# Other Stoich Calculations A. mole mass (mass mole) calculations. GIVEN mol A x CE mol B. PT g A CE mol A MOLE MASS :

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1 Chem. I Notes Ch. 12, part 2 Using Moles NOTE: Vocabulary terms are in boldface and underlined. Supporting details are in italics. 1 MOLE = 6.02 x representative particles (representative particles = ATOMS, IONS, MOLECULES, & FORMULA UNITS) CONVERSION FACTOR SUMMARY: 6.02 x representative particles 1 MOLE 1 MOLE 6.02 x representative particles MOLAR MASS (g) 1 MOLE 1 MOLE MOLAR MASS (g) 22.4 L (for a gas at STP) 1 MOLE 1 MOLE 22.4 L (for a gas at STP) stoichiometry using balanced chemical equations to obtain info. I. Mole - Mole (MOL MOL) Conversions A. the most important, most basic stoich calculation B. uses the coefficients of a balanced equation to compare the amounts of reactants and products C. coefficients are mole ratios D. the way to go from substance A to substance B E. mol mol is the only time the mole number in the conversion is not automatically 1. (Avogadro s #, molar mass, and 22.4 L (STP) are all equal to 1 mole.) MOL MOL : # mol A # mol B # = coefficients # mol B # mol A F. examples E1) How many moles of carbon monoxide are produced when mol of oxygen reacts with carbon? C + O 2 CO 2C + O 2 2CO mol O 2 x 2 mol CO = 1.50 mol CO 1 mol O 2 E2) Find the number of moles of the reactants, given mol of product is formed. Al + O 2 Al 2 O 3 4Al + 3O 2 2Al 2 O mol Al 2 O 3 x 4 mol Al = 1.32 mol Al mol Al 2 O 3 x 3 mol O 2 = mol O 2 2 molal 2 O 3 2 molal 2 O 3 II. MASS MASS Conversions Using molar mass in stoich problems to predict masses of reactants and/or products A. a balanced chemical equation can be used to compare masses of reactants and products B. mass mass cannot change which substance you are dealing with; only mol mol can do that 1

2 MASS MASS : GIVEN g A x 1 mol A x CE mol B x PT g B PT g A CE mol A 1 mol B C. examples E3) How many grams of hydrochloric acid are made from the reaction of g of hydrogen gas with excess chlorine gas? H 2 + Cl 2 HCl H 2 + Cl 2 2HCl GAME PLAN: KNOWN, PER.TABLE, COEFF., PER.TABLE g H 2 x 1 mol H 2 x 2 mol HCl x 36.5 g HCl = 18 g HCl 2.0 g H 2 1 mol H 2 1 mol HCl E4) Calculate the numbers of grams of products formed when 25.0 g of sodium nitrate decomposes into sodium nitrite and oxygen. NaNO 3 NaNO 2 + O 2 2NaNO 3 2NaNO 2 + O g NaNO 3 x 1 mol NaNO 3 x 2 mol NaNO 2 x 69.0 g NaNO 2 = 20.3 g NaNO g NaNO 3 2 mol NaNO 3 1 mol NaNO g NaNO 3 x 1 mol NaNO 3 x 1 mol O 2 x 32.0 g O 2 = 4.71 g O g NaNO 3 2 mol NaNO 3 1 mol O 2 III. Other Stoich Calculations A. mole mass (mass mole) calculations MASS MOLE : GIVEN g A x 1 mol A x CE mol B PT g A CE mol A MOLE MASS : GIVEN mol A x CE mol B CE mol A x PT g B 1 mol B E5) How many g of water are produced from the complete combustion of mol of C 2 H 2? C 2 H 2 + O 2 CO 2 + H 2 O 2C 2 H 2 + 5O 2 4CO 2 + 2H 2 O mol C 2 H 2 x 2 mol H 2 O x 18.0 g H 2 O = 12.3 g H 2 O 2 mol C 2 H 2 1 mol H 2 O E6) Using the equation 2C 2 H 2 + 5O 2 4CO 2 + 2H 2 O how many moles of O 2 would be needed to produce g of CO 2? g CO 2 x 1 mol CO 2 x 5 mol O 2 = 1.59 mol O g CO 2 4 mol CO 2 2

