Basic Chemistry Chapter 11 Gases Chapter 11 Lecture Fourth Edition Chapter 11 Gases 11.7 Volume and Moles (Avogadro’s Law) Learning Goal Use Avogadro’s law to determine the amount or volume of a gas when the pressure and temperature are constant.

Avogadro’s Law Avogadro’s law was formulated by Amedeo Avogadro in 1811 states that the volume of a gas is directly related to the number of moles of gas when temperature and pressure are held constant

Avogadro’s Law The volume of a gas is directly related to the number of moles of the gas. If the number of moles is doubled, the volume must double at constant temperature and pressure.

Learning Check If a 0.75 mole sample of helium gas occupies a volume of 1.5 L, what volume will a 1.2 mole sample of gas occupy at the same temperature and pressure?

Step 1 Organize the data in a table of initial and final conditions. Solution If a 0.75 mole sample of helium gas occupies a volume of 1.5 L, what volume will a 1.2 mole sample of gas occupy at the same temperature and pressure? Step 1 Organize the data in a table of initial and final conditions. Conditions 1 Conditions 2 V1 = 1.5 L V2 = ? n1 = 0.75 mole n2 = 1.2 moles

Solution If a 0.75 mole sample of helium gas occupies a volume of 1.5 L, what volume will a 1.2 mole sample of gas occupy at the same temperature and pressure? Step 2 Rearrange the gas law equation to solve for the unknown quantity.

Solution If a 0.75 mole sample of helium gas occupies a volume of 1.5 L, what volume will a 1.2 mole sample of gas occupy at the same temperature and pressure? Step 3 Substitute values into gas law equation and calculate.

STP and Molar Volume The volumes of gases can be compared at STP (standard temperature and pressure) when they have the same number of moles of gas. Standard temperature is exactly 0 °C (273 K). Standard pressure is exactly 1 atm (760 mmHg).

STP and Molar Volume At STP, 1 mole of any gas occupies a volume of 22.4 L, approximately equal to the volume of about three basketballs.

STP and Molar Volume When a gas is at STP conditions (0 °C and 1 atm), its molar volume can be written as a conversion factor and used to convert between moles of gas and its volume, in liters.

Guide to Using Molar Volume

What is the volume occupied by 2.75 moles of N2 gas at STP? Solution What is the volume occupied by 2.75 moles of N2 gas at STP? Step 1 State given and needed quantities. Given: 2.75 moles N2 at STP Need: liters of N2 at STP Step 2 Write a plan to calculate the needed quantity. Molar Volume moles of N2 liters of N2

Solution What is the volume occupied by 2.75 moles of N2 gas at STP? Step 3 Write the equalities and conversion factors.

Solution What is the volume occupied by 2.75 moles of N2 gas at STP? Step 4 Set up the problem with factors to cancel units.

Density at STP At STP, the density of a gas is calculated using the molar mass of the gas and its molar volume. Balloons rise in the air because helium is less dense than air.

Learning Check Calculate the density in g/L of O2 gas at STP.

Solution Calculate the density in g/L of O2 gas at STP. Step 1 State given and needed quantities. Given: O2 gas at STP Need: density of O2 at STP Step 2 Write a plan to calculate the needed quantity.

Solution Calculate the density in g/L of O2 gas at STP. Step 3 Write the equalities and conversion factors.

Solution Calculate the density in g/L of O2 gas at STP. Step 4 Set up the problem with factors to cancel units.