Step-by-Step Guidance for Sample Final Exam (Edmonds College GOB Chemistry)

Q15. Classify the molecules as polar or nonpolar.

Background

Topic: Molecular Polarity

This question tests your understanding of how to determine whether a molecule is polar or nonpolar based on its structure and the distribution of electrons.

Key Terms:

• Polar molecule: A molecule with an uneven distribution of charge, resulting in a dipole moment.

• Nonpolar molecule: A molecule with an even distribution of charge, so there is no overall dipole moment.

• Electronegativity: The tendency of an atom to attract electrons in a bond.

• Lewis structure: A diagram showing the arrangement of atoms and electrons in a molecule.

Step-by-Step Guidance

1. Examine the Lewis structures of the molecules provided. Identify the atoms and their arrangement.

2. Determine if there are any highly electronegative atoms (such as O, N, F) bonded to less electronegative atoms.

3. Assess the symmetry of the molecule. If the molecule is symmetrical and the dipoles cancel, it is likely nonpolar. If it is asymmetrical and the dipoles do not cancel, it is likely polar.

4. Consider the presence of lone pairs on central atoms, which can affect the shape and polarity.

Try solving on your own before revealing the answer!

Final Answer:

PBr3 is polar due to its trigonal pyramidal shape and the presence of a lone pair on phosphorus, which creates a net dipole moment. Formaldehyde (CH2O) is also polar because of the difference in electronegativity between carbon and oxygen, and its molecular geometry does not allow the dipoles to cancel.

Q16. Amino acids differ from each other by their unique branching “R” groups. Study the branching “R” group of the amino acid serine and choose the single most important type of intermolecular force that the side chain of serine experiences with water.

Background

Topic: Intermolecular Forces

This question tests your ability to identify the strongest intermolecular force between the side chain of serine and water molecules.

Key Terms:

• Intermolecular forces: Forces of attraction between molecules, including hydrogen bonds, dipole-dipole, dispersion (London) forces, and ionic bonds.

• Hydrogen bond: A strong dipole-dipole interaction between a hydrogen atom bonded to N, O, or F and another electronegative atom.

• Serine: An amino acid with a side chain containing an -OH group.

Step-by-Step Guidance

1. Identify the functional group present in the side chain of serine (the -OH group).

2. Recall that water also contains an -OH group, which is capable of forming hydrogen bonds.

3. Compare the possible intermolecular forces: dispersion forces (weak), dipole-dipole (moderate), hydrogen bonds (strong), and ionic bonds (only if ions are present).

4. Determine which force is most important between the -OH group of serine and water.

Try solving on your own before revealing the answer!

Final Answer:

The most important intermolecular force between the side chain of serine and water is hydrogen bonding, due to the presence of the -OH group in both molecules.