GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Chemical Reactions

Study concepts, example questions & explanations for GRE Subject Test: Biochemistry, Cell, and Molecular Biology

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All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources

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Example Question #87 : Gre Subject Test: Biochemistry, Cell, And Molecular Biology

Consider the following reaction:

Which atom is oxidized in the reaction?

Possible Answers:

Iron

Aluminum

Oxygen

None of the atoms are oxidized

Correct answer:

Aluminum

Explanation:

Oxidation is the process of losing electrons, or acquiring a more positive charge. We can determine which atom has lost electrons by comparing the oxidation number of the atom as a reactant and as a product. Aluminum is in elemental form as a reactant, so it has an oxidation number of 0. As a product, it is bonded with oxygen, giving each aluminum an oxidation number of +3. Since it has a more positive charge, the aluminum has been oxidized. 

Example Question #88 : Gre Subject Test: Biochemistry, Cell, And Molecular Biology

Which mnemonic would be most useful for remembering what happens during an oxidation-reduction reaction?

Possible Answers:

LEO says GER

BARF

ROME

PASS

OCEAN

Correct answer:

LEO says GER

Explanation:

LEO says GER is a mnemonic for Loss of Electrons is Oxidation, Gain of Electrons is Reduction. An oxidation-reduction reaction is a chemical interaction in which one substance is oxidized and loses electrons, and thus is increased in positive valance, while another substance gains an equal number of electrons and is reduced, thus decreasing in positive valance. This is called a redox system or reaction. Along with LEO says GER, another popular mnemonic for this system is "OIL RIG," which stands for Oxidation is Loss, Reduction is Gain (in reference to electrons).

BARF is a mnemonic for Break (a bond) Absorb (energy), Release (energy) Form (a bond). This is used to remember whether energy is required or released when chemical bonds are broken and formed. OCEAN describes the big five personality traits: openness, conscientiousness, extraversion, agreeableness, and neuroticism. PASS stands for pull, aim, squeeze, and sweep—how to use a fire extinguisher. ROME is a mnemonic for Respiratory Opposite, Metabolic Equal. The values for the respiratory form of acid-base balance are opposite; if pH is high, carbon dioxide will be low and vice versa. For the metabolic form, the values are equal. If pH is high, carbonic acid will be high. If pH is low, carbonic acid will be low. 

Example Question #1 : Chemical Reactions

Reduction of a compound results in a __________ of electrons, thus rendering the compound's charge to be more __________.

Possible Answers:

gain . . . positive

loss . . . positive

loss . . . negative

gain . . . negative

loss . . . dynamic

Correct answer:

gain . . . negative

Explanation:

The process of reduction causes a gain of electrons. Because electrons are negatively charged, any compound that becomes reduced will be more negative than it was prior to the reaction. 

Example Question #1 : Chemical Reactions

Which of the following is not a redox reaction?

Possible Answers:

I only

II only

II and III

I and II

III only

Correct answer:

II only

Explanation:

All of the given choices represent oxidation-reduction reactions that are important in cellular metabolism. Oxidation-reduction reactions involve the changing of oxidation states (commonly through the transfer of electrons). In the first reaction NAD+ is reduced. The second reaction shows the polymerization of two molecules of glucose via a condensation (dehydration synthesis) reaction. In the third reaction glucose is oxidized.

NADH and are important electron carriers that bring electrons to the electron transport chain and are formed during glycolysis and the Krebs cycle via reduction. The final choice represents the overall oxidation-reduction reaction that occurs for one molecule of glucose.

Example Question #1 : Help With Acid Base Chemistry And P H

What is the appropriate way to describe a solution with a pH of 3 compared to a solution with a pH of 6?

Possible Answers:

It is 30 times more acidic

It is 1000 times more acidic

It is 3 times more acidic

It is 3 times less acidic

Correct answer:

It is 1000 times more acidic

Explanation:

The pH scale is logarithmic, meaning every whole number change on the scale reflects a ten-fold change in acidity. Since a pH of 3 is three numbers higher than a pH of 6, we can find the change in acidity by taking 10 to the third power.

The solution with a pH of 3 is 1000 times more acidic than the solution with a pH of 6. 

Example Question #1 : Help With Acid Base Chemistry And P H

What is the pH of a 0.03M hydrochloric acid solution?

