Chemical reactions: chemical changes that occur when...
Transcript of Chemical reactions: chemical changes that occur when...
Lecture outline: Section 3Chemical reactions: chemical changes that occur when substances react to that occur when substances react to form new substances
1. Chemical equations2. Atomic and molecular mass3. Chemical calculations
1S. Ensign, stoichiometry and calculations
Law of conservation of mass:atoms are not created or destroyed during chemicaldestroyed during chemical reactions. They simply y yrearrange.
Mass before = mass after
2S. Ensign, stoichiometry and calculations
Stoichiometry: the quantitative nature of chemical formulas and reactionsreactions
Th t d f titi f - The study of quantities of materials consumed and produced in chemical reactions.
3S. Ensign, stoichiometry and calculations
Chemical Equations
++reacts with
to produce
Left of arrow: starting substances, “reactants” right of arrow: substances produced, “products”products”
Conservation of mass: atoms, left side = atoms, right side
4S. Ensign, stoichiometry and calculations
Chemical Equations
+ +
2H2 + O2 2H2O
Subscript after an element in a compound: number of atoms of that element in the compound
Stoichiometric coefficients: the number of compounds involved in the reaction
5S. Ensign, stoichiometry and calculations
Note the difference in meaning for a ffi i t i f t f f lcoefficient in front of a formula vs. a
subscript to a formula
6S. Ensign, stoichiometry and calculations
Balancing a chemical Equation•Identify the reactants and products
W it th b l d h i l ti•Write the unbalanced chemical equation
•Balance equation by addition of properBalance equation by addition of proper coefficients in front of reactants and products so that the atoms balance on bothproducts so that the atoms balance on both sides of equation
B l fi t th l t( ) th t i lBalance first the element(s) that occur in only one substance on each side of the equation
7S. Ensign, stoichiometry and calculations
•Sodium reacts with chlorine gas (Cl2) to produce sodium chloride
•Potassium reacts with water to produce potassium hydroxide and hydrogen gas (H2)
•Identify the reactants and products
•Write the unbalanced chemical equation
•Balance equation by addition of proper coefficients in front ofBalance equation by addition of proper coefficients in front ofreactants and products so that the atoms balance on both sides of equation
8S. Ensign, stoichiometry and calculations
Write a balanced chemical equation for the reaction of methane gas (CH4) with molecular g ( 4)oxygen (O2) to produce water and carbon dioxide
•Identify the reactants and products
•Write the unbalanced chemical equationWrite the unbalanced chemical equation
•Balance equation by addition of proper coefficients in front of reactants and products so that the atoms balance on both sides of equation
Balance first the element(s) that occur in only one substance on each side of the equation
9S. Ensign, stoichiometry and calculations
Additional information from b l d h i l ibalanced chemical equations
• Physical states of reactants and productsPhysical states of reactants and products
• Reactants or products that are dissolved in water
10S. Ensign, stoichiometry and calculations
Common types of chemical reactionCommon types of chemical reaction
• CombustionCombustion• Combination
d iti• decomposition
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Combustion: A chemical reaction where something burns in oxygen producing heatsomething burns in oxygen, producing heat
• Combustion of metals
• Combustion of diatomic molecules
• Combustion of nonmetal compoundsp
C b ti f h d b• Combustion of hydrocarbons
• Combustion of more complex organic molecules12
S. Ensign, stoichiometry and calculations
Hydrocarbon combustion• Reactants are a hydrocarbon (CxHy) and O2,
products are CO2 and H2O•
13S. Ensign, stoichiometry and calculations
Write the balanced chemical equation for the combustion of butane (tetracarbon decahydride)combustion of butane (tetracarbon decahydride)
•Identify the reactants and products
•Write the unbalanced chemical equation
•Start by balancing an element that only occurs in one compound on each side of the equation
14S. Ensign, stoichiometry and calculations
C b tiCommon types of chemical reaction
• Combustion• Combination
decomposition• decompositionhttp://biochemistryportal.com/chemistry%201210%20preview/chemistry1210resources/media/CONTAC%7E1.MOV
