MCAT Physical : Gases
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Example Questions
Example Question #1 : Partial Pressure
The molecular weight of gas A is 
Gas A is in a mixture with four other gases and has a partial pressure of 
What is the mass of gas A in this mixture?
Cannot be determined from the given information
Cannot be determined from the given information
Recall Dalton’s Law of partial pressure:
In this equation, 
From the given information, we can solve for mole fraction of gas A using the partial pressure of gas A and the total pressure of the system.
To find the mass of gas A we need to find the moles of gas A in the mixture and multiply it by the molecular weight of gas A; however, the information given in this question only allows us to solve for the mole fraction of gas A, or percentage of gas A in the system. To convert mole fraction to moles we would need to know the total number of moles in the mixture; therefore, the mass of gas A cannot be determined.
Example Question #1 : Partial Pressure
A container with a total pressure (
Partial pressure is given by the mole fraction of a gas multiplied by the total pressure in the container. Mole fraction is equal to the moles of a given compound divided by the total moles.
This is an application of Dalton's Law.
Example Question #2 : Partial Pressure
Which of the following is false regarding partial pressure?
Partial pressure can be calculated if you know the mole fraction of the gas and the total pressure of the system
Partial pressure signifies the pressure of a gas in a system at
Dalton’s Law of partial pressure is only valid for a mixture of inert gases
In Earth’s atmosphere, nitrogen has the largest partial pressure
Partial pressure signifies the pressure of a gas in a system at
Partial pressure of a gas is defined as the pressure of the gas in a mixture of gases. It is calculated by using Dalton’s Law of partial pressure, which states that the partial pressure is equal to the product of the mole fraction of the gas and the total pressure of the system.
One of the assumptions of this law is that the gases are independent and do not chemically react with each other. This means that the gases in the system have to be inert. If the gases were reactive with each other, then Dalton’s Law would become invalid. Recall that Earth’s atmosphere contains about 78% nitrogen, 21% oxygen, and 1% all other gases; therefore, nitrogen has the largest partial pressure in the atmosphere because it has the greatest mole fraction.
Partial pressure does signify the pressure of a gas in a mixture of gases, but it is only valid for the temperature of the system. The partial pressure of the gas will be different for different temperatures. This occurs because pressure is dependent on temperature. Changing the temperature will not alter the mole fraction (amount of gas); however, it will alter the total pressure which will subsequently alter the partial pressure of the gas according to Dalton's Law.
Example Question #31 : Gases
An ideal gas with a volume of one liter at STP is placed into a piston chamber that can freely change volume. The volume is measured as the temperature is lowered. The gas will achieve its minimal volume at __________.
its boiling point
its boiling point
The best answer is the compound's boiling point. At this point, it ceases obeying the gas laws and becomes a liquid.
If it were possible to remain in a gaseous state to zero Kelvin (
Example Question #62 : Fluids And Gases
Diffusion can be defined as the net transfer of molecules down a gradient of differing concentrations. This is a passive and spontaneous process and relies on the random movement of molecules and Brownian motion. Diffusion is an important biological process, especially in the respiratory system where oxygen diffuses from alveoli, the basic unit of lung mechanics, to red blood cells in the capillaries.
Figure 1 depicts this process, showing an alveoli separated from neighboring cells by a capillary with red blood cells. The partial pressures of oxygen and carbon dioxide are given. One such equation used in determining gas exchange is Fick's law, given by:
ΔV = (Area/Thickness) · Dgas · (P1 – P2)
Where ΔV is flow rate and area and thickness refer to the permeable membrane through which the gas passes, in this case, the wall of the avlveoli. P1 and P2 refer to the partial pressures upstream and downstream, respectively. Further, Dgas, the diffusion constant of the gas, is defined as:
Dgas = Solubility / (Molecular Weight)^(1/2)
At an alveoli-capillary diffusion equilibrium, which of the following is true?
There is a 4:1 exchange of carbon dioxide.
There is a 1:1 exchange of oxygen.
There is a 5:1 exchange of carbon dioxide.
There is a 2:1 exchange of oxygen.
There is a 1:1 exchange of oxygen.
This is also a straightforward question that can be answered from little to no information from the passage. The point of emphasis here is equilibrium. At a diffusion equilibrium there is no net change, and thus, a one-to-one oxygen molecule exchange fits the bill.
Example Question #32 : Gases
Gas X has a density of 
Gas X because it has a lower molar mass than gas Y
Gas Y because it has a higher molar mass than gas X
Gas X because it has a higher molar mass than gas Y
Gas Y because it has a lower molar mass than gas X
Gas X because it has a lower molar mass than gas Y
To solve this question, we can use Graham's law of effusion:
Graham's law demonstrates that the rate of effusion is inversely proportional to the square root of the molar mass. Essentially, lighter gases will effuse more quickly than heavier gases.
Given the densities and equal volumes of the two gases, we see that the gas with the greater density will account for more molar mass. The volume of gas Y weighs twice as much as the volume of gas X. Based on this information, we can conclude that gas X diffuses faster because it has a lower molar mass.
Example Question #33 : Gases
Which equation best describes Boyle's law?
Boyle's law describes the inverse relationship between a change and pressure and a change in volume, while a sample of gas is kept at constant temperature. This relationship can be mathematically written as:
or
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