1. Chapter 3 Calculations with Chemical Formulas and Equations

2. Contents and Concepts Mass and Moles of Substances
Here we will establish a critical relationship between the mass of a chemical substance and the quantity of that substance (in moles).
Molecular Weight and Formula Weight
The Mole Concept
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3. Determining Chemical Formulas Explore how the percentage composition and mass percentage of the elements in a chemical substance can be used to determine the chemical formula. 3. Mass Percentages from the Formula 4. Elemental Analysis: Percentages of Carbon, Hydrogen, and Oxygen 5. Determining Formulas
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4. Stoichiometry: Quantitative Relations in Chemical Reactions Develop a molar interpretation of chemical equations, which then allows for calculation of the quantities of reactants and products. 6. Molar Interpretation of a Chemical Equation 7. Amounts of Substances in a Chemical Equation 8. Limiting Reactant: Theoretical and Percentage Yields
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5. Learning Objectives Mass and Moles of Substances
Molecular Weight and Formula Weight
a. Define the terms molecular weight and formula weight of a substance.
b. Calculate the formula weight from a formula.
c. Calculate the formula weight from molecular models.
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6. a. Define the quantity called the mole.
2. The Mole Concept
a. Define the quantity called the mole.
b. Learn Avogadro’s number.
c. Understand how the molar mass is related to the formula weight of a substance.
d. Calculate the mass of atoms and molecules.
e. Perform calculations using the mole.
f. Convert from moles of substance to grams of substance.
g. Convert from grams of substance to moles of substance.
h. Calculate the number of molecules in a given mass of a substance.
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7. Determining Chemical Formulas 3. Mass Percentages from the Formula a
Determining Chemical Formulas 3. Mass Percentages from the Formula a. Define mass percentage. b. Calculate the percentage composition of the elements in a compound. c. Calculate the mass of an element in a given mass of compound.
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8. 4. Elemental Analysis: Percentages of Carbon, Hydrogen, and Oxygen a
4. Elemental Analysis: Percentages of Carbon, Hydrogen, and Oxygen a. Describe how C, H, and O combustion analysis is performed. b. Calculate the percentage of C, H, and O from combustion data.
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9. 5. Determining Formulas a. Define empirical formula. b
5. Determining Formulas a. Define empirical formula. b. Determine the empirical formula of a binary compound from the masses of its elements. c. Determine the empirical formula from the percentage composition. d. Understand the relationship between the molecular weight of a substance and its empirical formula weight. e. Determine the molecular formula from the percentage composition and molecular weight.
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10. Stoichiometry: Quantitative Relations in Chemical Reactions 6
Stoichiometry: Quantitative Relations in Chemical Reactions 6. Molar Interpretation of a Chemical Equation a. Relate the coefficients in a balanced chemical equation to the number of molecules or moles (molar interpretation).
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11. 7. Amounts of Substances in a Chemical Equation a
7. Amounts of Substances in a Chemical Equation a. Use the coefficients in a chemical equation to perform calculations. b. Relate the quantities of reactant to the quantity of product. c. Relate the quantities of two reactants or two products.
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12. 8. Limiting Reactant: Theoretical and Percentage Yield
a. Understand how a limiting reactant or limiting reagent determines how many moles of product are formed during a chemical reaction and how much excess reactant remains.
b. Calculate with a limiting reactant involving moles.
c. Calculate with a limiting reactant involving masses.
d. Define and calculate the theoretical yield of chemical reactions.
e. Determine the percentage yield of a chemical reaction.
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13. Molecular Weight (MW) Formula Weight (FW)
The sum of the atomic weights of all the atoms in a molecule of the substance.
Formula Weight (FW)
The sum of the atomic weights of all atoms in a formula unit of the compound, whether molecular or not.
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14. The answer rounded to three figures is 119 amu.
Calculate the formula weight of the following compound using its atomic weight (AW).
Chloroform, (CHCl3)
The answer rounded to three figures is 119 amu.
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15. NA = 6.02 × 1023 (to three significant figures).
Mole, mol
The quantity of a given amount of substance that contains as many molecules or formula units as the number of atoms in exactly 12 g of carbon-12.
Avogadro’s Number, NA
The number of atoms in exactly 12 g of carbon-12
NA = 6.02 × 1023 (to three significant figures).
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16. Molar Mass The mass of one mole of substance. For example,
Carbon-12 has a molar mass of 12 g/mol.
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17. Determine the mass of a chlorine atom in grams.
Atomic weight of Cl = 35.5 amu
Molar mass of Cl = 35.5 g/mol
Calculating the mass of one Cl atom,
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18. Questions to ask while converting from grams to moles of a substance
How much does a given number of moles of a substance weigh?
How many moles of a given formula unit does a given mass of substance contain?
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19. Convert 0.0654 mol of ZnI2 to grams.
Molar mass of ZnI2 = 319 g/mol
Converting mol of ZnI2 to grams,
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20. Percentage Composition
The mass percentage of each element in the compound. It helps determine the formula of new compounds.
Mass Percentage
Suppose that A is a part of something—that is, part of a whole.
The mass percentage of A in a substance is the parts of A per hundred parts of the total, by mass.
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21. Determining the mass of carbon in 83.5 g CH2O,
Determine the grams of carbon present in CH2O. Use the following data: C = 40.0 % H = 6.73 % O = 53.3 %
Determining the mass of carbon in 83.5 g CH2O,
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22. A sample of known mass of the compound is burnt to get CO2 and H2O.
