Some chemistry questions?

Question

I am a Bio major and am in Organic chem. These questions are review from Gen Chem 122. I'd really appreciate any and all help - if you can answer any one of the questions below, it'd really help me out. Explanations would be awesome, too, as I'm trying to LEARN this stuff - not just get answers for my homework.

1. Which of the following has nonpolar covalent bonds:

a)NaF b)H2O c)P4 d)NH3 e)BeCl3

2. What angle is associated with a central atom that has octahedral ELECTRONIC geometry?

a) 109.5 b)120 c)120 and 180 d)90 and 120 e)90 and 180

3. How many MOs are formed by the combination of the p atomic orbitals of 2 atoms?

a) 1 b) 2 c) 3 d) 6 e)12

4. What is the bond order of a species with 10 bonding electrons and 4 antibonding electrons?

a) 1 b) 1.5 c) 2 d) 2.5 e)3

5. The [OH-] = 1.3 X 10^-6 M for a 0.025 M solution of a weak base. Calculate the Kb for this weak base. (if someone could just explain how to do this, I had a pretty good grasp on acids/bases)

a)5.2 x 10^-5 b)3.1 x 10^-7 c)7.7 x 10^-9 d)4.0 x 10^-8 e)6.8 x 10^-11

Answer 1

1) NaF is an an ionic bond.  H20 is not symmetric, so it is polar.  NH3 is also polar.  BeCl3 (i'm guessing) is a salt.  P4 forms covalent bonds and is also symmetric (it's tetrahedral) so the answer is C.

2) if you have octahedral geometry, all vertices are 90 degrees.  (imagine the origin in 3-d cartesian space), so the answer is e) 90 and 180.

3) i'm not certain, but i think that since there are 3 p orbitals, you can form 3 molecular orbitals.

4) bond order is (#bonding e - #antibonding e)/2, so the answer is e)3

5) Kb = [B+] [OH-] / [BOH].

For each free OH-, there must be a complementary B+, so [B+] = [OH-].  Also, [BOH_equilibrium] = [BOH_initial] - [OH-], since each disassociation decreases the amount of bound BOH.  finally, if you plug and chug:

Kb = [1.3E-6][1.3E-6]/[.025-1.3E-6]

= voilà