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General, Organic, and Biological Chemistry
Fourth Edition Karen Timberlake Chapter 7 Gases 7.7 Volume and Moles (Avogadro’s Law) Lectures © 2013 Pearson Education, Inc.
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Avogadro’s Law: Volume and Moles
Avogadro’s law states that the volume of a gas is directly related to the number of moles (n) of gas. T and P are constant. 2
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Learning Check If 0.75 mole of helium gas occupies a volume of 1.5 L, what volume will 1.2 moles of helium occupy at the same temperature and pressure? A L B L C L 3
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Solution If 0.75 mole of helium gas occupies a volume of 1.5 L, what volume will 1.2 moles of helium occupy at the same temperature and pressure? Step 1 Organize the data in a table of initial and final conditions. Analyze the Problem. Conditions 1 Conditions 2 Know Predict V1 = 1.5 L V2 = ? V increases n1 = mole n2 = 1.2 moles n increases 4
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Solution If 0.75 mole of helium gas occupies a volume of 1.5 L, what volume will 1.2 moles of helium occupy at the same temperature and pressure? Step 2 Rearrange the gas law equation to solve for the unknown quantity. Step 3 Substitute values into the gas law equation and calculate. Answer is C. 5
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STP To make comparisons between different gases, we use arbitrary conditions called standard temperature (273 K) and standard pressure (1 atm). Standard temperature and pressure is abbreviated as STP. Standard temperature (T) = 0 C or 273 K Standard pressure (P) = 1 atm (760 mmHg) 6
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Molar Volume The molar volume of a gas measured at STP (standard temperature and pressure) is 22.4 L for 1 mole of any gas. Avogadro’s law indicates that 1 mole of any gas at STP has a volume of 22.4 L. 7
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Molar Volume as a Conversion Factor
The molar volume at STP has about the same volume as 3 basketballs. can be used to form 2 conversion factors. and 8
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Guide to Using Molar Volume
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Using Molar Volume What is the volume occupied by 2.75 moles of N2 gas
at STP? Step 1 State the given and needed quantities. Analyze the Problem. Step 2 Write a plan to calculate the needed quantity. moles N2 liters N2 Given Need 2.75 moles of N2 at STP Liters of N2 molar volume 10
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Using Molar Volume What is the volume occupied by 2.75 moles of N2 gas
at STP? Step 3 Write the equalities and conversion factors including 22.4 L/mole at STP. Step 4 Set up the problem with factors to cancel units. 11
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Learning Check What is the volume (L) at STP of 4.00 g of CH4?
A L B L C L 12
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Solution What is the volume (L) at STP of 4.00 g of CH4? Step 1 State the given and needed quantities. Analyze the Problem Step 2 Write a plan to calculate the needed quantity. g of CH4 moles of CH4 liters of CH4 Given Need 4.00 g of CH4 at STP volume of CH4 molar mass molar volume 13
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Solution What is the volume (L) at STP of 4.00 g of CH4?
Step 3 Write the equalities and conversion factors including 22.4 L/mole at STP. Step 4 Set up the problem with factors to cancel units. Answer: A 14
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Learning Check How many grams of He are present in 8.00 L of He at
STP? A g B g C g 15
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Solution How many grams of He are present in 8.00 L of He at STP? Step 1 State the given and needed quantities. Analyze the Problem. Step 2 Write a plan to calculate the needed quantity. liters of He molar moles of He molar g of He mass volume Given Need 8.00 L of He at STP grams of He 16
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Solution How many grams of He are present in 8.00 L of He at STP?
Step 3 Write the equalities and conversion factors including 22.4 L/mole at STP. Step 4 Set up the problem with factors to cancel units. Answer: C 17
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Gases in Chemical Reactions
The volume or amount of a gas at STP in a chemical reaction can be calculated from STP conditions. mole−mole factors from the balanced equation. 18
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Guide to Reactions Involving Gases
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Gases in Equations at STP
What volume (L) of O2 gas is needed to completely react with 15.0 g of aluminum at STP? Step 1 State the given and needed quantities. Analyze the Problem. 20
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Gases in Equations at STP
What volume (L) of O2 gas is needed to completely react with 15.0 g of aluminum at STP? Step 2 Write a plan to calculate the needed quantity. grams molar moles mole−mole moles molar liters of Al mass of Al factor of O2 volume of O2 molar mass mole-mole factor molar volume 21
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Gases in Equations at STP
What volume (L) of O2 gas is needed to completely react with 15.0 g of aluminum at STP? Step 3 Write the equalities and conversion factors including the molar volume. 22
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Gases in Equations at STP
What volume (L) of O2 gas is needed to completely react with 15.0 g of aluminum at STP? Step 4 Set up the problem and calculate
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Learning Check What mass of Fe will react with 5.50 L of O2 at STP?
A g of Fe B g of Fe C. 419 g of Fe 24
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Solution What mass of Fe will react with 5.50 L of O2 at STP? Step 1 State the given and needed quantities. Analyze the Problem. 25
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Solution What mass of Fe will react with 5.50 L of O2 at STP? Step 2 Write a plan to calculate the needed quantity. liters molar moles mole−mole moles molar moles of O2 volume of O2 factor of Fe mass of Fe2 molar volume mole-mole factor molar mass 26
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Solution What mass of Fe will react with 5.50 L of O2 at STP? Step 3 Write the equalities and conversion factors including the molar volume. 27
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Solution What mass of Fe will react with 5.50 L of O2 at STP?
Step 4 Set up the problem and calculate. Answer is B. 28
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