All GRE Subject Test: Chemistry Resources
Example Questions
Example Question #242 : Gre Subject Test: Chemistry
Oil and vinegar is a very popular salad dressing. It also is commonly used for dipping bread. The main component of the oil phase is olive oil, while the main part of the vinegar portion is aqueous acetic acid.
When oil and vinegar salad dressing is allowed to stand at room temperature, two distinct phases are observed. The main explanation for this phenomenon is the difference in _________ of the two phases.
polarity
acidity
solubility
buoyancy
density
polarity
Olive oil, like most oils, is non-polar, while aqueous acetic acid is very polar. These two phases do not mix because of their different solvent polarities.
Example Question #251 : Gre Subject Test: Chemistry
A gas sample is contained in a 4L vessel at a pressure of 3atm. Assuming all other conditions are kept constant, what is the new pressure in the vessel if the volume is reduced to 1.5L?
According to Boyle's law, pressure and volume are inversely proprotional to each other. This is represented by the equation:
In other words, as volume decreases in a vessel, the pressure will increase, and vice versa. Using the given conditions, we can solve for the final pressure in the vessel:
Example Question #1 : Gases
An unknown amount of neon gas is contained in a 3.00L vessel. At a temperature of , the gas exerts a pressure of 4.00atm.
Neon gas has a molar mass of .
Based on these conditions, what is the mass of neon gas in the vessel?
This question deals with the amount of gas present in a vessel for only one set of conditions. This makes the ideal gas law a suitable equation to use in order to determine the amount of gas in the vessel. The ideal gas law is written as:
Using this equation, we can solve for the molar quantity of gas in the vessel:
Knowing this, we can now solve for the mass of the gas in the vessel by multiplying this molar amount by the molar mass:
Example Question #1 : Gas Laws
Which gas follows the exact definition of the ideal gas law?
None of these
None of these
Though the ideal gas law gives a nearly close to real approximation of numbers, it oversimplifies its description of gases. No real gas follows the exact definition of the ideal gas law and is very complex because there are intermolecular forces that must be considered. An ideal gas described as a point mass in which the particles are so small that its volume is negligible. However, real gases have real volume. Also, ideal gases are considered elastic, having no attractive and repulsive forces with no energy transfer during collisions. Real gases actually collide and are non-elastic. Note that gases approach ideal behavior as their temperature increases and their pressure decreases.
Example Question #2 : Phases Of Matter
Which of the following assumptions is not made by the ideal gas law?
The molecules obey Newton's laws of motion at all times
The intermolecular interactions follow the Coulomb model of electric repulsion
The size of the molecules is much smaller than the container
The van der Waals forces are negligible
The molecules move randomly
The intermolecular interactions follow the Coulomb model of electric repulsion
Under the ideal gas law, we assume that the interactions between the molecules are very brief and that the forces involved are negligible. The assumption that the molecules obey Coulomb's law when interacting with each other is not necessary; rather, an ideal gas must disregard Coulomb's law.
The ideal gas law assumes only Newtonian mechanics, disregarding any intermolecular or electromagnetic forces.
Example Question #1 : Real Gases
Consider a real gas with a constant amount and a constant pressure. It has a temperature of and a volume of . If you double the temperature, what will happen to the volume?
The volume will become less than
The volume will become
The volume will become
The volume will become greater than
The volume will become less than
This question can be solved using either Charles's law or the ideal gas law (converted into the combined gas law).
Charles's Law:
Ideal Gas Law:
The question states that the pressure and moles are held constant; therefore, the volume and temperature are directly proportional. If the question were asking about an ideal gas, the volume would double when you double the temperature
The volume would double for an ideal gas; however, the question is asking about a real gas. To find the correct relationship between volume and temperature we need to look at the equation for real gas volume. Remember that the volume we are concerned with is the volume of the free space in the container, given by the container volume minus the volume of the gas particles. The equation for real gas volume accounts for the volume of the container and the volume of the gas particles. For a real gas, the volume is given as follows:
In this equation, is the number of moles of gas particles and is the bigness coefficient. This equation implies that the volume of free space for a real gas is always less than the volume for an ideal gas; therefore, doubling the temperature will produce a volume that is less than the predicted volume for an ideal gas. Our answer, then, must be less than double the initial volume.
Note that for an ideal gas the bigness coefficient, , would be zero and the volume of free space would be equal to the volume of the container . This occurs because the volume of the gas particles is negligible for an ideal gas.
Example Question #2 : Real Gases And Ideal Gases
Which of the following is relevant for real gases, but irrelevant for ideal gases?
I. Volume of gas particles
II. Intermolecular forces between gas particles
III. Volume of container
I and II
I and III
I only
III only
I and II
There are two main assumptions for an ideal gas (and a few smaller assumptions). First, the gas particles of the ideal gas must have no molecular volume. Second, the gas particles must exert no intermolecular forces on each other; therefore, forces such hydrogen bonding, dipole-dipole interactions, and London dispersion forces are irrelevant in ideal gases. Other small assumptions of ideal gases include random particle motion (no currents), lack of intermolecular interaction with the container walls, and completely elastic collisions (a corollary of zero intermolecular forces).
For real gases, however, these assumptions are invalid. This means that the real gas particles have molecular volume and exert intermolecular forces on each other.
Recall that the volume in the ideal gas law is the volume of the free space available inside the container. For ideal gases, the free space volume is equal to the volume of the container because the gas particles take up no volume; however, for real gases, the free space volume is the volume of the container minus the volume of the gas particles. Though the exact values of free space volume will differ, the volume of the container is important for both real and ideal gases.
Example Question #351 : Gre Subject Test: Chemistry
Use the following values for water as needed.
If burning wood releases of heat energy per gram of wood consumed, what mass of wood must be consumed to heat of water from to , and then to convert it to water vapor?
There are two processes requiring added heat in this problem:
1. Raising the temperature of the liquid water from to (use )
2. Boiling the water at a constant temperature of (use )
To use either of these equation, we need to find the mass of the water using the relation between mass, density, and volume.
Use this mass with the given specific heat and temperatures to find the heat for part 1 of the process.
Then, use the mass with the given heat of vaporization to find the energy needed to convert the water to water vapor.
Sum the energies for step 1 and step 2.
This is the total amount of energy needed from the burning wood. Use stoichiometry to find the grams of wood needed to produce this amount of energy.
Example Question #3 : General Thermodynamics
What was the final temperature of water if a sample of water absorbs of heat energy and heats up from ? (specific heat of water is )
Specific heat is the amount of heat needed to raise 1 gram of a substance by 1oC. Calorimeters used for these types of experiments because they are designed to be well-insulated, so no heat is gained from or lost to the surroundings.
Example Question #1 : Heat And Temperature
An metal at was put into of water at . The final temp of the water and metal was . Assuming heat was not lost to the surroundings find the specific heat of the metal? The specific heat of water is .
Specific heat is the amount of heat needed to raise 1 gram of a substance by 1oC. Calorimeters used for these types of experiments because they are designed to be well-insulated, so no heat is gained from or lost to the surroundings.
Heat is transferred from the metal to the water.
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