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
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HF is a weak acid so you will have to make an ICE table
HF + H2O —-> H3O+ + F-
0.300
-x……………………+x…..+x
0.300-x……………x……..x
x^2/(0.300-x) = 3.5×10^-4
Assume 0.300-x ~ 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.
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Ph Of Hf
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1) 3.5E-4=x2/(.3-x)
—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??
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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.
