Stoichiometry Notes

I. Problems Involving Compounds a. Compounds are measured in molecules (or formula units) and single elements are measured in atoms. 1 mole of CO2 = 6.022 X 10 23 molecules 1 mole of Ag = 6.022 X 10 23 atoms A mole is 6.022 X 10 23 of any substance!!!

Practice Problems: How many atoms of carbon are in 25.0 grams of carbon? 25 g C x 1 mol C / 12 g C x 6.02 x 10^23 atoms C / 1 mol C = ??? (use calculator & solve) How many molecules of calcium nitrate are in 0.200 moles of calcium nitrate? 0.200 mol Ca(NO3)2 x 6.02 x 10^23 molecules Ca(NO3)2 / 1 mol Ca(NO3)2 = ??? (use calculator & solve)

b. Recall that the molar mass of a compound is the mass, in grams, of one mole of that compound. To determine the molar mass of a compound, add together the molar mass of all of the elements (found on the periodic table) in the compound.

Practice problems: What is the molar mass of nitrogen dioxide (NO2)? (look @ chart & calculate answer) grams of 1 nitrogen = grams of 2 oxygen = + ______________ ________g/mol NO2

Problems involving molar mass of compounds. What is the mass in grams of 1.25 moles of sulfur trioxide? Molar mass: 1 S = 32; 3 O = 3 x 16 = 48; 32 + 48 = 80 g/mol SO3 1.25 mol SO3 X 80 g SO3 / 1 mol SO3 = ??? (use calculator to solve) When bees sting they release the compound isopentyl acetate, C7H14O2. How many molecules of isopentyl acetate are there in 2.00 grams of C7H14O2? Molar mass: 7 C = 98; 14 H = 14; 2 O = 32; 98 + 14 + 32 =144 g / mol C7H14O2 2.00 g C7H14O2 X 1 mol C7H14O2 / 144 g C7H14O2 = ??? (use calculator to solve)

II. Stoichiometry - shows the relationship between reactants & products in a chemical reaction. Now we will use our knowledge of chemical reactions and equations to solve problems.

a. One new conversion we need to know is the mole ratio – it relates the amounts of moles of any two substances involved in a chemical reaction. 2Al2O3  4Al + 3O2 2 mol Al2O3 2 mol Al2O3 4 mol Al 3 mol O2 4 mol Al 3 mol O2 Writing the inverse(the fraction “upside down”)of any of the above would make a total of 6 mole ratios.

Practice Problem: For the following chemical equation write all possible mole ratios. 2HgO  2Hg + O2 (hint: use the coefficients (front numbers) as your numbers in your ratios (fraction)) _______________ _______________ _______________ (write 3 fractions/ratios on the given fraction bars above)

b. Three types of problems involving stoichiometric calculations (mole ratios) are: 1. Mole to mole problems (1 step problems) 2. Mole to mass problems (2 step problems) 3. Mass to mass problems (3 step problems)

To solve these problems: 1st - write a balanced equation for the reaction. 2nd - set-up a dimensional analysis problem, starting with the known quantity written as a fraction. Use correct mole ratios to solve the problem.

Practice Problems (solve these): 8. According to the following equation how many moles of lithium hydroxide are required to react with 20.0 moles of CO2? CO2 + 2LiOH  Li2CO3 + H2O ? 9. How many moles of ammonia, NH3 are produced when 6.00 moles of hydrogen gas react with excess nitrogen gas? 3H2 + N2  2NH3 ? 10. What mass, in grams, of glucose is produced when 3.00 moles of water react with carbon dioxide? 6CO2 + 6H2O  C6H12O6 + 6O2 What mass in grams of magnesium oxide is produced when 2.00 moles of magnesium react with oxygen? ? 12.How many grams of SnF2 are produced from the reaction of 30.0 grams of HF with Sn? Sn + 2HF  SnF2 + H2 ? If mercury (II) oxide decomposes, how many grams of mercury (II) oxide are needed to produce 125 grams of oxygen? ? 14. What mass of aluminum is produced by the decomposition of 5000. grams of Al2O3? ?

III. Percent Yield a. Theoretical yield – maximum amount of product that can be produced from a given amount of reactant. This value can be calculated from working mass to mass problems.

Example: In Practice Problem 14 you determined that 2647 grams (your answer to #14) of Al could be produced from 5000. grams of Al2O3. This will occur if the reaction occurs without ANY type of error!!

b. Actual yield – actual amount of a product obtained from a reaction b. Actual yield – actual amount of a product obtained from a reaction. It must be experimentally obtained. It cannot be more than the theoretical yield.

Example: Let’s say we carry out the reaction in Practice Problem 14 in a lab and we only obtain 2100. grams of Al from 5000. grams of Al2O3.

c. Percent yield = Actual yield X 100 Theoretical yield 2100. grams X 100 = 79.36% 2647 grams

15. When 36.8 grams of C6H6 react with an excess of Cl2, the actual yield of C6H5Cl is 38.8 grams. What is the percent yield of C6H5Cl? C6H6 + C12  C6H5Cl + HCl Actual Yield (given)= _?_ Theoretical Yield (calculated from 36.8)= _?_ Percent Yield = _?_

16. If 75 grams of CO react to produce 68 16. If 75 grams of CO react to produce 68.4 grams of CH3OH, what is the percent yield of CH3OH? _?_ CO + 3H2  CH3OH

Stoichiometry Notes PART 2

IV. Limiting Reactants- The reactant that limits the amounts of the other reactants that can combine and the amount of the product that can form in a chemical reaction. The substance that is not used up completely in a reaction is called the excess reactant.

Let’s say you need the following to make one hamburger: 2 buns 1 piece of meat 1 piece of lettuce

If you have 16 buns, 16 pieces of meat, and 20 pieces of lettuce, how many hamburgers can we make?_________ What limits how many we can make (what do we run out of first)? _________ What is in excess (what do we have leftovers of)? _________

You should be able to recognize a limiting reactant problem because they have 2 amounts given – one for each reactant! To solve these problems first write the balanced equation for the reaction. Second determine which of the reactants the limiting reactant is. (To determine which the limiting reactant is use both amounts of reactants given and setup two dimensional analysis problems to solve for the product. The smaller answer will come from the limiting reactant and will be the correct answer.)

Practice Problems: 17. Silicon dioxide (quartz) is usually quite uncreative but reacts readily with hydrogen fluoride according to the following equation. SiO2 (s) + 4HF (g)  SiF4 (g) + 2H2O (l) If 2 moles of HF are exposed to 4.5 moles of SiO2, which is the limiting reactant? _?_ What is the excess reactant? _?_

18. If 20. 5 grams of chlorine is reacted with 20 18. If 20.5 grams of chlorine is reacted with 20.5 grams of sodium, how many grams of sodium chloride would be produced? _?_ which reactant is the limiting reactant? _?_ which is the excess reactant? _?_ How much of the excess reactant is left when the maximum amount of product is formed? _?_