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Chapter 3 Stoichiometry.notebook
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September 10, 2015
Oct 62:25 AM
Chapter 3: Stoichiometry
Oct 62:25 AM
The Meaning of the WordThe word Stoichiometry derives from two Greek words: stoicheion (meaning "element") and metron (meaning "measure"). Stoichiometry deals with calculations about the masses (sometimes volumes) of reactants and products involved in a chemical reaction. It is a very mathematical part of chemistry, so be prepared for lots of calculator use.
What is Stoichiometry?
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3.1 Counting by Weighing• Solve problems related to determining the number of atoms you have based on total mass and average mass.
Jellybeans can be counted by weighing
3.2 Atomic Mass• Calculate average atomic masses from isotope data• Calculate relative isotope abundance from pertinent mass spectral data.
Mass Spectrometer. 12C is assigned a mass of exactly 12 atomic mass units(amu), and the masses of all other atoms are given relative to this standard.
Oct 62:31 AM
Example 3.2 ACalculation of Average Atomic Masses from Isotopic Data.A sample of metal "M" is vaporized and injected into a mass spectrometer. The mass spectrum tells us that 60.10% of the metal is present as 69M and 39.90% is present as 71M. The mass value for 69M and 71M are 68.93 amu and 70.92 amu respectively. What is the average atomic mass of the element? What is the element?
To calculate Weighted Average use:
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Oct 63:14 AM
The element indium exists naturally as two isotopes. 113In has a mass of 112.9043 amu, and 115In has a mass of 114.9041 amu. The average atomic mass of indium is 114.82 amu. Calculate the percent relative abundance of the two isotopes of indium.
Example 3.2 B Calculation of Relative Isotope Abundance
Oct 34:00 PM
3.3 THE MOLE• Interconvert between mole, mass, and the number of particles of a given element.
The mole is the key to many chemical calculations. A mole is defined as the number equal to the number of carbon atoms in exactly 12 grams of pure 12C.
1mole of anything = 6.022 x 1023 units of that thing
Chemistry Lovers Celebrate Mole Day
One mole of ANY substance contains Avogardo's number of particles.
Key problem solving relationship: The average mass of one atom of a substance expressed in amu is the same number as the mass of one mole of a substance expressed in grams.
1 atom of 20Ne = 20.18 amu1 mole of 20Ne = 20.18 grams1 mole of 20Ne = 6.022 x 1023 atoms of Ne
The mole unit song
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Oct 1712:00 PM
Oct 23 10:03 PM
Samples containing one mole each of copper, aluminum, iron, sulfur, iodine, and mercury
Example 3.3 AConversion Among Grams, Atoms, and AMUsHow many grams does a sample containing 34 atoms of neon weight?
Example 3.3 BConversion Between Atoms, Moles, and MassA sample of elemental silver (Ag) has a mass of 21.46 g.How many moles of silver are in the sample?How many atoms of silver are in the sample?
Example 3.3 C Practice with ConversionsWhat is the weight of 7.81 x 1022 atoms of calcium?
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• Calculate the molar masses.• Interconvert among molar mass, moles, mass, and number of particles in a given sample.
3.4 Molar Mass
The Molar Mass of a substance is the mass in grams of one mole of the compound.
Example 3.4 ACalculations of the Molar mass of a compoundCalculate the molar mass of potassium dichromate.
How many micrograms are in 3.82 x 107 moles of pyrogallol, C6H6O3?
Example 3.4 BConverting Among Molar Mass, Moles, Mass, and Number of Particles
Oct 63:23 AM
Freon12, which has the formula of CCl2F2, is used as a refrigerant in air conditions and as a propellant in aerosol cans. Given a 5.6 mg sample of Freon12: a. Calculate the number of molecules of Freon12 in that sample.b. How many mg of chlorine are in the sample?
Example 3.4 C Practice with Conversions
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Oct 1010:19 AM
3.6 Percent Composition of Compounds• Calculate the mass percent of each element in the compound.
Calculate the mass percent of each element in Glucose, C6H12O6
Example 3.6 ADetermination of the mass percent in a Compound
There are 3 steps to calculating the mass percent of each element in a compound.a. Compute the molecular mass of the compound.b. Calculate how much the molecular mass comes from each elementc. Divide each element's mass contribution by the total molecular mass, and multiply by 100 to convert to percent.
3.5 Learning to Solve Problems• Solving problems in a flexible, creative way is called conceptual problem solving• Become independent problem solver.• With a new problem you need: a. Decide on a final goal; b. work backwards from the final goal; c. Ask" Does the answer makes sense?"
Oct 1010:19 AM
Example 3.6 B Practice with Mass Percent
Calculate the mass percent of each element in potassium ferricyanide, K3Fe(CN)6?