The first step in determining the formula of a new substance is obtaining its percentage composition.
Consider analyzing the composition of C, H, and O in compounds containing these three elements.
A sample of known mass of the compound is burnt to get CO2 and H2O.
Masses of CO2 and H2O are related to the mases of C and H.
Mass percentages of C and H are calculated.
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23. A 4. 24 mg sample of acetic acid is completely burned, giving 6
A 4.24 mg sample of acetic acid is completely burned, giving 6.21 mg of carbon dioxide and 2.54 mg of water. Determine the mass percentage of each element in acetic acid.
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24. Calculating grams of H (1 mol H2O yields 2 mol H),
Calculating grams of C,
Calculating grams of H (1 mol H2O yields 2 mol H),
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25. Calculating the mass percentage of O2,
The percentage composition of acetic acid is:
39.9% of carbon
6.72 % of hydrogen
53.4% of oxygen
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26. Empirical Formula (Simplest Formula)
The formula of a substance written with the smallest integer subscripts.
For example,
The empirical formula for H2O2 is HO.
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27. Determining the molecular formula requires: The percentage composition
The molecular weight
Determining the empirical formula for a substance involves converting the masses of the elements to moles.
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28. A compound of nitrogen and oxygen is analyzed and a sample weighing 1
A compound of nitrogen and oxygen is analyzed and a sample weighing g is found to contain g N and g O. Determine the empirical formula of the compound.
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29. Converting masses to moles,
Dividing the bigger mole number by the smaller one results in the smallest integers.
N = 1.00
O = 2.00
As the ratio of N to O is 1:2, the empirical formula is NO2
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30. Determining molecular formula from empirical formula For any molecular compound, n is the number of empirical formula units in the molecule. Multiplying the subscripts of the empirical formula by n gives the molecular formula.
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31. Determine the empirical formula and molecular formula of acetic acid using the following data. Percentage composition of acetic acid: C = 39.9% H = 6.72% O = 53.4% Molecular weight of acetic acid = 60.0 amu
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32. 100. 0 g of acetic acid contains 39. 9 g C, 6. 72 g H, and 5. 34 g O
100.0 g of acetic acid contains 39.9 g C, 6.72 g H, and 5.34 g O. Converting into moles gives: C = 3.33 mol H = 6.67 mol O = 3.34 mol Dividing the mole numbers by the smallest one gives: C = 1.00 H = 2.00 O = 1.00 The empirical formula of acetic acid is CH2O.
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33. Empirical formula weight of CH2O is 30. 0 amu
Empirical formula weight of CH2O is 30.0 amu. Calculating the molecular formula of CH2O, The molecular formula of acetic acid is (CH2O)2 or C2H4O2
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34. Interpreting a Chemical Equation
Stoichiometry
The calculation of the quantities of reactants and products involved in a chemical reaction.
Interpreting a Chemical Equation
A chemical equation may be interpreted in terms of either:
Numbers of molecules (or ions or formula units) or
(2) Numbers of moles
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35. Calculate the moles of product obtained from any moles of reactant.
A molar interpretation of the coefficients in a chemical reaction helps:
Calculate the moles of product obtained from any moles of reactant.
Answer questions of masses of reactants and products.
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36. Steps in a stoichiometric calculation
Figure Page 91
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37. Calculating the amount of Fe2O3 required to produce 1.00 kg of iron,
Consider the following reaction and determine the grams of iron that can be produced from 1.00 kg of Fe2O3.
Calculating the amount of Fe2O3 required to produce 1.00 kg of iron,
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38. Determine the grams of HCl that react with 5
Determine the grams of HCl that react with 5.00 g of manganese dioxide according to the following reaction.
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39. Limiting Reactant
The reactant that is entirely consumed when a reaction goes to completion.
Once one reactant has been completely consumed, the reaction stops.
The moles of product are always determined by the starting moles of the limiting reactant.
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40. Steps to solving a limiting-reactant problem
Identify the limiting reactant.
Calculate the amount of product from the amount of the limiting reactant.
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41. Consider the following reaction between zinc and hydrochloric acid
Consider the following reaction between zinc and hydrochloric acid. Determine the moles of H2 produced between 0.30 mol of zinc and 0.52 mol of HCl.
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42. Identifying the limiting reactant,
Zinc is the excess reactant.
Hydrochloric acid must be the limiting reactant.
Since Zinc is the limiting reactant, the amount of H2 produced must be 0.26 mol
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43. Consider the following reaction:
Determine:
The grams of acetic acid that can be produced in a reaction between 20.0 g of CH3CHO and 10.0 g of O2.
The remainder of excess reactant after the reaction is complete.
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44. Identify the limiting reactant using the following data:
Molar mass of acetaldehyde = 44.1 g/mol
Molar mass of oxygen = 32.0 g/mol
CH3CHO is the limiting reactant
Amount of HC2H3O2 produced = mol
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45. Converting mol to grams,
Determining the amount of excess reactant,
Initial amount of O2 was 10.0 g; the remaining quantity is:
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46. Theoretical Yield
The maximum amount of product that can be obtained by a reaction from given amounts of reactants.
This is a calculated amount.
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47. Actual Yield Percentage Yield
The amount of product that is actually obtained.
This is a measured amount.
Percentage Yield
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