# chemistry - exam practice help!!

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i'm not exactly sure how to do this question :/
i don't even know where to start.

"Determine the equilibrium [F-] of the following solution with initial concentrations od [HF]= 1.296 M and [NaF] = 1,045 M (ka for HF is 6.6 * 10 ^ -4)

options are

a) 1.046
b) 0.251
c) 2.344
d) 8.2 * 10^-4
e) 5.3*10^-4

all i know is that HF is a weak acid
and
that HF dissociates into H+ and F-

:S

any help at all is appreciated!!

• chemistry - exam practice help!! -

I assume the (NaF) = 1.045M and not 1,045M.
..........HF ==> H^+ + F^-
I........1.296...0......0
C.........-x.....x......x
E.......1.296-x...x.....x

.........NaF ==> Na^+ + F^-
I.......1.045.....0......0
C.....-1.045.....1.045..1.045
E........0......1.045..1.045

Ka for HF = (H^+)(F^-)/(HF)
(H^+) = x
(F^-) = x + 1.045
(HF) = 1.296-x

The easy of solving this is to substitute concns I've listed above, assume x + 1.045 = 1.045 and 1.296-x = 1.296; then solve for x in the Ka expression. When you obtain that, then total (F^-) = 1.045+x. If you want to do it up brown, set up the Ka expression and solve the quadratic equation.

• chemistry - exam practice help!! -

thank you! and yes the 1,045 was a typo ,

but how did you know that the change row in the ice table for NaF under Na+ and F- were gonna be 1.045 ? wouldn't they be x as well ? that's just the only part that messes me up. :S

• chemistry - exam practice help!! -

NaF is a salt, it is completely soluble in water, it is an ionic compound, and it is 100% ionized in aqueous solution. There is no unionized NaF left over as there is in HF, a weak acid and weak electrolyte.

• chemistry - exam practice help!! -

wow, i don't know how i missed that connection before. of course, its simple. i must be getting too nervous for my test lol. Thank you so much for clearing that up for me!

• chemistry - exam practice help!! -

It's ok to get nervous; just don't thing crooked, think straight.

• chemistry - exam practice help!! -

swag

• chemistry - exam practice help!! -

My name is victor yakubu am from bayelsa state i was among the successful candidates that wrote the Nigeria Air Force (NAF) exam in the year 2015, i am so happy as this was my fourth time of writing it and all my appreciation goes to Air Vice Marshal Sadique Baba Abubakar for his assistance i am very happy that few months from now i will be a member of Nigeria Air Force (NAF). if your name is still not out yet, i will advice you contact him to help you before the last list is out and it is only few names that will be released so hurry up and here is Air Vice Marshal Sadique Baba Abubakar number 08144702657. 08077687246. you can also contact him on facebook Avm SB Abubakar. thanks for all you have don in my life sir.

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