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Chapter 9 Chemical Quantities
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In order to avoid waste and to be economic, a chemist has to know how much to make, a doctor must know how much to prescribe, an engineer has to know how much fuel to mix with others, etc., etc., etc... Questions we answer here in Ch 9? How much can I get? How much do I need? The engineers who develop airbags have to make sure exactly the right amount of gas is made - no more, no less.
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Ch 9 is a compilation of Chs 1-8!
must name, balance, g to mol, mol to g… Notice that for the whole chapter, I’ll give you info on one thing, you’ll give me info on another but the only place they can meet is the balanced equation, the only language they speak is “molese”
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9.1 Information Given by Chemical Equations
The cook is a chemist! Cooking is: mixing ingredients (reactants) in a recipe (equation) to get a certain amount of food (product) Importance here? that info in a recipe can be used proportionally double amts? double product!
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The chemist is a cook! A 2:1 mole ratio from a balanced equation is just that - as long as the two substances are in 2:1 number ratio (not mass ratio!) it doesn’t matter how much you use
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9.2 Mole-Mole Relationships
Remember! The most common number ratio in chemistry is the mole ratio!
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Example How many moles of KClO3 must decompose to produce 9 moles of oxygen gas? (The other product is KCl.) 1) As always! First, get a balanced equation! 2KClO3 2KCl + 3O2 2) What is the mol ratio between KClO3 & O2?
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from balanced equation!
Example (cont’d) Remember this? 9 mol O2 2 mol KClO3 = 6 mol KClO3 3 mol O2 from balanced equation! Given
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Example When 5 mols sodium react with oxygen gas, how many mols of Na2O are formed? 4Na + O2 2Na2O 5 mol Na 2 mol Na2O = 2.50 mol Na2O 4 mol Na
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Example For this unbalanced equation, how many mols of NO do you need to make 7.5 mol H2O? NO + H2 N2 + H2O 2NO + 2H2 N2 + 2H2O it’s a 1:1 ratio! you need 7.5 mol!
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Example For this unbalanced equation, how many mols of SiH4 do you need to use with 4.2 mol NH3? SiH4 + NH3 Si3N4 + H2 3SiH4 + 4NH3 Si3N4 + 12H2 it’s a 3:4 ratio! you need 3.15 mol!
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9.3 Mass Calculations The study of the amount of substances consumed/produced = stoichiometry Remember that chemical equations tell you number ratios, not mass amounts! Your 3 friends now: 1) balanced chemical equations 2) mole ratios 3) math relationships
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We’ll start off simple with a problem in which I give you moles of one but you are asked about mass of another… But remember: the only place they can meet is the balanced equation, the only language they speak is molese
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CaC2 + 2H2O C2H2 + Ca(OH)2 Balanced equation!!!
CaC2 and water get together to make acetylene, C2H2, and calcium hydroxide. How many grams of water do you need to make 1.55 mol C2H2? Balanced equation!!! CaC2 + 2H2O C2H2 + Ca(OH)2 g mol H2O 1.55 mol C2H2
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Example cont’d mol bridge
They can only talk mole language from the balanced equation! 1.55 mol C2H2 2 mol H2O 18.0 g H2O 1 mol C2H2 1 mol H2O = 55.8 g H2O mol bridge
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Example How many grams of potassium chlorate (KClO3) must decompose to produce KCl and 1.45 mol oxygen gas? First! balance the equation!!! 2KClO3 2KCl + 3O2
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2KClO3 2KCl + 3O2 g mol KClO3 1.45 mol
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Example cont’d mol bridge
They can only talk mol language from the balanced equation! 1.45 mol O2 2 mol KClO3 122.6 g KClO3 3 mol O2 1 mol KClO3 = 119 g KClO3 mol bridge
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Example How many moles of copper must react with AgNO3 to produce 5.5 g silver and copper(II) nitrate? First! balance the equation!!! Cu + 2AgNO3 2Ag + Cu(NO3)2
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example cont’d Cu + 2AgNO3 2Ag + Cu(NO3)2 5.5g mol Cu mol Ag
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Example cont’d mol bridge
They can only talk mol language from the balanced equation! 5.5 g Ag 1 mol Ag 1 mol Cu 107.9 g Ag 2 mol Ag = mol Cu mol bridge
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(gramsA molsA ) (molsB gramsB )
This is for big people only; here I give you mass of one you tell me mass of the other key? start and end w/ grams; but must cross thru mol bridge!!! Basic method? (gramsA molsA ) (molsB gramsB ) PT BEq PT
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example How many grams of oxygen gas are required to completely react with 14.6 g of solid sodium to form sodium oxide, Na2O?
