# Grams A Moles A Moles B Grams B

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1 STOICHIOMETRY--The quantitative relationship among the reactants and products in a balanced chemical equation. The stoichiometric coefficients do not affect sig figs. Stoichiometric calculations can be divided into three categories. (1) Mass - Mass Relationship (2) Mass - Volume Relationship (3) Volume - Volume Relationship We will deal with (1) Mass-Mass relationships and (3) Volume Volume relationships now. Use coefficients from balanced chemical equation Grams AMoles AMoles BGrams B Find molar mass from periodic table Find molar mass from periodic table When the substance does not change, you are using molar mass to convert between moles and grams When the substance changes, you are using the mole to mole ratio from the chemical equation to convert from one type of substance to a new substance---unit won t change, compound will!! Molar mass is 1 mole A = mass from periodic table of A Example: How many moles of oxygen are used to make 35.0 moles of water when water is made? Step 1: Write what you are given and what you want mol H 2 0 = mol O 2 Step 2: Write balanced chemical equation.. 2 H 2 + O 2 2 H 2 O (remember, oxygen and hydrogen are diatomic) Step 3: T-grid 35.0 mol H 2 O = mol O 2 Check the diagram at the top: we are changing cmpds only, so use the mole to mole ratio for water and oxygen: 1 mole of oxygen for every 2 moles of water (moles are the coefficients in front of the formula) 35.0 mol H 2 O 1 mol O 2 = 35.0 mol O 2 = 17.5 mol O 2 2 mol H 2 2 mol H 2

2 Example: How many grams of sodium are needed to react with chlorine to make 12.5 moles of sodium chloride? Step 1: 12.5 mol NaCl = grams Na Step 2: Write bal chem eq: 2 Na + Cl 2 2 NaCl (don t forget that chlorine is diatomic) Step 3: T-Grid 12.5 mol NaCl = g Na (use diagram to figure out where to start and what conversion factors you will need: Starting with Moles A and converting to Grams B so you have to go from Moles A Moles B and then from Moles B to Grams B) 12.5 mol NaCl 2 mol Na 23.0 g Na = 288 g Na 2 mol NaCl 1 mol Na Example: How many moles of sulfuric acid are made from the reaction of grams of sulfur trioxide and excess water? g SO 3 = moles H 2 SO 4 H 2 O + SO 3 H 2 SO 4 (it is balanced) T-grid g SO 3 1 mol SO 3 1 mol H 2 SO 4 = mol H 2 SO g SO 3 1 mol SO 3 Example: How many grams of carbon dioxide are formed when 50.0 g C 6 H 12 O 6 completely combust? (You must remember the reactants and products of combustion.hydrocarbon + oxygen carbon dioxide + water) 50.0 g C 6 H 12 O 6 = grams CO 2 C 6 H 12 O O 2 6 CO H 2 O T-Grid 50.0 g C 6 H 12 O 6 1 mol C 6 H 12 O 6 6 mol CO g CO 2 = 73.3 g CO g C 6 H 12 O 6 1 mol C 6 H 12 O 6 1 mol CO 2 Molar mass: 6 * * * 16.0 = 180. g C 6 H 12 O 6 in 1 mol Mole Ratio: 6 mol CO 2 = 1 mol C 6 H 12 O 6 Molar Mass: 1 * * 16.0 = 44.0 g CO 2 in 1 mol NOTICE: g C 6 H 12 O 6 mol C 6 H 12 O 6 mol CO 2 g CO 2 it followed the diagram! All you have to do is follow the steps: Step 1: Write what you are given and what you want (this step can be skipped when you get it ) Step 2: Write a balanced chemical equation Step 3: T-Grid and follow your diagram Find molar mass from periodic table (always 1 mole = mass in grams)

