GRE Subject Test: Chemistry : Molecules and Compounds

Study concepts, example questions & explanations for GRE Subject Test: Chemistry

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Example Questions

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Example Question #31 : General Chemistry

Which of the following compounds is not held together by ionic bonds?

Possible Answers:

Correct answer:

Explanation:

Covalent bonds are formed when two nonmetals are bonded together. This covalent bond means that the electrons are shared by the two atoms in order to satisfy each atom's octet. There is very little difference in the electronegativities of the two atoms involved in the bond, so neither atom pulls the electrons closer to its nucleus.

Ionic bonds are formed between a metal and a nonmetal. Due to the dramatic difference between the electronegativities of metals and nonmetals, the electrons are pulled tightly to the nonmetal, and away from the metal nucleus. This results in each atom having a full octet, even though the electrons are not shared.

Carbon and oxygen are both nonmetals, so we would expect only covalent bonds in carbon dioxide.

Example Question #1 : Molecules And Compounds

Given the salt , what is the charge on the manganese ion in this molecule?

Possible Answers:

Correct answer:

Explanation:

In order to determine the charge of the transition metal manganese in the molecule, , we must first determine the net charge of the molecules it is bonded to. The manganese is bonded to three fluoride ions. Fluoride ion carries a negative one charge. since it is a halogen The subscript in front of the fluoride in the molecule tells us that we have three fluoride ions. Each fluoride ion carries a  charge and because we have 3 of them, there is a total charge of  from these fluoride ions. Molecules like to exist in their most stable state which gives them an overall charge of zero. Therefore, the manganese atom will carry a charge of  to counter the  charge from the three fluoride ions. 

Example Question #1 : Bonds And Forces

Which of the following solids is most ionic?

Possible Answers:

Correct answer:

Explanation:

An ionic bond is a bond that occurs between a metal and a nonmetal, which may dissociate into two ions of opposite charges (positive and negative). These bonds are formed by electrostatic forces. We can determine if a compound is ionic from what it is composed of. Ionic compounds can easily be identified because they are generally composed of a metal and non-metal such as . Metals are cations and non-metals are anions. In the options given, the substance that contains an ionic bond is . The ions involved are  and 

Example Question #1 : Molecules And Compounds

Given the salt , what is the charge on the  ion in this molecule?

Possible Answers:

Correct answer:

Explanation:

In order to determine the charge of the  ion in the molecule, , we must first determine the net charge of the molecules it is bonded to. The  ion is bonded to a  ions.

 ion is one a common anion and carries a -2 charge. Molecules like to exist in their most stable state which gives them an overall charge of zero.  has an overall charge of zero. Therefore, the  atom will carry a charge of +2 to counter the -2 charge from the  ion.

Example Question #1 : Molecules And Compounds

Given the salt , what is the charge on the barium ion in this molecule?

Possible Answers:

Correct answer:

Explanation:

In order to determine the charge of the barium ion in the molecule, , we must first determine the net charge of the molecules it is bonded to. The barium ion is bonded to two fluoride ions.

Fluoride ion carries a -1 charge. The subscript in front of the fluoride in the molecule tells us that we have two fluoride ions. Each fluoride ion carries a -1 charge and because we have 2 of them, there is a total charge of -2 from these fluoride ions. Molecules like to exist in their most stable state which gives them an overall charge of zero. Therefore, the barium atom will carry a charge of +2 to counter the -2 charge from the two fluoride ions.

Example Question #1 : Help With Molecular Properties

The degree of unsaturation for ephedrine (shown below) is __________.

Mcat_problem_set_1

Possible Answers:

Correct answer:

Explanation:

The degree of unsaturation is equal to the number of rings plus the number of pi bonds in a molecule. Ephedrine has one ring and three pi bonds, so its degree of unsaturation is four.

To arrive at this answer, one could also use the formula below, where  is the number of carbon atoms,  is the number of hydrogen atoms,  is the number of halogen atoms, and  is the number of nitrogen atoms.

 For ephedrine, , , , and .

Example Question #1 : Molecules And Compounds

2-butyne contains all of the following types of bonds except __________.

Possible Answers:

Correct answer:

Explanation:

2-butyne has the following chemical structure.

The end carbons have  hybridization (form single bonds only), while the middle two carbons have  hybridization (involved in a triple bond).  There are no  hybridized carbons in this molecule.  

Example Question #1 : Bonds And Forces

For the compound shown below, the hybridization for carbon A is __________ and the hybridization for carbon B is __________.

Mcat12

Possible Answers:

Correct answer:

Explanation:

Carbon A is  hybridized because this carbon is only bonded to two other atoms. Carbon B is bonded to four atoms, and would therefore be  hybridized.

Keep in mind that a carbon involved in a triple bond will always be hybridized, a carbon involved in a double bond will be hybridized, and a carbon involved only in single bonds will be hybridized.

Example Question #1 : Covalent Bonding

Which is not a property of covalent compounds?

Possible Answers:

They have low melting and boiling points

The elements involved generally have large differences in electronegativity

Many are not soluble in water, but are soluble in nonpolar liquids

They contain bonds formed by sharing one or more pairs of valence electrons between atoms

They are poor electrical conductors

Correct answer:

The elements involved generally have large differences in electronegativity

Explanation:

The incorrect statement is a property of ionic compounds rather than covalent. Recall that electronegativity is a measure of the ability of an atom to draw electrons to itself. Ionic compounds are formed by elements with very different electronegativities, since elements with different electronegativities will tend to form positive and negative ions (that is, they give away or gain electrons easily). In contrast, covalent bonds are formed by elements which are close in electronegativity and could exist as stable free molecules. All other statements are true of covalent compounds.

Example Question #1 : Covalent Bonds

Which of the following molecules contains polar, covalent bonds?

I. 

II.

III.

IV.

V.

Possible Answers:

I, II, IV, V

II, IV, V

II, IV

II, III, IV, V

III, IV, V

Correct answer:

II, IV, V

Explanation:

 is an ionic compound, while  is a nonpolar covalent compound. Remember that polarity results from a difference in the electronegativities of the atoms involved in the bond. Too great of a difference will result in an ionic bond; two of the same atoms will have zero difference, resulting in a nonpolar bond.

, , and  contain polar covalent bonds. In the first two, oxygen will carry a slight negative charge, leaving sulfate and phosphorus with slight positive charges. In , chlorine will carry a slight negative charge, leaving the carbon slightly positive. In  and , symmetry helps to balance the polar bonds, resulting in an overall nonpolar molecule, even though the individual bonds are nonpolar.

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