[Woopy]

Hello, just in need of some homework help on two problems regarding K expressions for acids. In the problems in given the pH, however I don't know what I'm suppose to do with it

1. Calculate the K_a of the weak acid, HNO₂, if the pH at equilibrium was 4.50 and the concentration of the HNO₂  at equilibrium was .500 M.

For this one, i'm not sure if your all taught the "ICE" chart method, but I (initial concentration), C (constant), and E (equilibrium concentration).

HNO₂ <--> H⁺ + NO₂^-
I       chunk    0                            0 
C      -x        +x                           +x
E      .500       x                             x

I don't know what to do from here since i'm given a pH, do I -log the pH to get the [H⁺], any help?



2. Aspirin is a weak acid. The forums HAs is used to represent the molecule. If the initial concentration of the aspirin was .1000M, at equilibrium, the pH of the solution was 2.244. Calculate the K_a for aspirin

This is what i've come up with so far, again I don't know where the pH comes into play with this problem

HAs <--> H⁺ + As ^-
I .1000          0                     0
C -x             +x                   +x
E who cares?   x                     x           

[Astrokel]

do I -log the pH to get the [H⁺], any help?

Right about the method but formula is wrong.

2. Aspirin is a weak acid. The forums HAs is used to represent the molecule. If the initial concentration of the aspirin was .1000M, at equilibrium, the pH of the solution was 2.244. Calculate the K_a for aspirin

This is what i've come up with so far, again I don't know where the pH comes into play with this problem

HAs <--> H⁺ + As ^-
I .1000          0                     0
C -x             +x                   +x
E who cares?   x                     x           

Same as 1, you can get a value for x

Try it out 

[The_Student]

i don't think you -log pH i think it is 10^-pH to find the concentration of H+