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Patterns of chemical reactivityPatterns of chemical reactivity
• Alkali metal + waterAlkali metal + water
• Alkali metal + halogen
16S. Ensign, stoichiometry and calculations
Concept check• Chemical formulas
– subscriptssubscripts• Chemical equations
– coefficients– coefficients• Balancing equations• Common reactions• Common reactions
– CombustionCombination– Combination
– Decomposition
17S. Ensign, stoichiometry and calculations
Chemical calculations
• Atomic massForm la mass• Formula mass
• The mole (mol)• Empirical and molecular formulas• Chemical equations and balancingChemical equations and balancing
18S. Ensign, stoichiometry and calculations
Atomic Mass: The mass of an atom is proportional to the number of p+ n and e- the atom hasthe number of p , n, and e the atom has
subatomicparticle
mass in grams mass in amuparticleproton 1.673 x 10-24 g 1.0073 amuneutron 1.675 x 10-24 g 1.0087 amugelectron 9.11 x 10-28 g 5.49 x 10-4
1 amu = 1 66054 x 10-24 g1 amu = 1.66054 x 10-24 g
19S. Ensign, stoichiometry and calculations
1 amu = 1 66054 x 10-24 g1 amu = 1.66054 x 10 g
1 1/12 th f 12C1 amu ≡ 1/12 the mass of 12C
th f 1 t f 12C 12the mass of 1 atom of 12C ≡ 12 amu
20S. Ensign, stoichiometry and calculations
Sum the masses of the subatomic particles of 12C and
By definition, the mass of 1 atom of 12C ≡ 12 amu
Sum the masses of the subatomic particles of 12C and what mass do you get?
p+ nn
subatomicparticle
mass in grams mass in amu
p+n p+p+
n p+p+nn
proton 1.673 x 10-24 g 1.0073 amuneutron 1.675 x 10-24 g 1.0087 amuelectron 9.11 x 10-28 g 5.49 x 10-4
21S. Ensign, stoichiometry and calculations
The masses of the elements are determined experimentally. They experimentally. They cannot be accurately predicted by the masses predicted by the masses of the constituent parts
22S. Ensign, stoichiometry and calculations
The greater the mass defect, the more stable an isotope of anmore stable an isotope of an
element is
23S. Ensign, stoichiometry and calculations
Average atomic masses•Most elements in nature are mixtures ofMost elements in nature are mixtures of more than 1 isotope•“Average atomic mass” incorporates the•“Average atomic mass” incorporates the relative abundance of each isotope in nature
p+
p+ nn p+
np+
n p+p+nn p+
p+ nn p+
np+
n p+p+nn n p+
p+ nn p+
np+
n p+p+nn n
n
6 6 66 protons6 neutrons
6 protons7 neutrons
6 protons8 neutrons
12C 13C98.892 % 1.108 % Trace amount (<0.001%)
14C(stable) (stable) (radioactive)
24S. Ensign, stoichiometry and calculations
Average atomic mass of C found in naturep+
p+ nn p+
np+
nn p+
p+ nn p+
np+
nn n p+
p+ nn p+
np+
nn nn
pn p+p+
nn pn p+p+
nn n pn p+p+
nn n
6 protons6 neutrons
6 protons7 neutrons
6 protons8 neutrons
12C 13C 14C
6 neutrons 7 neutrons 8 neutrons
C C98.892 %12 amu
1.108 %13.00335 amu
Trace amount (<0.001%)C
25S. Ensign, stoichiometry and calculations
Naturally occurring bromine is composed of two isotopes which have atomic masses of 78.918336 and 80.916289amu Isotope 1 is present at an abundance of 50 69%amu. Isotope 1 is present at an abundance of 50.69%while isotope 2 is present at an abundance of 49.31%. Use this information to determine the average atomic
f t ll i b imass of naturally occurring bromine
26S. Ensign, stoichiometry and calculations
The atomic weights provided in a Periodic Table are the average atomic weights of the naturally occurring elements
27S. Ensign, stoichiometry and calculations
Formula massFormula mass
• Atoms have atomic masses (atomicAtoms have atomic masses (atomic weights)
• Compounds have formula masses• Compounds have formula masses (formula weights)T l l t f l th• To calculate formula mass, sum the atomic masses of all atoms in the
l lmolecule
28S. Ensign, stoichiometry and calculations
Molecular massRecall that the formulas for ionic compounds are• Recall that the formulas for ionic compounds are always written as “empirical formulas” because there is no discrete molecular formulathere is no discrete molecular formula
• The masses of molecular compounds with defined formulas may be referred to as either “formulaformulas may be referred to as either formula masses” or “molecular masses” (molecular weights)
• The masses of empirical compounds are always p p yreferred to as “formula masses” (formula weights)
NaCl, FW = 58.44 C6H6, FW = 78.11 C2H2, FW = 26.03629
S. Ensign, stoichiometry and calculations
What is the formula weight of ammonium sulfate?
