GCHEM question, need explination please.

[TriCuspid]

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1) During a titration it was determined that 30.00 mL of a 0.100 M Ce4+ solution was required to react completely with 20.00 mL of a 0.150 M Fe2+ solution. Which​

reaction occurred?
*(The charges after the elements are supercripts, as in Ce^4+)

A. Ce4+ + 3Fe2+ + H2O 3Fe3+ + CeO- + 2H+
B. 2Ce4+ + Fe2+ Fe4+ + 2Ce3+
C. Ce4+ + Fe2+ Fe3+ + Ce3+
D. Ce4+ + 2Fe2+ 2Fe3+ + Ce2+
E. Ce4+ + 2Fe2+ 2Fe4+ + Cs2+ + 2e-


2) The concentrations of silver ion and chloride ion in an aqueous solution in equilibrium with solid silver chloride are
1.0 x 10-6 M. What is the value of Ksp for AgCl equal to?
​

A. (2.0 x 10-6)(1.0 x 10-6)
B. sq root of 1.0 x 10-6
C. 1.0 x 10-6
D. 1(1.0 x 10-6)​

E. (1.0 x 10-6)2

​

3) The electronic configuration of a particular neutral atom is 1​

​

​

​

s22s22p63s23p2. What is the number of unpaired electrons in this atom?

A. 1
B. 2
C. 3
D. 4​

E. 0

Ans:1)C 2)E 3)C, (I really thought answer was B, unless there was a special rule promotes the e- to p3 to make it a stable half making it 3s13p3?)​

 

[Pat Kelly]

1. if you find out the mols of each reactant, you would see that both are equal - thus, you look at the reactions to see which one gives you a 1:1 ratio for both reactants, hence the choice C.

2. ksp = [ag][cl], it's given that both [ag] and [cl] is 1x 10^-6, so you multiply both to get ksp - E

3. I agree with you - there's most likely a type such as they meant it to have 3s1 3p2, in which case there are 3 unpaired e-

 

[TriCuspid]

bump.

Thanks Pat.

Q#1) I still can't visualize how the answer is reasoned.

Q#2) I understand you explination. I wonder why I didn't see that before, it's actually a simple problem.

Q#3) Can others confirm? I know for the transitions elements you bump up the e- for group 6 and group 11, but is there any confirmation for the same effect for the P-block? EDIT: even if we promote the e- then its 3s13p3 then that would be 4 unpaired not 3 unpaired. This is confusing...

 

[UndergradGuy7]

bump.

Q#3) Can others confirm? I know for the transitions elements you bump up the e- for group 6 and group 11, but is there any confirmation for the same effect for the P-block?

1s22s22p63s23p2​

14 electrons = Silicon
http://chemistry.about.com/od/electronicstructure/ss/aufbau_3.htm

2 unpaired electrons.

 

[TriCuspid]

1s22s22p63s23p2​

14 electrons = Silicon
http://chemistry.about.com/od/electronicstructure/ss/aufbau_3.htm

2 unpaired electrons.

Thanks Undergrad. I shouldn't have doubt myself. I guess the ADA can be wrong with answers to their sample questions. I triple checked the key too.

Now if only I can understand the reason for Q#1, it's a shame they don't have an explination with their answer key.