Checking the FiguresAdding up the percentages does not add up to 100% due to the rounding to 2 sig. figs after the decimal point for the molar mass which leads to a loss of 0.01%. In such cases we say the answers are correct "within round off error"
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3.7 Determining the Formula of a Compound• Determine the empirical formula of a compound• Calculate the molecular formula of a compound
The empirical formula is represented by the simplest whole number ratio of atoms in a compound. Examples: CH, CH4, CH2O, K2Cr2O7
The molecular formula is the actual ratio of atoms in a compound.Examples: C6H6, C6H12O6, N2H4
The empirical and molecular formulas can be the same.
Examples of substances whose empirical and molecular formulas differ. Notice that:
molecular formula = (empirical formula)n, where n is an integer.
Oct 1112:37 PM
Example 3.7 ADetermination of the Empirical Formula of a CompoundThe analysis of rocket fuel showed that it contained 87.4% nitrogen and 12.6% hydrogen by weight. Mass spectral analysis showed the fuel to have a molar mass of 32.05 g/mol. What are the empirical and molecular formulas of the compound?
To solve empirical formula problems that do not involve combustion (added oxygen)1. Assume the compound has a mass of exactly 100 grams. You can therefore convert percentage to grams.2. Calculate the moles of each kind of atom present3. Determine the simplest whole number ratios by dividing the moles of each element by the smallest calculated mole value
Read the summary of methods for obtaining empirical and molecular formulas. p. 96
Numbers very close to whole numbers, such as 9.92 and 1.08, should be rounded to whole numbers. Numbers such as 2.25, 4.33, and 2.72 should NOT be rounded to whole numbers.
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Oct 1212:31 PM
Determine the empirical formula and molecular formulas for a deadly nerve gas that gives the following mass percent analysis:
C = 39.10% H = 7.67% O = 26.21% P = 16.82% F = 10.30%
The total mass is known to be 184.1 grams.
Example 3.7 BDetermination of the Empirical Formula of a Compound
Determination of the Empirical Formula of a Hydrate
In an experiment, 4.31 g of sodium carbonate hydrate are heated. Following the heating the dry remains (called anhydrate) weigh 3.22 g. What is the formula for the hydrate sodium carbonate?
Oct 1212:31 PM
A combustion device was used to determine the empirical formula of a compound containing ONLY carbon , hydrogen and oxygen. A 0.6349 g sample of the unknown produced 1.603g of CO2, and 0.2810 g of H2O. Determine the empirical formula of the compound.
Example 3.7 CDetermining the Empirical Formula From Combustion Data
Solution tips:1. Assume that the combustion was complete. Therefore, all the carbon was converted to CO 2 and all the hydrogen was converted to H2O.2. Calculate the grams of carbon in the given mass of CO 2 and the grams of hydrogen in the given mass of water. 3. Based on the law of conservation of mass and the given mass of the original compound you know that
grams O = total grams (grams C + grams H)4. Follow the steps for finding the empirical formula
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3.8 Chemical Equations• State the meaning of each part of a chemical reaction• State a variety of different relationships that are inferred from a chemical equation.
Chemical equation are all of the form
CH4 + 2O2 CO2 + 2H2OReactants Products
In a chemical reactions atoms have been reorganized. Bonds have been broken and new ones have been formed. Atoms are neither created nor destroyed. All atoms present in the reactants must be accounted for among the products.
Oct 138:50 PM
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Example 3.8 A Interpreting a Chemical Equation
State, completely, what is happening in the reaction given below. Include in your answer the physical states of each component of the reaction.
Example 3.8 BRelating Reactions and Products of a Chemical Equations
Show, by means of molar masses, that the matter is neither created nor destroyed in the equation given below.
Na2CO3(aq ) + 2HCl(aq ) CO2(g) + H2O(l) + 2NaCl(aq )
C2H6( l) + 3O2(g) 2CO2(g) + 3H2O(g)
Oct 138:50 PM
• Balance many chemical equations3.9 Balancing Chemical Equations
A balanced chemical equation means that the number of atoms of each element on the reactants' side equals the number of atoms of each element of atoms of each element on the products' side.
Balancing an Equation with the Jigsaw
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GUIDELINES FOR BALANCING A CHEMICAL EQUATION1. Use Correct formulas. Never change a molecular structure. Only use coefficients.2. Place a coefficient in front of each formula. The coefficients must be whole numbers. 3. Find the atoms that are in only one compound on the reactants' side. Balance those first.4. Balance polyatomic ions as a single unit. 5. In general leave O and H until the very end. 6. Always double check AFTER balancing!!!
