Chapter 3: Calculations with Chemical Formulas Renee Y. Becker Valencia Community College CHM 1045

Atomic and Molecular Mass The molecular mass is the sum of the masses of the atoms making up the molecule (units amu) (molecular compounds only!) calculate the molecular mass: H2O C2H4O2

calculate the formula mass: Formula Mass is the sum of atomic masses of all atoms in a formula unit of any compound, molecular or ionic (units amu) calculate the formula mass: NaCl MgCO3

Avogadro One mole of a substance is the gram mass value, molar mass, equal to the amu mass of a substance Molar mass, (MM), is in units grams/mole (conversion factor for gmol and molg) Avogadro’s Number (NA = 6.022  1023 particles) is the numerical value assigned to the unit, 1 mole

Example 1: Molar Mass Calc. the molar mass: Fe2O3 C6H8O7 C16H18N2O4

Example 2: Avogadro’s Number Li2SO4 1. How many Li2SO4 units are in 3 moles of Li2SO4? 2. How many Li atoms are in 3 moles of Li2SO4? 3. How many S atoms are in 3 moles of Li2SO4? 4. How many O atoms are in 3 moles of Li2SO4?

Calculate 2.35 moles C  g C 4.67 x10-3 moles H2O  g H2O Example 3: Molesg Calculate 2.35 moles C  g C 4.67 x10-3 moles H2O  g H2O 83.2 moles CaO  g CaO

Calculate 45.8 g Fe  moles Fe 6.54 x104 g C2H4  moles C2H4 Example 4: g Moles Calculate 45.8 g Fe  moles Fe 6.54 x104 g C2H4  moles C2H4

Stoichiometry Stoichiometry: Relates the moles of products and reactants to each other and to measurable quantities

Example 5: Stoichiometry 2 NaOH(aq) + Cl2(g)  NaOCl(aq) + NaCl(aq) + H2O(l) How many moles of Cl2 are needed to react with 2 moles of NaOH? How many moles of Cl2 are needed to react with 3 moles of NaOH? How many moles of NaCl are formed by this reaction? If you start with an excess of Cl2 and 5 moles of NaOH, how many moles of H2O can you produce?

Example 6: Stoichiometry 2 NaOH(aq) + Cl2(g)  NaOCl(aq) + NaCl(aq) + H2O(l) 1. How many moles of NaOH are needed to react with 25.0 g of Cl2? 2. How many grams of NaOH are needed to react with 25.0 g of Cl2?

Example 7: Stoichiometry Let’s make up some more examples 2 NaOH(aq) + Cl2(g)  NaOCl(aq) + NaCl(aq) + H2O(l)

% Yield = Actual yield X 100% Stoichiometry Yields of Chemical Reactions: If the actual amount of product formed in a reaction is less than the theoretical amount, we can calculate a percentage yield. % Yield = Actual yield X 100% Theoretical yield

Example 8: Stoichiometry CH2Cl2 is prepared by reaction of CH4 with Cl2 giving HCl. How many grams of CH2Cl2 result from the reaction of 1.85 kg of CH4 if the yield is 43.1%? CH4(g) + 2 Cl2(g)  CH2Cl2(l) + 2 HCl(g)

Stoichiometry Limiting Reagents: The extent to which a reaction takes place depends on the reactant that is present in limiting amounts--the limiting reagent. I need to make a fruit salad that is 1/2 apples and 1/2 oranges. I have 10 apples but only 7 oranges. What is the limiting fruit? How many apples and oranges can I use?

Example 9: Stoichiometry Limiting Reagent Calculation: Lithium oxide is a drying agent used on the space shuttle. If 80.0 kg of water is to be removed and 65 kg of lithium oxide is available, which reactant is limiting? Li2O(s) + H2O(l)  2 LiOH(s) Li2OMM = 29.88 g/mol H2OMM = 18.02 g/mol

Example 10: Stoichiometry K2PtCl4 + 2 NH3  Pt(NH3)2Cl2 + 2 KCl If 10.0 g of K2PtCl4 and 10.0 g of NH3 are allowed to react: a) which is limiting reagent? b) how many grams of the excess reagent are consumed? c) how many grams of cisplatin are formed? K2PtCl4 = 415.08 g/mol NH3 = 17.04 g/mol Pt(NH3)2Cl2 = 300.06 g/mol

Percent Composition Percent Composition: Identifies the elements present in a compound as a mass percent of the total compound mass. Empirical Formula: Determined from data about percent composition, tells only the smallest whole number ratio of atoms in a compound. Molecular Formula: Tells actual numbers of atoms in a molecule, can be same as empirical formula or a multiple of it.

Example 11: Percent Composition What is glucoses, C6H12O6, empirical formula, and what is the percentage composition of glucose?

Example 12: Percent Composition Saccharin has the molecular formula C7H5NO3S, what is its empirical formula and the percent composition?

Example 13: Empirical Formula A compound was analyzed to be 82.67% carbon and 17.33% hydrogen by mass. The molar mass is 58.11 g/mol. What is the empirical formula and molecular formula?