8(NH4)2SO46N
1H
1 008
8O
16.00
(NH4)2SO4
N14.01
1.008
16S
32 0632.06
30S. Ensign, stoichiometry and calculations
1 8
Determine the molecular mass of ethanol, C2H6O
6C
12 01
1H
1.008
O16.00
12.01
A.About 29 amu
B About 40 amuB.About 40 amu
C About 58 amuC.About 58 amu
D.About 46 amu
3131S. Ensign, stoichiometry and calculations
Percent composition: the percentage, by t ib t d b h l t imass, contributed by each element in a
substance
( )( ) 100AWElementofAtomsElement % ×= 00CompoundofFW
e e%
32S. Ensign, stoichiometry and calculations
What are the mass percentages of each element in ammonium sulfate?
(NH4)2SO4 61 8 16
( )( ) 100Compound of FW
AWElementofAtomsElement % ×=
( 4)2 4 6N
14.01
1H
1.008
8O
16.00
16S
32.06FW = 132.10
33S. Ensign, stoichiometry and calculations
How do we bridge the How do we bridge the microscopic world of amu p f
with the macroscopic ld f ?world of grams?
34S. Ensign, stoichiometry and calculations
This week in chemistry• Chapters 1 and 2 graded quizzes Chapter 3 graded• Chapters 1 and 2 graded quizzes,Chapter 3 graded
quiz available Wednesday• Recitations meet graded bring iclickersRecitations meet, graded, bring iclickers.
– Grading: two points participation, two points maximum for correct answers. I wrote this week’s questions
• Office hours: Mon. and Wed., 2:30-4 PM, other times by appt.
• Supplemental instruction: M, T, W, R• Read materials for chapters 3 and 4- overheads and
text• work problems in chapters 1-3 self tests, watch self
35
test tutorials to help in developing problem-solving skills. Work old exam problems. 3535
S. Ensign, stoichiometry and calculations
The mole (mol)• The S.I unit of “amount of a substance”• A collection of a very large number of• A collection of a very large number of
particles• “The amount of a substance that
contains as many objects (atoms, y j (molecules, or other particles) as the number of atoms in exactly 12 grams ofnumber of atoms in exactly 12 grams of the 12C isotope”
36S. Ensign, stoichiometry and calculations
1 mol = 6.022136736 x 1023
particles
“Avogadro’s number”
602,200,000,000,000,000,000,000
How big a quantity is the mol????
37S. Ensign, stoichiometry and calculations
• One 12C atom weighs 12 amu• 6 02 x 1023 12C atoms weigh 12 grams• 6.02 x 10 C atoms weigh 12 grams• One mol of 12C atoms weigh 12 grams
38S. Ensign, stoichiometry and calculations
Molar mass: the mass, in grams, of 1 mol (6.02 x 1023 particles) of a substancea substanceThe more a particular particle weighs, the p p g ,higher it’s molar mass
O l f i h thOne mol of pennies has more mass than one mol of carbon atoms
3939S. Ensign, stoichiometry and calculations
1 molecule of H2O: 1 dozen molecules of H2O: 1 mole of H2O molecules: 2mass = 18.0 amu = 2.99 x 10-23 g
2mass = 216 amu = 3.59 x 10-22 g
o e o 2O o ecu esmass = 1.08 x 1025 amu = 18.0 g
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Mass = 18.0 amu/molecule Mass = 18.0 g/mole
40S. Ensign, stoichiometry and calculations
Review some terms• Atomic mass: mass of a single atom of a single
isotope amu• Average atomic mass: average mass of a
collection of isotopes (as found in nature) amu• Formula mass: the sum of the average atomic
