1. 7.7 Volume and Moles (Avogadro’s Law)
Chapter 7 Gases
7.7 Volume and Moles (Avogadro’s Law)

2. 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
V1 = V2
n n2

3. 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?
1) L
2) 1.8 L
3) 2.4 L

4. Solution 3) 2.4 L STEP 1 V1 = 1.5 L V2 = ? V increases
Conditions Conditions 2 Know Predict
V1 = 1.5 L V2 = ? V increases
n1 = mole n2 = 1.2 moles n increases
STEP 2 Solve for unknown V2:
V2 = V1 x n2 n1
STEP 3 Substitute values and solve for V2:
V2 = 1.5 L x 1.2 moles He = L
0.75 mole He

5. 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 mmHg)

6. Molar Volume The molar volume of a gas
is measured at STP (standard temperature and pressure)
is 22.4 L for 1 mole of any gas

7. 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:
22.4 L and mole
1 mole L

8. Guide to Using Molar Volume

9. Using Molar Volume What is the volume occupied by 2.75 moles of N2 gas
at STP?
STEP 1 Given: moles of N2
Need: Liters of N2
STEP 2 Write a plan:
Use the molar volume to convert moles to liters.

10. Using Molar Volume (continued)
STEP 3 Write equalities and conversion factors:
1 mole of gas = L
1 mole gas and L
22.4 L mole gas
STEP 4 Substitute data and solve:
2.75 moles N2 x L = L of N2
1 mole N2

11. Learning Check What is the volume at STP of 4.00 g of CH4?
1) L 2) L 3) L

12. Solution
1) 5.6 L STEP 1 Given: 4.00 g of CH4 Need: liters of CH4 at STP STEP 2 Write a plan: g of CH4 moles of CH4 L of CH4 (at STP)
STEP 3 Write equalities and conversion factors:
1 mole of CH mole of gas = 22.4 L
1 mole CH4 and g CH mole gas and L
16.0 g CH mole CH L mole gas
STEP 4 Substitute data and solve:
4.00 g CH4 x 1 mole CH4 x L = 5.6 L of CH4 (STP)
16.0 g CH mole CH4

13. Learning Check How many grams of He are present in 8.00 L of He at
STP?
1) g 2) g 3) g

14. Solution
3) 1.43 g of He STEP 1 Given: 8.00 L of He Need: grams of He at STP STEP 2 Write a plan: L of He moles of He g of He (at STP)
STEP 3 Write equalities and conversion factors:
1 mole of gas = 22.4 L 1 mole of He = 4.00 g of He
1 mole gas and L 1 mole He and g He
22.4 L mole gas g He mole CH4
STEP 4 Substitute data and solve:
8.00 L He x 1 mole He x g He = g of He
22.4 L He mole He

15. Gases in Chemical Reactions
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

16. Guide to Problem Solving Reactions Involving Gases

17. Example of Gases in Equations
What volume (L) of O2 gas is needed to completely
react with 15.0 g of aluminum at STP?
4Al(s) + 3O2 (g) Al2O3(s)

18. Solution for Example of Gases in Equations
What volume (L) of O2 gas is needed to completely
react with 15.0 g of aluminum at STP?
4Al(s) + 3O2 (g) Al2O3(s)
STEP 1 Find moles of Al using molar mass.
STEP 2 Determine moles of O2 using mole–mole factor.
STEP 3 Convert moles of O2 to L using molar volume.
(STEP 1) (STEP 2) (STEP 3)
15.0 g Al x 1 mole Al x 3 moles O2 x L (STP)
27.0 g Al moles Al mole O2
= L of O2 (at STP)

19. Learning Check What mass of Fe will react with 5.50 L of O2 at STP?
4Fe(s) + 3O2(g) Fe2O3(s)
1) g of Fe
2) g of Fe
3) 419 g of Fe

20. Solution STEP 1 Find moles of O2 using molar volume.
STEP 2 Determine moles of Fe using mole–mole factor.
STEP 3 Convert moles of Fe to grams using molar mass.
(STEP 1) (STEP 2) (STEP 3)
5.50 L O2 x 1 mole O2 x 4 moles Fe x g Fe
22.4 L O moles O mole Fe
= g of Fe