Counting by mass: The Mole. Unit 8: Quantification of Chemical Reactions. Calculating molar mass. Particles. moles and mass. moles and particles

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1 Unit 8: Quantification of Chemical Reactions Chapter 10: The mole Chapter 12: Stoichiometry Counting by mass: The Mole Chemists can t count individual atoms Use moles to determine amounts instead mole is like dozen 1 mole = 6.022x10 23 particles of any substance It also relates to mass by comparing to atomic mass Carbon-12: 12gC 1kg 1amu 1000g 1.66x atomC = 6.02x10 23 atomsc kg 12amu 1 mole = atomic mass of any substance in grams 1 2 Particles atoms: single element ion: charged element or group molecules: covalently bonded nonmetals formula units: ionic compounds Molar Mass Calculating molar mass What is the molar mass of the following? sodium carbonate lead (II) nitrate bromine when we relate atomic mass to grams we call it molar mass. If you are dealing with a compound you add up all the atomic masses to get a total molar mass 3 4 moles and particles Convert 2.35 moles sodium carbonate to the proper particles. moles and mass How many grams are in 3.45x10-3 moles of potassium nitrate? Convert 5.67x10 21 molecules carbon dioxide to moles How many moles are in 2.23 grams magnesium chlorate? 5 6

2 moles and volume At standard temperature and pressure we can also relate moles to volume of gases. At STP 1 mole = 22.4 L of any gas How many moles are in 5500 ml of carbon monoxide gas at STP? 7 8 How many molecules are in 35.7 g of nitrogen? percent composition If you want 1.77x10 24 formula units of sodium chloride how many grams do you need? We can determine information about compounds from their percent composition You have 45.0g of oxygen gas. What volume in ml would you have at STP? 9 10 Percent composition percent by mass of individual elements in a compound. mass of element in compound mass % = 100% x mass of compound Percent of iron in iron (III) oxide. mass % Fe = 100% = % x Determining Percent composition from Formula or lab data 1. Determine the molar mass of compound (or total mass from a lab sample). 2. Determine the mass that each element contributes to the molar mass (or individual masses from lab sample) 3. Divide the mass each element contributes by the molar mass (or total mass from lab sample) 4. Multiply by 100 to get a percentage

3 Try it! Try it! What is the percent composition of each element in lithium sulfate? You decompose a g of an unknown nitrogen oxide compound. The product produces g of nitrogen gas. What is the percent composition of each element in this compound? Formulas empirical formula = smallest whole number ratio of a molecule. Not actual formula. example: CH2 molecular formula = (empirical formula)n [n = integer] If our integer is 6 then: molecular formula = (CH2)6 = C6H12 Determining Empirical Formulas from Percent composition. 1. List the elements known to be in the compound. Identify the percent composition of each element in the compound. 2. Assume you have 100 g of substance and convert the percent composition to a mass. 3. Convert the mass calculated from above to mole using atomic mass of each substance. 4. Determine the simplest whole number ratio of your moles for each element by dividing each number by the smallest one. Then use multipliers to make each element s number of moles a whole number. 5. Write empirical formula using subscripts as the number of moles for each element Try it! Determining molecular formula from an empirical formula. Go back to our unknown nitrogen compound practice problem. Using that data calculate the empirical formula. 1. Identify empirical formula of compound 2. Calculate the molar mass of the empirical formula 3. Look up the actual molar mass of your substance from information provided. 4. Divide actual molar mass by the empirical molar mass to get a multiplier for subscripts. 5. Multiple the subscripts in empirical formula by multiplier to get molecular formula

4 Identify mole ratios! C 3 H 8 + 5O 2 " 3CO 2 + 4H 2 O! 1 mol + 5 mol " 3 mol + 4 mol! If I have 4 moles of propane how many mols of carbon dioxide can I produce? Stoichiometry! When solving stoichiometry problems, you must start from a balanced equation.! All stoichiometry problems use mole ratios at some point.! Solving Stoichiometry problems: (Three steps method)! Organize what you know! Create the proper dimensional analysis sequence! Solve and verify! If I want to make 18 moles of water, how many mols of oxygen will be used? Example Problem! You have 25.6 g HCl and plenty (excess) sodium metal. How many grams hydrogen gas can be produced in this reaction?! Start by writing the reaction equation and balance: 2HCl + 2Na " H 2 + 2NaCl! Next, write what you know Example Problem! Finally, Set up the problem and solve: g HCl = 25.6 g Molar Mass H 2 = 2.02 g/mol Mole ratio HCl:H 2 = 2:1 Molar Mass HCl = g/mol Try these based on the following Al 2 + Cu 3 2 SO 4 " Al 2 (SO 4 ) 3 + Cu 6 1. If you have 50.0 g of Aluminum and excess of copper (I) sulfate, how many grams of copper can you produce? Using Density! Just adds one more step to get from grams to ml. Hg(NO 3 ) 2(aq) + Li 2 (s) " Hg (l) + LiNO 2 3(aq) 2. If you only have 35.3 g of copper (I) sulfate, how many grams of aluminum can you react? 1. If you have 43.8 g lithium and an excess of mercury (II) nitrate, how many ml of mercury can you produce? (Density of mercury = 13.5 g/ml) 3. If you produce 45.4 g of copper, how many grams aluminum sulfate will you make? 2. To produce 5.80 ml of Hg, how many grams of mercury (II) nitrate will you need? 23 24

5 Mole bridge Chart Limiting reactants! Calculations involving limited amounts of both reactants.! Determining the limiting reactant Limiting Reactant: runs out first; dictates the amount of product that can be produced. Excess Reactant: more present than what is needed.! No new math! Just need do calculation twice and compare results! Sample Problem! P 4 O 10 + H 6 2 O " H 4 3 PO 4! How many grams of phosphoric acid can you produce if you react 135 g of tetraphosphorus decoxide with 70.0 g of water?! How many grams of your excess reactant will be left over? Percent Yield What should happen vs. What DOES happen! Chemical equations tell you what should happen, but it isn t always what really happens.! Theoretical yield is the amount that you get from stoichiometry calculations. (math)! Actual yield is what you actually obtain, and is usually less than what is expected. (lab)! This might happen for several reasons.! Percent yield (efficiency) is a way to measure this difference: actual yield theoretical yield 100 = percent yield Recall Previous Problem How many grams of phosphoric acid can you produce if you react 135 g of tetraphosphorus decoxide with 70.0 g of water. P 4 O H 2 O " 4H 3 PO 4 Try another one! You combust 25.0 ml of liquid butane (C4 H10) with an excess of oxygen gas. If the reaction is known to be only 89.0 % efficient how many ml of carbon dioxide gas at STP will you actually make? (Density of butane 0.599g/ml)! If 145 g of phosphoric acid is actually produced what is the percent yield?