3 Find mole to mole ratio from chem eq (always balance equation!!!) Example: When calcium carbonate reacts with 25 grams of copper (I) bromide, how many grams of copper (I) carbonate will be formed? 25 g CuBr = g Cu 2 CO 3 CaCO CuBr CaBr 2 + Cu 2 CO 3 T-Grid 25 g CuBr 1 mol CuBr 1 mol Cu 2 CO g Cu 2 CO 3 = 16 g Cu 2 CO 3 (2 sig figs) g CuBr 2 mol CuBr 1 mol Cu 2 CO 3 NOTICE: g CuBr mol CuBr mol Cu 2 CO 3 g Cu 2 CO 3 Molar mass: 1 * * 79.9 = g CuBr in 1 mol Mole to Mole ratio: 1 mol Cu 2 CO 3 to 2 mol CuBr Molar mass: 2 * * * 16.0 = g Cu 2 CO 3 in 1 mol Gay-Lussac s law the stoichiometric coefficients in a chemical reaction relate to the volumes of gases in the equation as well as the number of moles of substances in the equation. So, not only can you change from Moles A Moles B, you can also change from Liters A Liters of B if all substances in the chemical reaction that you use are gases!!!!! OYO 6.5 Na 2 CO 3 (s) + 2 HCl (g) 2 NaCl (s) + CO 2 (g) + H 2 O (g) If the chemist wants to make 3.67 L of carbon dioxide gas, how many liters of HCl gas must be used? Since the given in the problem is a gas and what is wanted is a gas, you can use Gay-Lussac s law. This means that 1 L CO 2 gas is equal to 2 Liters of HCl. You work the problem in the same way as the previous examples: 3.67 L CO 2 2 L HCl = 7.34 L HCl 1 L CO 2 LIMITING REACTANT NOTES In real lab situations, one or more reactants is present in excess; that is, in more than the xact amounts required to react with the given amount of the other reactant according to the chem eq. When one of the reactants is used up, no more product can be formed, even if there is a leftover amount of the other reactant. The substance that is completely used up first is called the LIMITING REACTANT. It limits the amount of product that can be formed. The substance that is not used up is the EXCESS REACTANT. Example: To make one batch of cookies, you need 2 eggs per batch and 1oz salt. If you have a dozen eggs and 64 oz salt, how many batches of cookies can be made? o One dozen eggs can only make 6 batches, and will use 6 oz salt. o 64 oz salt will make 64 batches cookies and will use 128 eggs. o Eggs are the limiting reactant, and salt is in excess. This is exactly how you will determine LR and excess as well as product made!!!! Take reactant A and find out how much of B is needed Take reactant B and find out how much of A is needed Determine which one is LR and which has excess Use LR to determine amount of product made

4 Example: Silicon dioxide is usually quite stable but reacts with hydrogen fluoride according to the following reaction: SiO 2 (s) + 4HF (g) SiF 4 (g) + 2H 2 O (l) If 2.0 mol of HF is combined with 4.5 mol of SiO 2, which is the Limiting Reactant (LR)? Determine the amount of SiF 4 formed. Take reactant A and find out how much of B is needed 2.0 mol HF 1 mol SiO 2 = 0.5 mol SiO 2 4 mol HF Take reactant B and find out how much of A is needed 4.5 mol SiO 2 4 mol HF = 18 mol HF 1 mol SiO 2 Determine which one is LR and which has excess If we use all of the HF, we only need 0.5 mol SiO 2. We have 4.5 mol of SiO 2, so this rxn would have 4.0 moles of leftover, or excess, SiO 2. If we use all of the SiO 2, we need 18 mol HF. Problem: We only have 2 mol of HF, thus we don t have enough HF to go with the 4.5 mol SiO 2. Therefore, 2.0 mol HF must be the LR. Use LR to determine amount of product made 2.0 mol HF 1 mol SiF 4 = 0.5 mol SiF 4 would be made 4 mol HF Example: Some rocket engines use a mixture of hydrazine (N 2 H 4 ) and hydrogen peroxide (H 2 O 2 ) as the propellant system. The reaction is given by the equation..n 2 H 4 + 2H 2 O 2 N 2 + 4H 2 O What is the LR when mol N 2 H 4 reacts with mol H 2 O 2? How much nitrogen, in moles, will be formed? Take reactant A and find out how much of B is needed.750 mol N 2 H 4 2 mol H 2 O 2 = 1.50 mol H 2 O 2 1 mol N 2 H 4 Take reactant B and find out how much of A is needed.500 mol H 2 O 2 1 mol N 2 H 4 =.250 mol HF 2 mol H 2 O 2 Determine which one is LR and which has excess If we use all of the hydrazine, we only need 1.5 mol peroxide. We have.5 mol of peroxide, so this rxn would use more peroxide that we have. This means peroxide is the LR. If we use all of the peroxide, we need.250 mol hydrazine. This means that if we use all the peroxide, we will only use.250 mol of the hydrazine so we will have leftover hydrazine since we have.750 moles of it. Therefore,.500 mol of peroxide must be the LR. Use LR to determine amount of product made.500 mol H 2 O 2 1 mol N 2 = 0.25 mol N 2 would be made 2 mol H 2 O 2

