Jul 24, 2009
3
0
Status
Pre-Dental
Hello, everyone!
This is my first time posting here:)
It's been so long since I took my general chemistry, and I cannot remember much.
While I was doing Destroyer, I had several questions that I cannot understand. So please please help me~

#128
A solution of NaOH and HF are titrated to the endpoint of a titration. The final solution will contain?

(a) Na+ and F-
(b) H2O, H+, OH-, HF, Na+, and F-
(c) H2O, F-, and Na+
(d) Na+ and F-, NaOH
(e) NaOH and HF in a 1:1 mole ratio

Also in #203, it says Sr++ is smaller than S-- since it forms a positive ion.
I understand that if an element is a positive ion, it will have smaller radius, but what I don't understand is... when we look at periodic table, the trend is going left and down direction, the radius gets bigger. And it seems like Sr and S are too far apart for charges to S bigger than Sr. They are even separated by one whole period. Please explain.

These are my questions. Please help me :'(
 

Kneecoal

10+ Year Member
Mar 2, 2009
870
0
Long Island, NY
Status
Pre-Dental
for 128, NaOH is a strong base, so it'll dissociate completely into Na+ and OH- ions. HF, however, is a weak acid, so it'll only partly dissociate. that means you'll have some H+, some F-, and also some HF (and the H+ and OH- floating around will also be forming H2O here and there)
 

dl9006

10+ Year Member
5+ Year Member
Jul 2, 2008
541
1
Status
Dental Student
Hello, everyone!
This is my first time posting here:)
It's been so long since I took my general chemistry, and I cannot remember much.
While I was doing Destroyer, I had several questions that I cannot understand. So please please help me~

#128
A solution of NaOH and HF are titrated to the endpoint of a titration. The final solution will contain?

(a) Na+ and F-
(b) H2O, H+, OH-, HF, Na+, and F-
(c) H2O, F-, and Na+
(d) Na+ and F-, NaOH
(e) NaOH and HF in a 1:1 mole ratio

Also in #203, it says Sr++ is smaller than S-- since it forms a positive ion.
I understand that if an element is a positive ion, it will have smaller radius, but what I don't understand is... when we look at periodic table, the trend is going left and down direction, the radius gets bigger. And it seems like Sr and S are too far apart for charges to S bigger than Sr. They are even separated by one whole period. Please explain.

These are my questions. Please help me :'(
question 1: water will always be in equilibrium so you will have h20, H+, OH-.
HF is a weak acid so it won't completely dissociate. thus u have HF and some F- from neutralization with OH-

NaOH is soluble so will exist as Na+ and OH-


hmm for the 2nd question, i think it may have to do with the fact even though those two ions are a period apart, they still have the same # of protons pulling the electrons as when they were neutral atoms

S2- has 16 protons pulling an extra 2 electrons in

whereas Sr 2+ now has 2 less electrons to pull in. Sr's 38 protons can now pull on only 36 electrons vs 38 when it was neutral.

the average force that can be exerted on each electrons is much greater in Sr2+



that's my take on it..
 
Last edited:
Jun 14, 2009
800
3
Status
Pre-Dental
Personally, I would have guessed that S-- was the smaller ion, owing to the fact that Sr++ is an entire electron shell larger. I looked them up on chemicool.com, and Sr++ is a full 38pm smaller than S--... S-- is larger than S, and Sr++ is smaller than Sr, but I wouldnt have imagined by that much. I guess the proton-electron difference is enough to pull the entire electron cloud smaller by a period's worth of ionic radius

=/