Stoichiometry AP Chemistry – Unit 2

What is Stoichiometry? Given the mass or moles of one reactant or product determine how much reactants (in mass of moles) will be needed or products produced in the following reactions. Given the mass or moles of one reactant or product determine how much reactants (in mass of moles) will be needed or products produced in the following reactions. Basic Steps: Basic Steps: 1. Get into the mole 1. Get into the mole 2. Apply mole ratio (from coefficients in balanced chemical equation) 2. Apply mole ratio (from coefficients in balanced chemical equation) 3. Get out of the mole 3. Get out of the mole

There are four steps involved in solving these problems Make sure you are working with a properly balanced equation. Make sure you are working with a properly balanced equation. If you are given grams, convert grams of the substance given in the problem to moles.(Divide by the molar mass) If you are given grams, convert grams of the substance given in the problem to moles.(Divide by the molar mass) Use the coefficients from the balanced equation to set up a mole ratio between what is given and what you need to find. (if the problem asks for moles, this is where you stop!) Use the coefficients from the balanced equation to set up a mole ratio between what is given and what you need to find. (if the problem asks for moles, this is where you stop!) Convert moles of the substance just solved for into grams (multiply by the molar mass!) Convert moles of the substance just solved for into grams (multiply by the molar mass!)

Things to watch out for Double check the equation. Lots of students go right ahead and solve using the unbalanced equation supplied in the problem (or test question for that matter). Double check the equation. Lots of students go right ahead and solve using the unbalanced equation supplied in the problem (or test question for that matter). DON'T use the same molar mass in steps two and four. Your teacher is aware of this and, on a multiple choice test, will provide the answer arrived at by making this mistake. You have been warned! DON'T use the same molar mass in steps two and four. Your teacher is aware of this and, on a multiple choice test, will provide the answer arrived at by making this mistake. You have been warned!

Don't multiply the molar mass of a substance by the coefficient in the problem BEFORE using it in one of the steps above. For example, if the formula says 2 H 2 O, DON'T use 36.0 g/mol, use 18.0 g/mol. Don't multiply the molar mass of a substance by the coefficient in the problem BEFORE using it in one of the steps above. For example, if the formula says 2 H 2 O, DON'T use 36.0 g/mol, use 18.0 g/mol. Don't round off until the very last answer. In other words, don't clear your calculator after step two and write down a value of 3 or 4 significant figures to use in the next step. Round off only once after all calculations are done. Don't round off until the very last answer. In other words, don't clear your calculator after step two and write down a value of 3 or 4 significant figures to use in the next step. Round off only once after all calculations are done.

Example #1 If I start with 1.7 moles of sodium metal, how many moles of chlorine gas do I need to turn it all to salt? Na(s) + Cl 2 (g)  NaCl(s) If I start with 1.7 moles of sodium metal, how many moles of chlorine gas do I need to turn it all to salt? Na(s) + Cl 2 (g)  NaCl(s) 1.7 moles Na 1 mol Cl moles Na 1 mol Cl 2 2 mole Na 2 mole Na 22 = 0.85 moles Cl 2

Example 2: Given the following Reaction: AgNO 3 + BaCl 2  AgCl + Ba(NO 3 ) 2 How many moles of AgCl will form from 20.0 grams of BaCl 2 ? AgNO 3 + BaCl 2  AgCl + Ba(NO 3 ) 2 How many moles of AgCl will form from 20.0 grams of BaCl 2 ? 20.0g BaCl g BaCl 2 = moles AgCl = moles AgCl 22 1mol BaCl g BaCl 2 2 mol AgCl 1 mole BaCl 2

How many grams of silver chloride are produced from 50.0g of silver nitrate reacting with an abundance of barium chloride? How many grams of silver chloride are produced from 50.0g of silver nitrate reacting with an abundance of barium chloride? 2AgNO 3 + BaCl 2  2 AgCl + Ba(NO 3 ) 2 2AgNO 3 + BaCl 2  2 AgCl + Ba(NO 3 ) g AgNO 3 = 42.2 grams AgCl will form 1mol AgNO gAgNO 3 2 mol AgCl 2 molAgNO g AgCl 1mol AgCl

#3. Silicates are compounds on earth. They consist of silicon attached to two or four oxygen molecules. Silicon is refined by heating silicates and releasing the oxygen in this equation: SiO 4  Si(s) + O 2 (g). SiO 4  Si(s) + O 2 (g). If you start with 1.4 x 10 7 g of silicates, how many grams of each product will be produced? If you start with 1.4 x 10 7 g of silicates, how many grams of each product will be produced? 4.3 x 10 6 grams Si 4.3 x 10 6 grams Si 9.7 x 10 6 grams O x 10 6 grams O 2 2