All GRE Subject Test: Chemistry Resources
Example Questions
Example Question #1 : Gas Laws
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 #2 : Gas Laws
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 : Gases
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 #1 : Real Gases And Ideal Gases
Which of the following assumptions is not made by the ideal gas law?
The size of the molecules is much smaller than the container
The van der Waals forces are negligible
The intermolecular interactions follow the Coulomb model of electric repulsion
The molecules move randomly
The molecules obey Newton's laws of motion at all times
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 #1 : Orbitals And Bonding
Which of the following sets of quantum numbers () could correspond to a orbital?
Since we're looking at the orbital, we know . The range of possible values for is 0 to . Possible values for range to . Therefore, among the answer choices, is the only possible combination of quantum numbers corresponding to an orbital.
Example Question #2 : Orbitals And Bonding
Which of the following sets of quantum numbers could correspond to a orbital?
Since we're looking at the orbital, we know . The range of possible values for is 0 to . Possible values for range to . Therefore, among the answer choices,
is the only possible combination of quantum numbers corresponding to a orbital.
Example Question #1 : Reaction Rates And Reaction Order
A scientist is studying a reaction, and places the reactants in a beaker at room temperature. The reaction progresses, and she analyzes the products via NMR. Based on the NMR readout, she determines the reaction proceeds as follows:
In an attempt to better understand the reaction process, she varies the concentrations of the reactants and studies how the rate of the reaction changes. The table below shows the reaction concentrations as she makes modifications in three experimental trials.
Reaction rates depend on reaction concentrations, as well as the reaction constant . The Arrhenius equation is used to calculate this constant. Which of the following factors is used in the Arrhenius equation?
I. Temperature
II. Activation energy
III. Reactant concentrations
II and III
I and II
I only
I, II, and III
III only
I and II
If you thought all of these factors are used in the Arrhenius equation, consider that the rate constant is independent of the reactant concentrations. The concentration effect is considered in the rate law by multiplying the concentrations by the constant, but the concentrations themselves are independent of the actual Arrhenius calculation.
The calculation shows that . In this equation, is the activation energy and is the temperature.
Example Question #1 : Reaction Rate And Rate Law
You want to find the order of each reactant by manipulating the reactant concentrations in multiple trials. The table below shows the effect of altered reactant concentrations on initial reaction rate.
Using the above trials, write the rate law for the reaction.
Keep in mind that the chemical equation and its coefficients have NOTHING to do with the rate law. The order of each reactant must be determined by experiment.
To find the order of each reactant, compare the initial reaction rates of two trials in which only one of the three reactants' concentrations is altered. For example, trials 1 and 4 keep A and B equal, but C is doubled. When C is doubled, we see that the initial reaction rate is quadrupled. As a result, we determine that the order of reactant C is 2. When the reactant is altered, but the initial reaction rate is kept constant, as seen in trials 1 and 3 with respect to A, the order of that reactant is 0. Finally, when the reactant is multiplied by the same factor that the initial reaction rate is multiplied, as seen in trials 1 and 2 with respect to B, the order of the reactant is 1.
Putting the data together: A is zeroth order, B is first order, and C is second order. Our rate law can thus be written .