Gas Stoichiometry Formula
Stoichiometric calculations involving gases using molar volume and ideal gas law
Key Formulas
Molar Volume at STP
1 mol gas = 22.4 L at STP
STP: 0°C (273 K) and 1 atm
Ideal Gas Law
PV = nRT
n = PV / RT (for non-STP conditions)
Volume-Volume Relationships
V₁/a = V₂/b (at same T & P)
Gas volumes proportional to mole ratios
Constants & Conditions
STP Conditions
Temperature: 0°C = 273.15 K
Pressure: 1 atm = 101.325 kPa
Molar Volume: 22.4 L/mol
Gas Constant (R)
0.0821 L·atm/(mol·K)
8.314 J/(mol·K)
8.314 kPa·L/(mol·K)
62.4 L·mmHg/(mol·K)
Worked Examples
Example 1: Volume at STP
2H₂ + O₂ → 2H₂O
Problem: What volume of O₂ at STP reacts with 10.0 g H₂?
Solution:
Step 1: Convert H₂ to moles
n(H₂) = 10.0 / 2.02 = 4.95 mol
Step 2: Use mole ratio
n(O₂) = 4.95 × (1/2) = 2.48 mol
Step 3: Convert to volume at STP
V(O₂) = 2.48 mol × 22.4 L/mol
V(O₂) = 55.5 L at STP
Example 2: Non-STP Conditions
2KClO₃ → 2KCl + 3O₂
Problem: 49.0 g KClO₃ decomposes. Find V(O₂) at 25°C and 1.5 atm.
Solution:
n(KClO₃) = 49.0 / 122.55 = 0.400 mol
n(O₂) = 0.400 × (3/2) = 0.600 mol
Use PV = nRT:
V = nRT/P
V = (0.600)(0.0821)(298) / 1.5
V = 14.7 / 1.5
V(O₂) = 9.8 L
Example 3: Gas-to-Gas Volume Ratio
N₂ + 3H₂ → 2NH₃
Problem: What volume H₂ reacts with 50.0 L N₂? (same T & P)
Solution:
At same T & P, volume ratios = mole ratios
V(H₂)/V(N₂) = 3/1
V(H₂) = 50.0 × (3/1)
V(H₂) = 150 L
(No need for PV=nRT when gases are at same conditions!)
Example 4: Mass to Gas Volume
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Problem: 22.0 g propane burns. Find V(CO₂) at STP.
Solution:
n(C₃H₈) = 22.0 / 44.10 = 0.499 mol
n(CO₂) = 0.499 × (3/1) = 1.50 mol
V(CO₂) = 1.50 × 22.4
V(CO₂) = 33.6 L at STP
Common Mistakes
Using 22.4 L/mol at Non-STP
22.4 L/mol ONLY valid at STP! Use PV=nRT for other conditions
Temperature in Celsius
Must use Kelvin in PV=nRT: K = °C + 273.15
Wrong R Value
Match R units with pressure units: 0.0821 for atm, 8.314 for kPa
Gas-to-Gas Shortcut
At same T & P, gas volumes are in same ratio as mole coefficients