Gas Stoichiometry Flashcards
What if our gas is a mixture of multiple different gases?
Consider adding O2 at 3 Pa to N2 at 2 Pa:
The new pressure is the sum of the original pressures!
Dalton’s Law of Partial Pressures
The total pressure of a mixture of non-reacting gases is equal to the sum of the partial pressures of the individual gases:
Ptotal = P1 + P2 + P3…
This works because we are assuming that individual gas particles do not interact with one another
Collecting gas over water
In the lab, if a gas is produced in a reaction, we often collect it by bubbling it through water
We use the change in water level as evidence that the gas is in fact being produced
Collecting gas over water
Calculations
The problem with this is that water vapour gets mixed in with our gas product
We can solve this problem through using Dalton’s Law of Partial Pressures!
Ptotal = Pwater + Pgas product
Ptotal - Pwater = Pgas product
Ptotal can be measured directly, but what about Pwater?
Collecting gas over water table
Pwater depends on the temperature of the mixture
The higher the temperature, the greater the number water molecules that have evaporated into the gas state, the greater the partial pressure of water
Luckily for us, chemists have measured the partial pressure of water at various temperatures, so we just need to look up the value for the temperature we are working at
Reactions: For gases, mole ratios are also volume ratios!
This is because, at a constant T and P, the volume of a gas is directly determined by the amount (moles) of the gas present (from Avogadro’s Law)
Consider the reaction below:
N2(g) + 3 H2(g) 🡪 2 NH3(g)
This reaction can be read in two ways:
1 mol of N2 reacts with 3 mol of H2 to produce 2 mol of NH3, OR
1 L of N2 reacts with 3 L of H2 to produce 2 L of NH3
This does NOT mean that 1 mol equals 1 L!
Reactions: For gases, mole ratios are also volume ratios!
Law of Combining Volumes
(true for constant T and P)
Use mole ratios!
