Chemistry I Notes Unit 7: Stoichiometry Notes Stoichiometry Relating Mass to Numbers of Atoms The Mole The mole is the SI unit for amount of substance. A mole (abbreviated mol) is the amount of a substance that contains as many particles as there are atoms in exactly 12 g of carbon- 12. Avogadro s Number Avogadro s number 6.02 10 23 is the number of particles in exactly one mole of a pure substance. Gram/Mole Conversions Chemists use molar mass as a conversion factor in chemical calculations. For example, the molar mass of helium is 4.00 g He/mol He. To find how many grams of helium there are in two moles of helium, multiply by the molar mass. 2.00 mol He!.!!!!"!!"#!" = 8.00 g He
A chemical formula indicates: 1) the elements in a compound 2) the number of atoms of an element Chemical formulas also allow chemists to calculate a number of other values for a compound: Formula mass Molar mass Percentage Composition Empirical Formulas
Formula Mass The formula mass is the sum of the average atomic masses of all the elements as shown in the chemical formula. The formula mass is measured in atomic mass units (amu). Image Source: http://www.digipac.ca/chemical/molemass/images/table5.gif Molar Mass The mass of one mole of a pure substance is called the molar mass of that substance. Molar mass is usually written in units of g/mol. The molar mass of an element is numerically equal to the formula mass of the element in atomic mass units and they are calculated the same way (only the units differ). Image Source: http://www.amazingrust.com/experiments/background_knowledge/images/molar_weight_water.gif Molar Mass as a Conversion Factor The molar mass is used as a conversion factor to relate an amount in moles to a mass in grams. To convert moles to grams, multiply the amount in moles by the molar mass: Amount in moles molar mass (g/mol) = mass in grams To convert grams to moles, divide by the molar mass: Image Source: http://opentextbc.ca/introductorychemistry/wp- content/uploads/sites/17/2014/07/screen- Shot- 2014-07- 21- at- 5.16.11- PM.png
Composition stoichiometry deals with the mass relationships of elements in compounds. Percentage composition Empirical formulas Percentage Composition the percentage makeup of a compound. To calculate the percent composition of a component in a compound: 1. Find the molar mass of the compound by adding up the masses of each atom in the compound using the periodic table. 2. Calculate the mass due to the component in the compound you are for which you are solving by adding up the masses of these atoms. 3. Divide the mass due to the component by the total molar mass of the compound and multiply by 100. Percentage Composition =!"##!"!"!#!$%!"!#$!"##!"!!!!"#$"%&' 100 Empirical Formulas Empirical Formula the lowest whole number ratio of elements in a compound. To calculate empirical formulas: 1. Convert from grams to moles for each element in the compound. If percentages are given, assume 100 g sample and the percentages are the amounts in grams. 2. Divide each number of moles by the smallest number of moles. 3. Write empirical formula. Example #1 Empirical Formula = CaO 2H 2 or Ca(OH) 2 Image Source: https://www.chem.tamu.edu/class/majors/tutorialnotefiles/emp1c.gif
Example #2 Empirical Formula = C 14H 18N 2O 5 Image Source: https://www.chem.tamu.edu/class/majors/tutorialnotefiles/emp2d.gif Reaction stoichiometry involves the mass relationships between reactants and products in a chemical reaction. Reaction stoichiometry is always based on a balanced chemical equation. Mole Ratios A mole ratio is a conversion factor that relates the amounts in moles of any two substances involved in a chemical reaction Example: 2Al2O3(l) 4Al(s) + 3O2(g) Mole Ratios: 2 mol Al2O3 4 mol Al 2 mol Al2O3 3 mol O2 4 mol Al 3 mol O2
4 Types of Reaction Stoichiometry Problems
Molar Volume Molar Volume is the volume of 1 mol of any gas. 1 mol of any gas = 22.4 L Limiting Reactants The limiting reactant is the reactant that limits the amount of product that can form in a chemical reaction. The excess reactant is the substance that is not used up completely in a reaction. Percentage Yield The theoretical yield is the maximum amount of product that can be produced (from a calculation). The actual yield of a product is the measured amount of that product obtained (from experiment). The percentage yield ratio of actual yield to theoretical yield, multiplied by 100. percentage yield = actual yield theorectical yield 100