masses of the elements in a compound amu• Molecular mass: the same as formula weight,
applied to covalent (molecular) compounds amuM l th f l f ti l / l• Molar mass: the mass of a mol of particles g/mol
41S. Ensign, stoichiometry and calculations
Interconversion of mass mols and Interconversion of mass, mols, and number of particles
Molar mass
Avogadro’snumbermass moles moleculesmass number
42S. Ensign, stoichiometry and calculations
Using molar mass and Avogadro’s number as conversion factors
12.01 g C atoms 18.02 g H2O moleculesg1 mol C atoms
g 2
1 mol H2O molecules
1 mol C atoms 1 mol H O molecules12.01 g C atoms 18.02 g H2O molecules
1 mol H2O molecules
1 mol C atoms
12.01 g C atoms
1 mol C atoms6.02 x 1023 C atoms
6.02 x 1023 H2O molecules1 mol H2O molecules
12.01 g C atoms6.02 x 1023 C atoms
6 02 x 1023 C atoms
6.02 x 1023 H2O molecules
1 mol H2O molecules
12.01 g C atoms6.02 x 1023 C atoms 2
43S. Ensign, stoichiometry and calculations
An experiment calls for 0.36 mol of Al. What mass of Al is needed?? 13What mass of Al is needed?? 13
Al26.99
26.99 amu or 26.99 g/mol
44S. Ensign, stoichiometry and calculations
How many moles of silicon correspond to 454 g Si??14
28 09 28 09 / l Si28.09
28.09 amu or 28.09 g/mol
45S. Ensign, stoichiometry and calculations
80You have a graduated cylinder containing 25.4 ml Hg. How many moles of Hg are in the cylinder?
Hg200.6
How many atoms of Hg are in the cylinder?
dHg = 13.534 g/ml
46S. Ensign, stoichiometry and calculations
Stoichiometric relations of atoms in a moleculeSucrose is C12H22O1112 22 11
•1 molecule of sucrose contains 12 atoms of C
1 l l f i 22 f H•1 molecule of sucrose contains 22 atoms of H
•1 molecule of sucrose contains 11 atoms of O•1 mol of sucrose contains 12 mols of C
•1 mol of sucrose contains 22 mols of H1 mol of sucrose contains 22 mols of H
•1 mol of sucrose contains 11 mols of OW it th l ti i f tWrite these relations as conversion factors
47S. Ensign, stoichiometry and calculations
Which conversion factor is incorrect for sucrose, C12H22O11 :
A. Catoms 12sucrose molecule 1
l lB.Hatoms 22
sucrose 1molecule
C. sucrose 1moleculeHmol 22
sucrose1molD.O mol 11
sucrose 1mol48S. Ensign, stoichiometry and calculations
Interconversion of mass mols and Interconversion of mass, mols, and number of particles
l l lMolar mass
Avog.#l
Stoich.relationmass molesa moleculesmass #molesbrelation
49S. Ensign, stoichiometry and calculations
How many C atoms are present in 46.0 grams of sucrose, C12H22O11?grams of sucrose, C12H22O11?
6C
1H
1 008 12.011.008
8OO
16.00
50S. Ensign, stoichiometry and calculations
You have 23.6 grams of ethanol, C2H6O.•How many moles of ethanol do you have?
•How many molecules of ethanol do youHow many molecules of ethanol do you have?
•How many atoms of carbon do you have?•How many atoms of carbon do you have?
•What is the mass of one molecule of th l i ?ethanol in grams?
5151S. Ensign, stoichiometry and calculations
You have 23.6 grams of ethanol, C2H6O.
•How many moles of ethanol do you have?mass molesa
moleculesor atoms
Molar mass
Avog.#
molesb
Stoich.relation
•How many molecules of ethanol do you have?
•How many atoms of carbon do you have?
•What is the mass of one molecule of ethanol in grams?