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4Na O2 2Na2O g 14.6 g mol Na mol O2 notice that there are always non-players. but they are needed for balancing the equation!
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example cont’d mol bridge
remember: they can only talk mol language from the balanced equation! 14.6 g Na 1 mol Na 1 mol O2 32.0 g O2 23.0 g Na 4 mol Na 1 mol O2 mol bridge = 5.08 g O2
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example When 20.4 grams of sodium metal are mixed with chlorine gas, are 52.0 g of sodium chloride produced?
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2Na Cl2 2NaCl g 20.4 g mol NaCl mol Na
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example cont’d mol bridge
They can only talk mol language from the balanced equation! 20.4 g Na 2 mol Na 2mol NaCl = 51.9 g NaCl 1 mol NaCl 58.5 g NaCl mol bridge 23.0 g Na 1 mol Na
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example Limestone, CaCO3, is heated to produce lime, calcium oxide, CaO, and carbon dioxide. How much limestone is required to produce 10.0 g of lime?
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CaCO3 CaO + CO2 g 10.0 g mol CaO mol CaCO3
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example cont’d mol bridge
They can only talk mol language from the balanced equation! 10. g CaO 1 mol CaO 1mol CaCO3 = 17.8 g CaCO3 1 mol CaCO3 100.1g CaCO3 mol bridge 56.1 g CaO
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summary gA gB PT PT molA molB BE
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9.5 Mass Calculations: Comparing Two Reactions
we can use all this to compare; for example, which store-bought antacid is more effective by weight… sodium bicarbonate and magnesium hydroxide are both used as antacids if you have 1.00 g of both which will eat the most acid?
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NaHCO3 + HCl NaCl + H2O + CO2
1) sodium bicarb? NaHCO3 + HCl NaCl + H2O + CO2 1.00 g NaHCO3 1 mol NaHCO3 1 mol HCl = 1.19 x 10-2 mol mol bridge 84.01 g NaHCO3 1mol NaHCO3
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2) magnesium hydroxide? mol bridge Mg(OH)2 + 2HCl MgCl2 + 2H2O
1.00 g Mg(OH)2 1 mol Mg(OH)2 2 mol HCl 58.33 g Mg(OH)2 1 mol Mg(OH)2 mol bridge = 3.42 x 10-2 mol
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Sorry, but equations Do Not describe:
1) Exact conditions * Temperature * Pressure * Volume 2) What the atoms are doing (e.g. not all of them react)
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9.6 The Concept of Limiting Reactants
Just as if you were to make sandwiches there will be something left over, Impossible for every atom/molecule to react, so… must add one reactant in there in excess (xs) to react as much of the other as possible
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what is the limiting ingredient here. i. e
what is the limiting ingredient here? i.e. what will stop you from making more sandwiches?
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the “limiting reactant” here were hammers
how many sets of 1 pliers, 2 screwdrivers, and 1 hammer can you make, and what is left over?
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Mg reacts w/ O2 in 2:1 ratio So the 6 Mg on left only need 3 O2’s; but there are 7 O2’s! what happens to the remaining oxygens???
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4 O2’s will not play, but they assure that (essentially) all Mg’s will be found and destroyed!
Mg’s will run out first; they limit the amount of product that can be made; Mg is the limiting reactant
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Here N2 is in excess, H2 was the limiting reactant
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9.6 EOCs 29
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9.7 Calculations Involving a Limiting Reactant
If reactants in a reaction are not present in their mol ratios, one will be used up before the other = limiting reactant; the other one = excess but you can still predict how much prodruct will be made; yippee! [hint: limiting reactant probs betray themselves in asking how much product you can get when they give amts of 2 reactants]
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You must determine which of the 2 is limiting and finish the prob w/ that one only, like this:
1) change amts of both reactants (masses) to mols (old stuff) 2) check mol ratios for which one will be used up first (limiting reactant), (new thing!) 3) use that one only to finish prob (forget about the xs reactant) (old stuff)
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example We’ll start simple:
1.21 mol zinc are added to 2.65 mol HCl. Zinc chloride and hydrogen gas are formed. Which reactant is in XS? Calculate the amt of mols of zinc chloride produced. START W/ BALANCED EQUATION!
