Determine the pH of an HF solution of each of the following concentrations?
1. 0.300 M
2. 5.10×10^−2 M
3. 2.60×10^−2 M
In which cases can you not make the simplifying assumption that x is small?
Ka of HF = 3.5*10^4
Please, please help!!!!
4 Answers

HF is a weak acid so you will have to make an ICE table
HF + H2O —> H3O+ + F
0.300
x……………………+x…..+x
0.300x……………x……..x
x^2/(0.300x) = 3.5×10^4
Assume 0.300x ~ 0.300
log[X] to find pH
if x / 0.300 is less than 5%, then your assumption is good. Otherwise, you cannot assume x is small and you will have to solve the quadratic equation.
This will get you going on how to solve the questions.

Ph Of Hf

1) 3.5E4=x2/(.3x)
—equation from forming ice table
x=[H+]=.010073
log.010073 = 1.99684
pH =2
2) pH=2.39
3) pH=2.55
in b and c you cant make that assumption because the actual concentrations produced are more than 5% that the error allows for when using the small number approximation
would you happen to know how to do the buffers in medicine problem??

pH= log[concentration]
pH= log[0.3M]=0.52
pH= log[5.10 x 10^2]= 1.29
pH= log[2.60 x 10^2]= 1.59
pH=log[3.5 x 10^4]= 3.46
These are all acidic because they are under 7. Anything above 7 is basic.
x is smallest in the first one.