Possible Answers:

Correct answer:

Explanation:

Since hydrochloric acid is considered a strong acid, we can assume that the acid will dissociate completely, leaving us with a 0.03M concentration of hydronium ions. Knowing this, we can calculate the pH by taking the negative log of the hydronium concentration:

 

Example Question #13 : Biochemistry

What is the concentration of hydroxide ions in a 1M hydrochloric acid solution?

Possible Answers:

Correct answer:

Explanation:

To solve this question we need to first look at the relationship between hydroxide and hydrogen ion concentrations:

where  is concentration of hydrogen ions and  is the concentration of hydroxide ions. Solve for :

We can solve for the concentration of hydrogen ions in this solution by writing the dissociation of hydrochloric acid in solution:

The question states that it is a 1M solution. Since  and  are in 1:1 ratio, the concentration of hydrogen ions will equal the concentration ; therefore, . Note that this won’t be the case if the acid was weak. For a weak acid, the concentration of  will be less than the concentration of acid, even if it was 1:1 ratio. This occurs because a weak acid does not dissociate completely.

We can now use the  to solve for .

Therefore, concentration of hydroxide ions in this solution is

Example Question #14 : Biochemistry

Which of the following substances is considered a Lewis acid?

Possible Answers:

Both of these compounds

None of these compounds

Correct answer:

Explanation:

Recall that acids and bases can be defined in three different ways. First, the Arrhenius definition states that an acid is a substance that increases the concentration of hydrogen ions in solution whereas a base decreases hydrogen ions concentration. Second, Bronsted-Lowry definition states that an acid is any substance that donates a proton whereas a base is any substance that accepts a proton. Finally, Lewis definition states that an acid is any substance that accepts an electron pair whereas a base is a substance that donates an electron pair.

We have two substances in this question. , or borane, is considered a Lewis acid because it accepts an electron pair. Boron has four orbitals in its outermost shell. If we look at Boron in borane, we will notice that three orbitals are occupied by the electrons in the three single bonds (bound to hydrogen) whereas the last orbital is empty and is free to accept electrons. Since it accepts electrons, borane is considered a lewis acid. , or ammonia, is a lewis base. If we look at the nitrogen atom in ammonia, we will notice that all of its orbitals are occupied (three orbitals are occupied with the three bonds to hydrogen atoms and the last orbital is occupied with a lone electron pair). Since it has extra electrons in the last orbital, ammonia can donate electrons and is considered a lewis base.

Example Question #15 : Biochemistry

Solutions A and B are both made up of an acidic solution. After analysis, it is observed that the  of solution A is higher whereas the  of acid in solution B is lower. What can be concluded about these results?

Possible Answers:

More products are produced in solution A

The acid dissociation constant of solution A is higher

More hydrogen ions are produced in solution B

Results seem invalid

Correct answer:

Results seem invalid

Explanation:

The question states that the  of solution A is higher. The definition of  is:

If we rearrange and solve for concentration of hydroxide ions, we get

This means that  decreases as  increases. The relationship between hydroxide ion and hydrogen ion is as follows

Note that the hydroxide ion concentration is inversely proportional to that of hydrogen ions; therefore, lower hydroxide ion concentration leads to higher hydrogen ion concentration and higher acidity. Since solution A has the higher , its hydroxide ion concentration is lower than solution B. This means that solution A has higher hydrogen ion concentration and, therefore, is more acidic than solution B.

The question also states that the  of solution B is lower. Recall that as  of an acid decreases it becomes more acidic. Based on this, we conclude that solution B is more acidic. The two results given in this question conflict each other; therefore, these results seem invalid.

Example Question #16 : Biochemistry

The pH of a solution __________ when the acid dissociation constant __________.

Possible Answers:

increases . . . decreases

does not change . . . increases

does not change . . . decreases

increases . . . increases

Correct answer:

increases . . . decreases

Explanation:

Acid dissociation constant, or , for a prototypical acid-base reaction is defined as

where  is an acid and  is its conjugate base. Based on this equation, we can determine that  and  are directly proportional to one another; therefore, increasing  will increase  and vice versa.

Recall the definition of 

Solving for  gives us

This means that increasing pH will decrease  and vice versa (has the opposite effect on each other); therefore,  and  also have an opposite effect on each other.

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