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example cont’d Zn + 2HCl ZnCl2 + H2 the Zn:HCl is 1:2
Only one reactant can take us to product (= LR) Will it be Zn or HCl???
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example cont’d Zn + 2HCl ZnCl2 + H2
One way is to pretend you don’t know about one of the reactants, e.g... If there are 1.21 mol Zn, how many mols of HCl would you need? the Zn:HCl is 1:2, so 1.21 mol Zn need twice as much HCl = 2.42 mol HCl
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example cont’d Zn + 2HCl ZnCl2 + H2 Do we have 2.42 mol HCl???
YESSIREEBOB!, and more! (2.65 mol) We have plenty of HCl (= XS) :) but we will run out of Zn (= LR) :( Zn will take us to products!...
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example cont’d Zn + 2HCl ZnCl2 + H2 then, back to the old ways...
1.21 mol Zn 1 mol Zn 1 mol ZnCl2 = 1.21 mol ZnCl2
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example Zn + 2HCl ZnCl2 + H2 What if reactants are given in grams?
No problem! Just change to mols first... 79.1 g Zn react with 76.5 g HCl. 1) Which is Limiting Reactant? 2) How much H2 will be formed?
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example cont’d Zn + 2HCl ZnCl2 + H2 Balanced equation!
Change the reactants to mols and compare: 79.1 g Zn 1.21 mol Zn 76.5 g HCl 2.10 mol HCl
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example cont’d Zn + 2HCl ZnCl2 + H2
pretend not to know about one of the reactants, this time Zn if there is a 2 HCl : 1 Zn ratio, and we have 2.10 mol of HCl, we should have at least half as much Zn (1.05 mol) for a complete reaction we have 1.21 mol Zn = PLENTY! = XS
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example cont’d HCl will take us to product! = 2.12 g H2 2 mol HCl
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example 1.00 g Zn reacts with 6.2 x 10-3 mol Pb(NO3)2 to form Zn(NO3)2 and Pb… 1) Which is LR? 2) How much Pb will be formed? first, the balanced equation!...
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example cont’d Zn + Pb(NO3)2 Pb + Zn(NO3)2
find the mols of both players: 1.00 g Zn mol Zn (done already!) 6.2 x 10-3 mol Pb(NO3)2
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example cont’d Zn + Pb(NO3)2 Pb + Zn(NO3)2
again, pretend we have only one (Zn) if we have only mol Zn, we need mol Pb(NO3)2 (1:1 ratio) we don’t have enough!!! So Pb(NO3)2 is the Limiting Reactant and will take us to the products...
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example cont’d Zn + Pb(NO3)2 Pb + Zn(NO3)2
same old way to our destination... .0062 mol Pb(NO3)2 1 mol Pb(NO3)2 1 mol Pb = 1.3 g Pb 207.2 g Pb
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what happens when there is not enough of one reactant (l)
not enough O2 means sooty flame
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9.7 EOCs 35 all, 38
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9.8 Percent Yield Up to this point, all the calculations we’ve done only predicts what we’d get in a perfect world, but… Reactants aren’t always 100% pure; there are spills, etc., therefore, what we really get will be less
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The predicted yield is called a theoretical yield (the one we do on paper)
What we actually get is called actual yield (the one we get from the lab) percent yield is... percent yield = actual yield theoretical yield • 100
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example In the reaction of Zn with HCl, g ZnCl2 are formed, although the theoretical yield was 143 g. What was the % yield? percent yield = g 143 g • 100 percent yield = 98.0 %
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Big Person example For the reaction: K2CO3 + 2HCl 2KCl + H2O + CO2
if 45.8 g K2CO3 are added to xs HCl, 46.3 g of KCl are recovered. 1) What is the theoretical yield? 2) What is the percent yield?
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example cont’d theoretical yield: = 49.4 g KCl 45.8 g K2CO3
1 mol K2CO3 2 mol KCl = 49.4 g KCl 1 mol KCl 74.6 g KCl 138.2 g K2CO3
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example cont’d Now we can find % yield... percent yield = 46.3 g
• 100 percent yield = 93.7 %
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chemists shoot for 100% but are often satisfied with < 50%
chemists involved in manufacturing, medicine, plastics, etc., constantly strive for the highest profit margin; i.e. they seek the elusive 100% yield
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9.8 EOCs 42, 45, 52ab, 58, 59XC
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