Mg to 3 significant figures.**The Mole**• Example 2-2: Calculate the number of atoms in one-millionth of a gram of Mg to 3 significant figures.**The Mole**• Example 2-3. How many atoms are contained in 1.67 moles of Mg?**The Mole**• Example 2-3. How many atoms are contained in 1.67 moles of Mg?**The Mole**• Example 2-3. How many atoms are contained in 1.67 moles of Mg?**The Mole**• Example 2-3. How many atoms are contained in 1.67 moles of Mg?**The Mole**• Example 2-3. How many atoms are contained in 1.67 moles of Mg?**The Mole**• Example 2-4: How many moles of Mg atoms are present in 73.4 g of Mg? You do it!**The Mole**• Example 2-4: How many moles of Mg atoms are present in 73.4 g of Mg?**The Mole**• Example 2-4: How many moles of Mg atoms are present in 73.4 g of Mg?**The Mole**• Example 2-4: How many moles of Mg atoms are present in 73.4 g of Mg? IT IS IMPERATIVE THAT YOU KNOW HOW TO DO THESE PROBLEMS**Formula Weights, Molecular Weights, and Moles**• How do we calculate the molar mass of a compound? • add atomic weights of each atom • The molar mass of propane, C3H8, is:**Formula Weights, Molecular Weights, and Moles**• The molar mass of calcium nitrate, Ca(NO3)2 , is: You do it!**Formula Weights, Molecular Weights, and Moles**• One Mole of Contains • Cl2 or 70.90g 6.022 x 1023 Cl2 molecules 2(6.022 x 1023 ) Cl atoms • C3H8You do it!**Formula Weights, Molecular Weights, and Moles**• One Mole of Contains • Cl2 or 70.90g 6.022 x 1023 Cl2 molecules 2(6.022 x 1023 ) Cl atoms • C3H8 or44.11 g 6.022 x 1023 C3H8 molecules 3 (6.022 x 1023 ) C atoms 8 (6.022 x 1023 ) H atoms**Formula Weights, Molecular Weights, and Moles**• Example 2-5: Calculate the number of C3H8 molecules in 74.6 g of propane.**Formula Weights, Molecular Weights, and Moles**• Example 2-5: Calculate the number of C3H8 molecules in 74.6 g of propane.**Formula Weights, Molecular Weights, and Moles**• Example 2-5: Calculate the number of C3H8 molecules in 74.6 g of propane.**Formula Weights, Molecular Weights, and Moles**• Example 2-5: Calculate the number of C3H8 molecules in 74.6 g of propane.**Formula Weights, Molecular Weights, and Moles**• Example 2-8. Calculate the number of O atoms in 26.5 g of Li2CO3. You do it!**Formula Weights, Molecular Weights, and Moles**• Example 2-8. Calculate the number of O atoms in 26.5 g of Li2CO3.**Percent Composition**• % composition = mass of an individual element in a compound divided by the total mass of the compound x 100% • Determine the percent composition of C in C3H8.**Percent Composition and Formulas of Compounds**• What is the percent composition of H in C3H8? You do it!**Percent Composition**• What is the percent composition of H in C3H8?**Percent Composition**• Example 2-10: Calculate the percent composition of Fe2(SO4)3 to 3 significant figures. You do it!**Percent Composition**• Example 2-10: Calculate the percent composition of Fe2(SO4)3 to 3 sig. fig.**Empirical and Molecular Formulas**• Empirical Formula - smallest whole-number ratio of atoms present in a compound • Molecular Formula - actual numbers of atoms of each element present in a molecule of the compound • We determine the empirical and molecular formulas of a compound from the percent composition of the compound.**Empirical Formulas**• Example 2-11: A compound contains 24.74% K, 34.76% Mn, and 40.50% O by mass. What is its empirical formula? • Make the simplifying assumption that we have 100.0 g of compound. • In 100.0 g of compound there are: • 24.74 g of K • 34.76 g of Mn • 40.50 g of O**Empirical Formulas**• Example 2-12: A sample of a compound contains 6.541g of Co and 2.368g of O. What is empirical formula for this compound? You do it!**Empirical Formulas**• Example 2-12: A sample of a compound contains 6.541g of Co and 2.368g of O. What is empirical formula for this compound?**Empirical Formulas**• Example 2-12: A sample of a compound contains 6.541g of Co and 2.368g of O. What is empirical formula for this compound?