# Volume and Moles (Avogadro’s Law)

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Chapter Gases 6.7d Volume and Moles (Avogadro’s Law)

In Avogadro’s Law the volume of a gas is directly related to the number of moles (n) of gas. T and P are constant. V1 = V2 n n2

Learning Check If 0.75 mole helium gas occupies a volume of 1.5 L, what volume will 1.2 moles helium occupy at the same temperature and pressure? 1) L 2) 1.8 L 3) 2.4 L

Solution STEP 1 Conditions 1 Conditions 2 V1 = 1.5 L V2 = ???
n1 = mole He n2 = 1.2 moles He STEP 2 Solve for unknown V2 V2 = V1 x n2 n1 STEP 3 Substitute values and solve for V2. V2 = 1.5 L x moles He = 2.4 L 0.75 mole He

STP The volumes of gases can be compared at STP, Standard Temperature and Pressure, when they have the same temperature. Standard temperature (T) 0°C or 273 K the same pressure. Standard pressure (P) 1 atm (760 mm Hg)

Molar Volume At standard temperature and pressure (STP), 1 mole of a gas occupies a volume of 22.4 L, which is called its molar volume.

Molar Volume as a Conversion Factor
The molar volume at STP can be used to form conversion factors. 22.4 L and mole 1 mole L

Using Molar Volume What is the volume occupied by 2.75 moles N2 gas
at STP? The molar volume is used to convert moles to liters. 2.75 moles N2 x L = L 1 mole

Guide to Using Molar Volume

Learning Check A. What is the volume at STP of 4.00 g of CH4?
1) L 2) L 3) 44.8 L B. How many grams of He are present in 8.00 L of gas at STP? 1) g 2) g 3) 1.43 g

Solution A. 1) 5.60 L 4.00 g CH4 x 1 mole CH4 x 22.4 L (STP) = 5.60 L
16.0 g CH mole CH4 B. 3) 1.43 g 8.00 L x 1 mole He x g He = g He 22.4 L mole He

Gases in Equations The volume or amount of a gas at STP in a chemical
reaction can be calculated from STP conditions. mole factors from the balanced equation.

STP and Gas Equations What volume (L) of O2 gas is needed to completely react with 15.0 g of aluminum at STP? 4 Al(s) O2 (g) Al2O3(s) Plan: g Al mole Al mole O L O2 (STP) 15.0 g Al x 1 mole Al x 3 moles O2 x L (STP) 27.0 g Al moles Al mole O2 = L O2 at STP

Learning Check 4 Fe(s) + 3 O2(g) 2 Fe2O3(s)
What mass of Fe will react with 5.50 L O2 at STP? 4 Fe(s) + 3 O2(g) Fe2O3(s)

Solution 4Fe(s) + 3O2(g) Fe2O3(s) ? L at STP 5.50 L O2 x 1 mole x 4 moles Fe x g Fe = g 22.4 L 3 moles O mole Fe