5252S. Ensign, stoichiometry and calculations
Problem solving strategy for stoichiometric calculationsMolar AvogStoich Stoich
M l i i f t ( / l)
mass molesamoleculesor atoms
Molar mass
Avog.#molesb
Stoich.relation
Stoich.relation molecules
or atoms
• Molar mass is a conversion factor (g/mol)• For a compound, the relationship between atoms
and molecules is a conversion factorand molecules is a conversion factor
• For a compound the relation between numbersFor a compound, the relation between numbers of mols is a conversion factor
• Avogradro’s number can be used as a converion foactor to convert mols and molecules (or atoms)
53S. Ensign, stoichiometry and calculations
Example to illustrate problem solving strategy for stoichiometric calculations
B j t b i thi bik i f t b• By just observing this bike, no conversion factors can be deduced equating the mass of the bike, the bike frame, the wheels and pedalsp
• By just observing the bike, we can deduce conversion factors relating the numbers of the components of the bike
• Scale these relationships up to a collection of parts equal to a• Scale these relationships up to a collection of parts equal to a mol
54S. Ensign, stoichiometry and calculations
Empirical formula from experimental analysisp p y
1. Base calculation on 100 grams of compound.
2. Determine moles of each element in 100 grams of compound.
3. Divide each value of moles by the3. Divide each value of moles by the smallest of the values.
4 Multiply each number by an integer to4. Multiply each number by an integer to obtain all whole numbers.
5555S. Ensign, stoichiometry and calculations
The compound hydrazine (FW = 32) consists f N d H Th t f N dof N and H. The mass percentages of N and
H in hydrazine are 87.42% and 12.58%, respecti elrespectively.
What is the empirical formula for hydrazine?hydrazine?What is the molecular formula for hydrazine?hydrazine?
5656S. Ensign, stoichiometry and calculations
The compound hydrazine (FW = 32) consists of N and H. The mass percentages of N and H in hydrazine are 87.42% and 12.58%, respectively.
What is the empirical formula for hydrazine?What is the empirical formula for hydrazine?What is the molecular formula for hydrazine?
1. Base calculation on 100 grams of compound.2. Determine moles of each element in 100 grams of compound.3. Divide each value of moles by the smallest of the values.4. Multiply each number by an integer to obtain all whole numbers.
5757S. Ensign, stoichiometry and calculations
What is the molecular formula for hydrazine?•Determine true molar mass of hydrazine experimentallyy p y
•Compare true molar mass to the molar mass based on empirical formula
•The true molar mass will be the empirical mass multiplied by some number “n”
•Multiply “n” by the empirical formula to get the molecular formula
5858S. Ensign, stoichiometry and calculations
Mothballs are made of napthalene, a C and H containing compound. The mass % of C in naphthalene is 93.71%, while the mass % of H is 6.29%. Naphthalene has a true molar mass of 128 amu.What is the empirical formula for naphthalene?What is the empirical formula for naphthalene?What is the molecular formula for naphthalene?
5959S. Ensign, stoichiometry and calculations
Mothballs are made of napthalene, a C and H containing compound. The mass % of C in naphthalene is 93.71%, while the mass % of H is 6.29%. Naphthalene has a true molar mass of 128 amu.What is the empirical formula for naphthalene?What is the molecular formula for naphthalene?What is the molecular formula for naphthalene?
6060S. Ensign, stoichiometry and calculations
Quantitative information from b l d h i l tibalanced chemical equations
2H2 + O2 2H2O2 2 2
6161S. Ensign, stoichiometry and calculations
Quantitative information from balanced chemical equations
2H2 + O2 2H2Obalanced chemical equations
Stoichiometricall eq i alent q antities are sed• Stoichiometrically equivalent quantities are used as conversion factors
• Molecules relate to molecules as mols relate to• Molecules relate to molecules as mols relate to mols
• How many mols of H O are produced from the• How many mols of H2O are produced from the reaction of excess H2 with 3.5 mols of O2?