NaOH(aq ) + H3PO4(aq ) Na3PO4(aq ) + H2O(l)
Balance the following chemical equation.
Example 3.8A Balancing Chemical Equation
Example 3.8B Practice with Balancing
Balance the following chemical equation.
C4H10( g) + O2(g) CO2(g) + H2O(g)
Oct 138:50 PM
3.10 Stoichiometric Calculations: Amounts of Reactants & Products
• Calculate the amount of products obtained from a given amount of reactant• Calculate the amount of reactants required to generate a desired amount of product.
1. Balance the equation for the reaction.2. Convert the known mass of the reactant or the product to moles of that substance.3. Use the balanced equation to set up the appropriate mole ratios.4. Use the appropriate mole ratio to calculate the number of moles of the desired reactant or product.5. Convert from moles back to grams if required by the problem.
Calculating Masses of Reactants and Products in Chemical reactions
Remember....• Double check the balanced chemical equation. • Don't multiply the molar mass of a substance by the coefficient in the problem BEFORE using it in one of the steps above. For example, if the formula says 2H2O, DON'T use 36.0 g/mol, use 18.0 g/mol. • Round off only once after all calculations are done.
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Analogies in Stoichiometry Bike by Parts Consider the following Analogy: 2 Wheels + 1 Body + 1 Handle bar + 1 Gear Chain = 1 bike How many bikes can be produce given, 8 wheels, 5 bodies, 6 handle bar and 5 gear chain.
Oct 138:50 PM
Example 3.10A Amount of product from a Given Amount of Reactant
Given the following reaction:
Na2S(aq ) + AgNO3(aq ) Ag2S(g) + NaNO3(aq )
How many grams of Ag2S can be generated from the reaction of 3.94 g of AgNO3 with excess Na2S?Strategy1. Always balance the chemical equation!The mole ratio conversion factor acts as the bridge between AgNO3 and Ag2S. This leads to the following strategy:
molar mass
mole ratio
molar massmoles Ag2S g Ag2Sg AgNO3 moles AgNO3
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Oct 1510:01 AM
C7H6O3 + C4H6O3 C9H8O4 + HC2H3O2
Aspirin is prepared by the reaction of salicylic acid (C7H6O3) and acetic acid anhydride (C4H6O3). How many grams of salicylic acid (sal) are needed to make 500 aspirin tablets weighing 1.00 g each (assuming 100% yield)?
Example 3.10 BAmount of Reactant Needed to Produce a Product
Salicylic acid
acetic anhydrate
aspirin acetic acid
number of aspirin tablets
g of aspirin
moles of aspirin
moles of salicylic acid
g of salicylic acid
Strategymass of tablets
molar mass
mole ratio
molar mass
Oct 2010:19 PM
• Identify the limiting reactant.• Solve problems involving a limiting reactant.• Calculate percent yield of a product
3.11 Calculations Involving a Limiting Reactant
Solving Stoichiometry Problems Involving Masses of Reactants and Products.
1. Write and balance the equation for the reaction.2. Convert the known mass of the substances to moles.3. Determine which reactant limiting.4. Using the amount of the limiting. reactant and the appropriate mole ratios, compute the number of moles of the desired product.5. Convert from moles to grams using molar mass.
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Example 3.11A A Calculations Involving a Limiting Reactant
Sodium hydroxide reacts with phosphoric acid to give sodium phosphate and water. If 17.80 g of NaOH is mixed with 15.40 g of H3PO4,
a. How many grams of Na 3PO4 can be formed?b. How many grams of the excess reactant remain un reacted?c. If the actual yield of Na3PO4 was 15.00 g, what is the percent yield of Na3PO4?
3NaOH(aq ) + H3PO4(aq ) Na3PO4(aq ) + 3H2O(l)
2. Determine which reactant limits the amount of product formed by calculating the amount of product formed by each reactant. The reactant that forms less product is limiting.
molar mass
mole ratio
molar mass
moles of product
g of product
g of reactant
moles of reactant
Strategy1. Write a balanced chemical equation for this reaction
3. To determine grams of excess reactant, you must calculate how many moles of excess reactant were actually used.
# moles excess = # moles original # moles usedThen convert moles to grams.
4. The percent yield = actual yieldtheoretical yield
x 100%
Oct 2112:23 AM
The space shuttle environmental control system handles excess CO2 (which the astronauts breathe out it is 4% by mass of exhaled air) by reacting it with lithium hydroxide pellets to form lithium carbonate and water. If there were 7 astronauts on board the shuttle, and each exhales 20 liters of air minute, how long could clean air be generated if there were 25,000 g of lithium hydroxide pellets available for each shuttle mission? Assume the density of air is 0.0010 g/mL.
Example 3.11B Tying it all together.
