MCAT Physical : MCAT Physical Sciences
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Example Questions
Example Question #1 : Ions In Solution
Hydrochloric acid and sodium sulfite react in aqueous solution via the process:
Which of the following correctly expresses the net ionic equation for this reaction?
Start by writing the total ionic equation, using the following steps.
1. Break all soluble strong electrolytes, denoted by "(aq)," into their ions, indicating the number and charge of each ion.
2. Bring along unchanged any compounds denoted as "(s)," "(l)," or "(g)."
Remove any ions appearing on both sides of the reaction (any "spectator" ions) to get the net ionic equation.
Spectator ions:
Example Question #1 : Solution Chemistry
All of the following are true regarding a solution except __________.
A saturated solution is in a state of equilibrium
A solution is composed of a solute in one phase dissolved in a solvent in another phase
Solvation is the process by which a solute dissolves into a solvent and becomes a solution
A solution can contain any of the three phases of matter
An under-saturated solution is a solution that contains less solute than the solvent is capable of dissolving
A solution is composed of a solute in one phase dissolved in a solvent in another phase
Solutions can occur in all three phases of matter. They can also occur between two compounds in a single phase. For example, brass is a solution of two metals: zinc and copper.
Example Question #5 : Solubility And Ions
Electrolytes play a big role in maintaining blood pressure. Loss of electrolytes often leads to a drop in blood pressure. What can a doctor prescribe to a patient who has very low blood pressure?
Decrease the consumption of salt
Increase the consumption of lithium chloride
Increase the consumption of sodium chloride
Spend an hour a day in a hot sauna
Increase the consumption of sodium chloride
The question states that the patient has a low blood pressure, suggesting that the patient has a low concentration of electrolytes in his blood. To counter this, the doctor must prescribe a solution that will increase the concentration of electrolytes in blood. The best solution would be to increase the consumption of salt, or sodium chloride. Sodium chloride is a strong electrolyte and will dissociate into sodium ions and chlorine ions. This will increase the blood pressure of the patient and will remedy the problem.
Spending an hour a day in a hot sauna will cause excessive sweating. Remember that salts are excreted through sweat; therefore, the electrolyte concentration, and the patient's blood pressure, will decrease further if the patient spends time in a sauna. Although lithium chloride is a strong electrolyte (like sodium chloride), the doctor will not prescribe lithium chloride because it is a toxic substance.
Example Question #1 : Solubility And Ions
Adding ammonia to a solution containing a copper hydroxide precipitate has the effect of dissolving the precipitate because __________.
an increase in hydronium ions shifts the reaction of copper and hydroxide to the left
the addition of ammonia allows more hydroxide and hydrogen ions to form water molecules, increasing the solubility of copper hydroxide
hydroxide ions bond ionically with ammonia
covalent bonds form between copper ions and ammonia molecules
covalent bonds form between copper ions and ammonia molecules
The copper ion is able to form four covalent bonds. This can be explained with electron configurations.
The copper atom configuration:
The copper ion configuration:
The copper ion can accept electrons that fill empty 
Example Question #1 : Solution Chemistry
both involve coordinate covalent bonds that utilize empty electron orbitals in the metal ion
both involve hybridization of the 
both compounds are able to form coordinate covalent bonds with a metal ion
both involve ionic bonds that form due to polarity
both involve coordinate covalent bonds that utilize empty electron orbitals in the metal ion
Complex ions typically form when coordinate covalent bonds are formed between ligands, such as water or chloride, and metal ions that have lost electrons and have available orbitals. While one answer choice, "Both compounds are able to form coordinate covalent bonds with a metal ion," may seem correct, this happens within the compound. The compounds themselves are ions that will form ionic compounds.
Example Question #7 : Ions In Solution
Which is an inaccurate depiction of the central iron atom in a hemoglobin molecule?
It is covalently bonded to five water molecules that can easily be replaced with oxygen molecules.
It has a coordination number of 6.
It is covalently bonded to five nitrogen atoms, one of which is a component of the globin protein.
It is able to form coordinate covalent bonds with water, oxygen, and carbon monoxide.
It is covalently bonded to five water molecules that can easily be replaced with oxygen molecules.
The central iron in hemoglobin is the middle of a complex compound and is involved in six covalent bonds (coordination number of 6). It is bonded to five nitrogen atoms, one of which is part of an amino acid in the globin protein. This leaves one remaining bond, which can be with water, oxygen, or carbon monoxide (which forms a very stable bond, making it toxic).
Example Question #1 : Acid Base Equilibrium
Acids and bases can be described in three principal ways. The Arrhenius definition is the most restrictive. It limits acids and bases to species that donate protons and hydroxide ions in solution, respectively. Examples of such acids include HCl and HBr, while KOH and NaOH are examples of bases. When in aqueous solution, these acids proceed to an equilibrium state through a dissociation reaction.
All of the bases proceed in a similar fashion.
The Brønsted-Lowry definition of an acid is a more inclusive approach. All Arrhenius acids and bases are also Brønsted-Lowry acids and bases, but the converse is not true. Brønsted-Lowry acids still reach equilibrium through the same dissociation reaction as Arrhenius acids, but the acid character is defined by different parameters. The Brønsted-Lowry definition considers bases to be hydroxide donors, like the Arrhenius definition, but also includes conjugate bases such as the A- in the above reaction. In the reverse reaction, A- accepts the proton to regenerate HA. The Brønsted-Lowry definition thus defines bases as proton acceptors, and acids as proton donors.
Which of the following expressions most closely approximates the equilibrium constant of pure water?
The autionization of water proceeds as follows:
The equilibrium constant is calculated by the following formula.
The equilibrium expression is 
Example Question #2 : Acid Base Equilibrium
What is the hydroxide concentration in an aqueous solution with a pH of 2?
There are no hydroxide ions in the solution
The hydroxide concentration can be determined by considering the autoionization of water in a solution. At 
Since the reaction is in terms of proton and hydroxide ion concentrations, we can set the expression equal to this value in order to determine the concentration of hydroxide ions at the given pH. We start by determining the concentration of protons in the solution by using the following equation:
Now, we can solve for the hydroxide concentration.
This problem can also be solved by calculating the pOH of the solution and using this value to find the hydroxide ion concentration.
Example Question #2 : Acid Base Chemistry
What is the pH of a solution which has a hydroxide ion concentration of 5 * 10-4M?
First convert concentration of OH- into concentration of H+. Remember that Kw is 1*10-14.
Then, convert concentration of H+ into pH.
Alternatively, you can use the hydroxide concentration to solve for pOH and convert to pH.
Example Question #2 : Acid Base Equilibrium
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)
Which following pair gives the correct equation, and change in pH, when carbon dioxide diffuses into blood?
CO2 + H2O H2CO3 | pH decreases
CO2 + H2O HCO3– | pH decreases
O2 + CO2 + H2O H2O2 + CO32– | pH decreases
CO2 + H2O H2CO3 | pH increases
CO2 + H2O H2CO3 | pH decreases
This is essentially a two-part question. The first asks you to generate the production of carbonic acid; and the second, requires you to infer that an acid will lower blood pH. Only one answer choice is both balanced correctly and gives an accurate change in pH. Although this knowledge is not required for the MCAT, your body constantly monitors blood pH to determine if concentrations of CO2 are too high, or alternatively, if O2 concentrations are too low.
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