# Uses of the Ideal Gas Law and Stoichiometry with gases ```Uses of the Ideal Gas Equation:
I.
Solving for Molecular Mass of an unknown gas (Butane Lighter Lab)
n = number of moles (units: mol)
m = mass (units: grams)
M = molecular mass (units: grams/mol)
Note: M = m
n
therefore n = m
M

PV = nRT
PV = mRT
M
Example: Solve for molecular mass (M) given m = 0.800 g, P = 106.7 kPa, T =
100&deg;C, V = 0.372 L, R = 8.31 L∙kPa
mol∙K
II.
Gas Stoichiometry
A
+
B

C
+
1) V  V
2) Mass  V
3) V  Mass
4) Mass  Density
5) Density  Volume
6) Limiting Reactants
7) Mass  Molecular Mass
1
D
Example 1: Volume to Volume Stoichiometry Gas Law Problem
What volume of gas is produced from x liters substance?
How many liters of hydrogen chloride gas will be produced from 2.00 L
hydrogen gas?
H2 (g) + Cl2(g)  2 HCl (g)
2.00 L
?? L
Example 2: Mass to Volume Stoichiometry Gas Law Problem
What volume of gas is produced from x grams substance?
Steps: 1. Balance equation and compound formulas
2. Find moles
3. Find mole ratio
4. Convert moles gas  volume of gas (1 mol = 22.4 L)
How many liters of hydrogen gas will be produced from 6.54 g of zinc?
2 HCl (aq) + Zn (s)  H2 (g) + ZnCl2 (aq)
6.54 g
?? L
2
Example 3: Volume to Mass Stoichiometry Gas Law Problem
What mass of gas is produced from x liters substance?
What mass of magnesium will react with excess sulfuric acid to produce 500
mL of hydrogen?
Mg
?? g
+
H2SO4

H2
+
500 mL
3
MgSO4
III. Determining Gas Density
Step 1: Start with PV = mRT and solve for the molecular mass (M).
M
Step 2: Density (D) is mass (m) per unit volume (V).
Write in equation form: ________________
Step 3: You can introduce the density equation into the equation from step 1. Find
m in M = mRT equation from step 1. Replace m with D.
PV
V
Step 4: Solve for density (D).
4
Example 1: Calculate the density of sulfur dioxide at STP to 3 significant figures.
(hint: from balloon worksheet) (Ans: 2.87 g/L)
Example 2: What is the density of a sample of ammonia gas, NH3, if the pressure
is 0.928 atm and the temperature is 63.0&deg;C?
(Ans: 0.527 g/L)
5
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