3 B. mass volume (volume mass) calculations Using molar volume in stoich problems MASS VOLUME : STP) GIVEN g A x 1 mol A x CE mol B x 22.4 L B PT g A CE mol A 1 mol B VOLUME MASS : STP) GIVEN L A x 1 mol A x CE mol B x PT g B 22.4 L A CE mol A 1 mol B E7) How many L of hydrogen are produced from the decomposition of 3.50 g of water at STP? H 2 O H 2 + O 2 2H 2 O 2H 2 + O g H 2 O x 1 mol H 2 O x 2 mol H 2 x 22.4 L H 2 = 4.36 L H g H 2 O 2 mol H 2 O 1 mol H 2 E8) Using the equation 2C 2 H 2 + 5O 2 4CO 2 + 2H 2 O how many liters of water vapor are produced when 5.02 g of C 2 H 2 undergoes complete combustion under STP conditions? 5.02 g C 2 H 2 x 1 mol C 2 H 2 x 2 mol H 2 O x 22.4 L H 2 O = 4.32 L H 2 O 26.0 g C 2 H 2 2 mol C 2 H 2 1 mol H 2 O C. volume volume calculations VOLUME VOLUME : STP) GIVEN L A x 1 mol A x CE mol B x 22.4 L B 22.4 L A CE mol A 1 mol B (SHORT CUT: compare coefficients!) E9) How many liters of CO 2 are produced from L of HCl reacting with NaHCO 3? Use the equation NaHCO 3 + HCl NaCl + CO 2 + H 2 O L HCl x 1 mol HCl x 1 mol CO 2 x 22.4 L CO 2 = L CO L HCl 1 mol HCl 1 mol CO 2 SHORTCUT: coefficients = 1 mol HCl to 1 mol CO 2., so L HCl = L CO 2 E10) How many L of sulfur trioxide are produced from the reaction of 36.1 L of oxygen with sulfur dioxide at STP? SO 2 + O 2 SO 3 2SO 2 + O 2 2SO L O 2 x 1 mol O 2 x 2 mol SO 3 x 22.4 L SO 3 = 72.2 L SO L O 2 1 mol O 2 1 mol SO 3 SHORTCUT: coefficient of O 2 = 1; coefficient of SO 3 = L x 2 = 72.2 L SO 3 3

4 D. mass particle (particle mass) calculations MASS PARTICLE : GIVEN g A x 1 mol A x CE mol B x (6.02 x ) r.p. B PT g A CE mol A 1 mol B PARTICLE MASS : GIVEN r.p. A x 1 mol A x CE mol B x PT g B (6.02 x ) r.p. A CE mol A 1 mol B E11) How many formula units of barium sulfate are made from reacting 5.33 g of barium hydroxide with sulfuric acid? Ba(OH) 2 + H 2 SO 4 BaSO 4 + H 2 O Ba(OH) 2 + H 2 SO 4 BaSO 4 + 2H 2 O 5.33 g Ba(OH) 2 x 1 mol Ba(OH) 2 x 1 mol BaSO 4 x 6.02 x f.units BaSO 4 = 1.87 x g Ba(OH) 2 1 mol Ba(OH) 2 1 mol BaSO 4 f.units BaSO 4 E12) How many molecules of NH 3 are produced from reacting 2.07 g of H 2 with excess N 2? N 2 + H 2 NH 3 N 2 + 3H 2 2NH g H 2 x 1 mol H 2 x 2 mol NH 3 x 6.02 x molecules NH 3 = 4.2 x molecules NH g H 2 3 mol H 2 1 mol NH 3 IV. Percent Yield A. percent yield percentage of product recovered; comparison of actual and theoretical yields B. actual yield amount of product obtained in lab C. theoretical yield amount of product predicted by the math (theory) % YIELD = ACTUAL YIELD x 100 THEORETICAL YIELD D. examples E14) 35.0 g of product should be recovered from an experiment. A student collects 22.9 g at the end of the lab. What is the percent yield? 22.9 g x 100 = 65.4% 35.0 g E15) What is the percent yield if 2.89 g of NaCl is produced when 1.99 g of HCl reacts with excess NaOH? Water is the other product. HCl + NaOH NaCl + H 2 O Actual yield = 2.89 g NaCl Theoretical yield =? Percent yield implies product yield g HCl x 1 mol HCl x 1 mol NaCl x 58.5 g NaCl = 3.19 g NaCl theoretical yield 36.5 g HCl 1 mol HCl 1 mol NaCl % YIELD = ACTUAL YIELD x 100 = 2.89 g NaCl = 90.6% THEORETICAL YIELD 3.19 g NaCl 4

