112
Chapter Chemistry 3b
Fetzer Gislason
Empirical Formulas: the smallest, whole number mole-ratio of elements in a compound. C10H24O6 C12H9Cl3 Examples: 1) A sample of an anti-tumor drug was analyzed and found to contain 65.01% Pt, 9.33% N, 23.63% Cl and 2.01% H. Determine its empirical formula.
2) Barium and nitrogen form two binary compounds containing 90.745% and 93.634% barium, respectively. Determine the empirical formulas of these two compounds. Empirical formula from combustion data: given masses of CO2 and H2O, find moles of C, H, and possibly some other element. Example: An organic acid contains C, H, and O. Burning a 4.24 mg sample of this acid in oxygen produced 6.21 mg CO2 and 2.54 mg H2O. Determine the empirical formula of the acid.
Molecular formula..the same as or some multiple of the empirical formula Empirical Formula HO BH3 BH4Cl C2H8F Formula Mass Molar mass 34 27.687 150.925 255.326 Molecular formula
Example: Adipic acid contains 49.30% C, 6.91% H and the rest oxygen. The molar mass of adipic acid is about 146 g. Determine the molecular formula for adipic acid.
Mass relationships in chemical reactions (Stoichiometry) A coefficients in a balanced equation give us the molar ratio of reactants and products to each other in a chemical reaction. Once established, this ratio is constant. This allows us to use the coefficients of the balanced equation to predict mass or volume quantities of reactants needed and/or quantities of product formed. General types of stoichiometry problems. 1. Calculations using only mole-mole ratios 2. Calculations involving volumes of gas (Law of combining Volumes: "equal volumes, equal moles" --temp. and pressure must be constant) 3. Calculations involving grams moles conversion Example 1: Consider the reaction, 2 CaCN2(s) + 7 O2(g) 2 CaO(s) + 2 CO2(g) + 4 NO2(g) a) How many moles of CO2 can be produced by the complete of reaction 4.95102 moles of oxygen?
Chemistry 112
Chapter 3b
Fetzer Gislason
2 CaCN2(s) + 7 O2(g) 2 CaO(s) + 2 CO2(g) + 4 NO2(g) b) What volume of oxygen is needed to produce 153.5 liters of nitrogen dioxide?
c)
How many grams of CO2 will result from the complete reaction of 22.7 g CaCN2?
The meaning of some "special" vocabulary used in stoichiometric problems. 1. One of the reactants is in excess. (Meaning of the word "excess" in this context: don't worry about that reactant. There is enough of it present so that the reaction will proceed. The question is about some other reactants or products.) 2. The problem gives mass quantities for more than one reactant. (Meaning: Now you do have to worry about which reactant is used up and which is in excess. The reactant that is in short supply limits the reaction. It is the only one that can be used in your calculations. It is called a limiting reagent problem.) 3. The word "maximum" mass means actual mass of the substance. (In a limiting reactant problem, the actual mass of a product is the smaller of two calculated masses.) 4. The "the...