How dimensional analysis is used to solve stoichiometry problems.How to gather equalities for use in stoichiometry problems.How to use Molarity.How to use the molar volume
f t STP
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of a gas at STP.How to find the limiting reactant.How to find percent yield.
Use the KUDOS method and dimensional analysis to solve stoichiometry problems.
Identify the unknown.
Identify needed definitions.
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DFor dimensional analysis, the “definitions” are the equalities:Ratio from balanced equation (one compound ↔ another compound)Molar mass (grams ↔ moles)
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Find the output - perform the dimensional analysis.
Substantiate your answer.
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Molar mass (grams ↔ moles)Molarity (grams ↔ liters of a solution)Molar volume of a gas (moles ↔ liters of a gas)
Example:
Mole-Mole ProblemsIf 4.2 mole of H2 reacts completely with O2, how many moles of O2 are needed?
2 H2 + O2 2 H2O
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4.2 mole H2
mole H
mole O2 = ________ mole O22
1 2.1
From balanced equation: 2 mole H2 1 mole O2
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mole H22
2.1 is a reasonable answer when 4.2 is given.“mole O2” is the correct unit.2 sig. fig. given 2 sig. fig. in answer.
Using MolarityMolarity is used to convert between moles and liters.Example: If 0.85 moles NaOH are needed and you have a 1.5 M
solution, how many liters of the solution do you need?
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0.85 mol NaOH
mol NaOH
L = ________ L1.5
1 0.57
From concentration: 1.5 moles NaOH = 1 L
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0.57 is a reasonable answer for L.“L” is the correct unit.2 sig. fig. given 2 sig. fig. in answer.
From balanced equation:
Example:
Mass-Volume Problems (Solutions)If you need to precipitate 15.7 g of Ba(OH)2, how many liters of 2.50 M NaOH solution is needed? 2 NaOH + BaCl2 Ba(OH)2 + 2 NaCl
q2 mol NaOH 1 mol Ba(OH)2
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15.7 g Ba(OH)2 mol Ba(OH)21
Concentration of NaOH:2.50 mole NaOH = 1 L
mol NaOH2 L NaOH1
Molar Mass of Ba(OH)2:1 mol Ba(OH)2 = 171.35 g
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g ( )2
g Ba(OH)2
mol Ba(OH)2
= ________ L NaOH
171.35
0.07330.0733 is a reasonable answer for L (73.3 mL).“L NaOH” is the correct unit.3 sig. fig. given 3 sig. fig. in answer.
Mass-Volume Problems (Gases)If you need react 1.5 g of zinc completely, what volume of gas will be produced at STP?2 HCl (aq) + Zn (s) ZnCl2 (aq) + H2 (g)
q1 mole Zn 1 mole H2
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1.5 g Zn mole Zn1
Molar volume of a gas:1 mole H2 = 22.4 L
mole H21 L H222.4
Molar Mass of Zn:1 mole Zn = 65.39 g
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0.51 is a reasonable answer for L.“L H2” is the correct unit.2 sf given 2 sf in answer.
Planning a MealYou go to the grocery store and you buy 1 package of Brats (5 Brats), 1 package of cheese (16 slices) and 1 package of hot dog buns (8 buns).p g g ( )
If you use all of these… You can make this many…
5 Brats 5 meals
16 slices of cheese
8 hot dog buns
16 meals
8 meals
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So you have the possibility of making 5, 16 or 8 meals…which is it?
You’ll never get the chance to make 8 or 16 meals…you’ll run out of Brats after 5.
Once you run out of one component, you have to stop making meals.
Definition: Limiting Reactant
Limiting Reactant – The reactant that runs out firstreactant that runs out first and causes the reaction to stop.
In the previous example, the Brats were the limiting reactant—once they were gone you had
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limiting reactant—once they were gone, you had to stop!
Once one of the reactants runs out, the reaction stops…it can’t make any more product.
You’ll never get to 4.6 moles of H2O because H2 runs out when 2.3 moles H2O is made.2.3 is reasonable for moles.
“moles H2O” is the correct unit.2 sig. fig. given 2 sig. fig. in answer. S
Example:
Identifying the Limiting ReactantWhat is the limiting reactant in the last problem?2 H2 + O2 2 H2O …Calculate and compare the amount of products produced by each reactant .
2.3 mol O2 mol H2O l H O2 4 6
The reactant that produces a lesser amount of product is the limiting reactant.
2
mol O2
2 = ________ mol H2O1
4.6
2.3 mol H2
mol H2
mol H2O = ________ mol H2O2
2 2.3Less product
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limiting reactant.H2 was the reactant that ran out (caused the smallest number of products) therefore, H2 is the limiting reactant.
Simple Limiting Reagent Finder Mnemonic: Find the moles of each reactant and divide the moles of each reactant by its coefficient. The reactant with the smallest number is the limiting reagent = “Smallest m/c is Limited”.
Definition: Percent YieldPercent Yield – Compares how much the reaction actually produced (the “actual yield”) to how much the stoichiometry says you should get if things react completely 100% (the “theoretical yield”).
actual yield% i ld ×100
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y% yield = ×100theoretical yield
% yield is always calculated with masses—not moles!
Percent Yield ProblemA student works out a stoichiometry problem and determines that they should produce 1.56 g AgCl in the lab. They complete the lab and find that they obtained 1.25 g AgCl. Find the percent yield of the lab.
Theoretical yield = 1.56 g AgClActual yield = 1.25 g AgCl
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D% yield = ?
actual yield% yield = ×100theoretical yield
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80.1 is a reasonable answer for percent yield.“%” is the correct unit.3 sig. fig. given 3 sig. fig. in answer.