5 Ideal Gas Law will be covered later Percent Composition already covered in part 1 V. Empirical Formula Determining Chemical Formulas A. Empirical Formula the simplest whole-number ratio of elements in a cmpd. B. it is a non-reducible ration of moles C. problem procedure 1. convert % to grams directly 2. find numbers of moles 3. make mole ratios using the smallest mole number as the denominator 4. use these whole number ratios as the subscripts of the formula (If a number with 0.5 is observed, multiply everything by 2. Don t round up!) D. examples E16) Calculate the empirical formula of a compound composed of 67.6% mercury, 10.8% sulfur, and 21.6% oxygen. (check the PROBLEM PROCEDURE steps) STEP 1 mercury: 67.6% Hg = 67.6 g Hg out of 100 g cmpd. sulfur: 10.8% S = 10.8 g S out of 100 g cmpd. oxygen: 21.6% O = 21.6 g C out of 100 g cmpd. STEP 2... Hg: 67.6 g C x 1 mol Hg = mol Hg S: 10.8 g S x 1 mol S = mol S g Hg 32.1 g S O: 21.6 g O x 1 mol O = 1.35 mol O 16.0 g STEP 3 Hg = = 1 S = = 1 O = 1.35 mol = mol STEP 4 Hg 1 S 1 O 4 = HgSO 4 E17) What is the empirical formula of a compound with 25.9% nitrogen and 74.1% oxygen? 25.9% N = 25.9 g N out of 100 g cmpd. 74.1% O = 74.1 g O out of 100 g cmpd. N: 25.9 g N x 1 mol N = 1.85 mol N O: 74.1 g O x 1 mol O = 4.63 mol O 14.0 g N 16.0 g O N: 1.85 = 1 O: 4.63 = 2.5 N 1 O 2.5 can t have.5 subscripts x 2 = N 2 O

6 VI. Molecular Formula A. a multiple of the empirical formula B. still whole number ratios C. examples E18) A compound with an empirical formula of CH has a molecular weight of 78.0 g/mol. What is the molecular formula? molar mass of CH = = 13.0 g 78 / 13 = 6 molecular formula = C 6 H 6 E19) A compound is 75.46% carbon, 4.43% hydrogen, and 20.10% oxygen by mass. It has a molecular mass of g/mol. What is the molecular formula for this compound? 67.6% C = g C out of 100 g cmpd. 4.43% H = 4.43 g H out of 100 g cmpd % O = g O out of 100 g cmpd. (75.46 g C) (1 mol/ 12.0 g C) = 6.29 mol C (4.43 g H) (1 mol/ 1.0 g H) = 4.4 mol H (20.10 g O) (1 mol/ 16.0 g O) = 1.26 mol O (6.29 mol C)/ (1.26) = 4.99 = 5 mol C (4.4 mol H)/ (1.26) = 3.49 = 3.5 mol H (1.26 mol O)/ (1.26) = 1 mol O.5 value means multiply subscripts by 2: empirical fmla. = C 10 H 7 O 2 Now that you have the emp.fmla., you can find the molecular fmla like in problem E18. emp. fmla. mass = 10(12.0) + 7(1.0) + 2(16.0) = g/mol The problem says the molecular mass is g per mole g/mol = = 2 ratio g/mol Since there are two empirical units in a molecular unit, the molecular formula = C 20 H 14 O 4 6

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