6262S. Ensign, stoichiometry and calculations
A camping propane cylinder contains one pound of propane (C H ) What mass ofpound of propane (C3H8). What mass of CO2 is produced from the complete
b ti f ll th i thcombustion of all the propane in the cylinder? 1 lb = 454 gg
Write a chemical equation for this reactionBalance the equationBalance the equationUse the proper stoichiometric relationships
from the balanced equation
massa molesa massb
Molar mass
Molar massmolesb
Stoich.relation
63
massa molesa massbmolesb
63S. Ensign, stoichiometry and calculations
A camping propane cylinder contains one pound of propane (C3H8). What mass of CO2 is produced from the complete combustion of all the propane in the cylinder? 1 lb = 454 gcombustion of all the propane in the cylinder? 1 lb 454 g
mass moles massb
Molar mass
Molar massmolesb
Stoich.relationmassa molesa massbmolesb
6464S. Ensign, stoichiometry and calculations
A camping propane cylinder contains one pound of propane (C3H8). What mass of CO2 is produced from the complete combustion of all the propane in the cylinder? 1 lb = 454 gcombustion of all the propane in the cylinder? 1 lb 454 g
mass moles massb
Molar mass
Molar massmolesb
Stoich.relationmassa molesa massbmolesb
6565S. Ensign, stoichiometry and calculations
Limiting reactants2H + O 2H O2H2 + O2 2H2O
66
No limiting reactant66S. Ensign, stoichiometry and calculations
Limiting reactants2H + O 2H O2H2 + O2 2H2O
67
No limiting reactant67S. Ensign, stoichiometry and calculations
Limiting reactants2H + O 2H O2H2 + O2 2H2O
68
O2 is the limiting reactant (H2 is in excess)68S. Ensign, stoichiometry and calculations
Limiting reactants2H + O 2H O2H2 + O2 2H2O
69
H2 is the limiting reactant (O2 is in excess)69S. Ensign, stoichiometry and calculations
Silicon is used in computer chips and solar ll Th fi l t i th f t i fcells. The final step in the manufacturing of
silicon “metal” is SiCl + 2M Si + 2M ClSiCl4(l) + 2Mg(s) Si(s) + 2MgCl2(s)
225 g of SiCl4 is mixed with 225 g Mg.Is one reagent present in a limiting
amount, and if so, which one?Wh t f Si( ) i f d if thWhat mass of Si(s) is formed if the
reaction goes to completion?What mass of the excess reagent is leftWhat mass of the excess reagent is left
over after the reaction goes to completion?Molar MolarStoich.
70massa molesa massb
Molar mass
Molar massmolesb
Stoich.relation
70S. Ensign, stoichiometry and calculations
SiCl4(l) + 2Mg(s) Si(s) + 2MgCl2(s)225 g of SiCl4 is mixed with 225 g Mg.I t t i li iti t d if hi h ?
massa molesa massb
Molar mass
Molar mass
molesb
Stoich.relation
Is one reagent present in a limiting amount, and if so, which one?What mass of Si(s) is formed if the reaction goes to completion?What mass of the excess reagent is left over after the reaction
goes to completion?g p
7171S. Ensign, stoichiometry and calculations
SiCl4(l) + 2Mg(s) Si(s) + 2MgCl2(s)225 g of SiCl4 is mixed with 225 g Mg.I t t i li iti t d if hi h ?
massa molesa massb
Molar mass
Molar mass
molesb
Stoich.relation
Is one reagent present in a limiting amount, and if so, which one?What mass of Si(s) is formed if the reaction goes to completion?What mass of the excess reagent is left over after the reaction
goes to completion?g p
7272S. Ensign, stoichiometry and calculations
Methanol can be synthesized from carbon monoxide and dihydrogen (a combination reaction): CO(g) + 2H2(g) CH3OH(l)365 g of CO are mixed with 65 0 g H365 g of CO are mixed with 65.0 g H2.
Which is the limiting reagent?What is the maximal mass of methanol that can be formed??What mass of the excess reagent is left over after the reaction goes to completion?What mass of the excess reagent is left over after the reaction goes to completion?
7373S. Ensign, stoichiometry and calculations
Methanol can be synthesized from carbon monoxide and dihydrogen (a combination reaction): CO(g) + 2H2(g) CH3OH(l)365 g of CO are mixed with 65 0 g H365 g of CO are mixed with 65.0 g H2.
Which is the limiting reagent?What is the maximal mass of methanol that can be formed??What mass of the excess reagent is left over after the reaction goes to completion?What mass of the excess reagent is left over after the reaction goes to completion?
7474S. Ensign, stoichiometry and calculations
Theoretical vs. actual yield• “Theoretical yield” is the quantity of
product calculated to form when all of theproduct calculated to form when all of the limiting reagent reacts
• The “actual yield” is the amount of product• The actual yield is the amount of product actually obtained experimentallyA t l i ld ≤ th ti l i ld• Actual yield ≤ theoretical yield
001yieldactual% i ld 001yield ltheoretica
y%yield x=
7575S. Ensign, stoichiometry and calculations
Theoretical vs. actual yield
001 i ldlth ti
yield actual%yield x=yield ltheoretica
7676S. Ensign, stoichiometry and calculations