5 Example: Aluminum reacts with hydrogen chloride to produce aluminum chloride and hydrogen gas. If 18 g of aluminum are combined with 75 g of hydrogen chloride, how much aluminum chloride will be produced? (This is a bigger problem. You will need to: write a balanced chemical equation, convert grams of aluminum to grams of hydrogen chloride, convert grams of hydrogen chloride to grams of aluminum, determine which reactant is the LR, then use the LR to find the grams of aluminum chloride produced.) 2 Al + 6 HCl 2 AlCl H 2 18 g Al 1 mol Al 6 mol HCl 36.5 g HCl = 73 g HCl 27 g Al 2 mol Al 1 mol HCl 75 g HCl 1 mol HCl 2 mol Al 27 g Al = 19 g Al 35.5 g HCl 6 mol HCl 1 mol Al The LR is 18 g Al, since using 75 g HCl requires the use of more Al than was given. 18 g Al 1 mol Al 2 mol AlCl g AlCl 3 = 89 g AlCl 3 27 g Al 2 mol Al 1 mol AlCl 3 Empirical and Molecular Formula, % Composition 1. Empirical formula is a chemical formula that tells you a simple, whole number ratio for the atoms in a molecule. It is the lowest possible subscripts. 2. Molecular formula is a chemical formula that provides the number of each type of atom in a molecule. 3. % Composition is the percent by mass of each element in a compound Example: C 4 H 12 is a molecular formula; CH 3 is the empirical formula. The first formula can be reduced by a factor of 4 to yield the second formula. Finding Empirical Formula given % Composition 1. Convert % s to grams by assuming a 100 g sample. There fore, 78% of 100 g would be 78 g. 34% of 100 g would be 34 g. Easy. 2. Convert grams to moles using a T-grid and molar mass from periodic table. 3. Divide each molar amount by the smallest number of moles. This is the smallest whole number ratio of atoms. Example: A compound contains 26.56% potassium, 35.41% chromium, and 38.03% oxygen. Find the empirical formula of this compound. 1. Convert % to grams: g K, g Cr, g O g K 1 mol K =.679 mol K g Cr 1 mol Cr =.681 mol Cr g O 1 mol O = 2.38 mol O 39.1 g K 52.0 g Cr 16.0 g O 3. Divide each mole amount by the smallest number of moles: /.679 = 1 K.681/.679 = 1.00 Cr 2.38 /.679 = 3.51 O 1 mol K: 1 mol Cr: 3.5 mol O, 3.5 must be doubled to get to the closet whole number so 2 mol K: 2 mol Cr: 7 mol O K 2 Cr 2 O 7

6 Example: g sample of a compound known to contain only phosphorus and oxygen yields g oxygen. What is the empirical formula of this compound? g O and g P g O 1 mol O =.3573 mol O g P 1 mol P =.143 mol P 16.0 g O 31.0 g P.3573 /.143 = 2.50 mol O.143 /.143 = 1 mol P 2.50 mol O: 1 mol P 5 mol O: 2 mol P P 2 O 5 Calculation of Molecular Formula 1. (Empirical Formula) x = Molecular Formula 2. X = mass of molecular formula / mass of empirical formula 3. Multiply the subscripts of the empirical formula by X. Example: Given empirical formula of P 2 O 5. Experiment shows the molecular formula mass to be g. Determine the molecular formula. 1. (P 2 O 5 ) x =? 2. X = g / 142 g = = 2 3. ( P 2 O 5 ) 2 = P 4 O 10 is the molecular formula Example: A compound with a molecular mass of g is found to be 85.64% C and 14.36% H by mass. Find its molecular formula. 1. First find empirical formula, then the empirical formula mass. Then proceed to calculate molecular formula g C 1 mol C =7.14 mol C g H 1 mol H = 14.2 mol H 12.0 g C 1.01 g H 7.14 /7.14 = 1 mol C 14.2/ 7.14 = = 2 mol H i.e: CH 2 is empirical formula 2. Calculate empirical formula mass: 1 * * 1.01 = g g / g = (CH 2 ) 3 = C 3 H 6 is the molecular formula Percent Composition 1. (Mass of element / molar mass of compound) x 100 = % of element Example: Find the percent composition of copper (I) sulfide, which occurs as the copper ore chalcolite. Cu 2 S is the chemical formula i.e. 2 mol Cu and 1 mol S is the makeup. Mass of compound is g. 2 mol Cu 63.5 g Cu = 127 g Cu x 100 = 79.8 % Cu 1 mol S 32.1 g S = 32.1 g S = 20.2% S 1 mol Cu g Cu 1 mol S g S Example: Find percent composition of each element in Ba(NO 3 ) 2. 1 mol Ba g Ba = g Ba x100= 52.5% Ba 2 mol N 14 g N = 28 g N x 100 = 10.7% N 1 mol Ba g 1 mol N mol O 16.0 g O = 96 g O x 100 = 36.7% O Mass of Ba(NO 3 ) 2 is g 1 mol O 261.3

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