Equation:  C_{8}H_{18}  +  O_{2}  CO_{2}  +  H_{2}O  
Balancing:  2 C_{8}H_{18}  +  25 O_{2}  16 CO_{2}  +  18 H_{2}O  
Stoichiometry (n or V):  2  :  25  :  16  :  18 
Before reaction:  0.025 L  0.600 L  0  0  
From the stoichiometry can see that octane is limiting  some octane will be left over


After reaction:  0^{} L  0.288 L  0.200 L  0.225 L 
Now lets find the pressure as requested:
Use Gas Laws to solve: PV = nRT, putting constants together,
PV/nT = R, or P_{1}V_{1}/n_{1}T_{1} = P_{2}V_{2}/n_{2}T_{2}
Rearranging: P_{2} = (P_{1})(V_{1}/V_{2})(n_{2}/n_{1})(T_{2}/T_{1})
(Note that V is the volume of the cylinder, so what is V_{2}? Do you know what to use for n?)
P_{2} = 2.43 x 10^{4} mmHg = 32.0 atm (or 470 psi)
Effusion refers to the passage of a substance through a small orifice.
This law states that the effusion of a gas through a small orifice is inversely proportional to the square root of its density.
or, since the density of a gas is proportional to its molecular weight
(n = mass/MW; from PV = nRT, n/V = const. = (mass/MW)/V; multiplying both sides by MW gives (MW)(const.) = mass/V = density.)
Equivalently, the relative rates of effusion of two gases at the same pressure and temperature is given by the inverse square roots of their densities.
Example: What is the relative rate of effusion of H_{2} vs. O_{2}?
Rate_{H2}/Rate_{O2} = (32/2)^{1/2} = 16^{1/2} = 4
What is the MW of a molecule that effuses 6.5 times slower than nitrogen?
Rate_{N2}/Rate_{unk} = 6.5 = (MW_{unk})^{1/2}/(MW_{N2})^{1/2} = (MW_{unk})^{1/2}/(28_{})^{1/2}
(MW_{unk})^{1/2} = (6.5)(28_{})^{1/2}, square both sides,
MW_{unk} = (42.2)(28) = 1183g/mol = 1.2 x 10^{3}g/mol
Diffusion refers to the passage of one substance through another. An example for gases would be the passage of an aroma, such as a perfume or skunk smell, through still air. Given that gases are mostly empty space this interpenetration is not surprising. What we want to look at now is the rate of this process:
This law states that "The rate of diffusion of a gas is inversely proportional to the square root of its density."
or, since the density of a gas is proportional to its molecular weight
Unlike in effusion, this turns out to be not quite the case for diffusion. That is, the ratios of rates of diffusion of different gases will not quite fit prediction. The problem is that, although the average velocities of the molecules follow the inverse proportionality, as in effusion, the molecules are impeded by collisions with the gas they are passing through. Not surprisingly, the description of this more complex process is not quite the simple law originally postulated by Graham. It does still give a useful first order picture however.
We have been looking at the various properties of gases, now we want to look at a theory to explain those behaviors. A simple model is the kineticmolecular theory. There are four basic postulates:
public domain image via Wikipedia Creative Commons‡
Syllabus / Schedule 
© R A Paselk
Last modified 2 March 2015