gchem Qs

[tRNA]

Advertisement - Members don't see this ad

I am having trouble with this type of problems:

1) what will be the hydrogen ion concentration of a 1M solution of a very weak acid?
a) .5Ka
b) 2Ka
c) Ka^2
d) Ka^1/2
(answer=d)
I understand that it will be less than the normal Ka since it is a very weak acid, but why isn't .5Ka correct?

and what if the solution was a strong acid solution, would the hydrogen ion concentration be 2Ka or Ka^2?? why?

thanks for helping

 

[Mia305]

Ka = ([x][x])/M... since M = 1, then Ka = x^2, which can then be solved for X

x= sqrtKa or Ka^1/2

 

[tRNA]

thanks, but what if the solution is a strong acid would you have the same answer, ie sqrtKa? i guess iam asking are there any conditions under which the ka might be changed to 2Ka or Ka^2 ?

 

[Nasem]

nevermind... I tried to draw an ICE table but it didn't come out well...

 

[Mia305]

I know what you are asking, but I am really not positive. If it was me I would put the same answer, because like you I am not sure if there is a difference with acid strength.

 

[doc toothache]

thanks, but what if the solution is a strong acid would you have the same answer, ie sqrtKa? i guess iam asking are there any conditions under which the ka might be changed to 2Ka or Ka^2 ?

With a strong acid you would have 100% dissociation. Therefore, a 1 M would give you 1M H+ and 1M A-. You solve the problems as Mia305 did above giving a Ka= 1.

 

[scottyhoop]

With a strong acid you would have 100% dissociation. Therefore, a 1 M would give you 1M H+ and 1M A-. You solve the problems as Mia305 did above giving a Ka= 1.

I agree...strong acids/bases don't really have a Ka so the above method is the way to go.
Scott

 

[Mia305]

Don't strong acids have a high Ka and low pKa?

 

[doc toothache]

Don't strong acids have a high Ka and low pKa?

????

pKa= -log Ka

Perhaps you a are thinking high Ka low pKb or low pKa high pKb.

 

[jdmsamurai]

actually he is right...strong acids have high Ka and low pKa...basically remember this

strong acids
LOW pH
LOW pKa

HIGH H conc
HIGH Ka

 

[jdmsamurai]

please dont mix up Ka and Kb...they are two different things...here is my quicke

Ka X Kb = 1.0 e-14 always

Acids
HA --> H+ + A-
Ka = ([H+][A-])/[HA]

Bases
B- + H20 ---> HB + OH-
Kb = ([HB][OH])/[B-]

some from any of those equations you can get pH or pOH by
pH = -log[H+]
pOH = -log[OH-]

and pH + pOH = 14 always

 

[lor]

add these to your flash cards
WK acid/base only
[H+]=sqaure rt (ka*[WK acid])
for OH replace H+ with OH-, Ka with Kb and WK acid to WK base
you are good to go
good luck to you