What stoichiometric concentration of the indicated substance is required to obtain an aqueous solution with the pH value shown below?
aniline (C6H5NH2) for PH of 9.05
I converted pH to pOH so pOH=4.98
[OH-] =1.122*20^-5
C6H5NH2 + H2O --------> C6H5NH3+ + OH-
I: a N/A 0 0
C: -x N/A +x +x
E: a-x N/A x x
Kb=[C6H5NH3+][OH-]/[C6H5NH2 ]
4.2*10^-10 = x^2/(a-x)
Substituting in the equilibrium values from the ICE table and rearranging I found "a" which is the concentration of C6H5NH2 to be 0.299M but this is the wrong answer. Does anyone know what I did wrong?
The only thing I can find is that if pH = 9.05 then pOH is 4.95 and not 4.98 which makes (aniline) = 0.2997 which I would round to 0.300M. I assume you are using Kb found in your text or notes. My OLD OLD text lists it as 3.98E-10 but you should go with the number in your text or notes. The correct answer is 0.300M. See my solution a couple posts above yours under Ashley. It looks almost the same as yours except for the pOH thing.
On second thought I do know what the problem is. You have only two significant figures in Kb; therefore, the answer must have no more than two. So the answer should be rounded to 0.03M
Let me know is this is correct. I think that must be the problem.
No, that didn't work. I even tried using your Kb value but that still wasn't right.
I made a typo in my response (transposed the numbers). The concn we calculated is 0.298 or so and to two s.f. that rounds to 0.30 (not 0.03 as I posted in my second post). I think 0.30 will get it for you. I would appreciate a new post to DrBob222 since this one is getting buried.
Surely 0.30 is right.
No, 0.30 isn't right either. I'm going to ask my prof about it tomorrow.
Based on your calculation, it seems that you have set up the correct equilibrium expression and attempted to solve for the concentration of aniline (C6H5NH2) by substituting the values from the ICE table. However, there might be a mistake in the calculations that led to the wrong answer. Here are a few things you can check:
1. Check your initial concentration (I) of C6H5NH2.
Double-check if you have entered the correct initial concentration of aniline (C6H5NH2) in the ICE table. It should be consistent with the concentration you found in your final answer.
2. Verify your quadratic equation.
Ensure that you have correctly set up the quadratic equation by rearranging the equilibrium expression. Check if you made any mistakes in converting the pOH to the concentration of OH- ions.
3. Check your calculation for x.
Make sure you correctly solved for x since it represents the change in concentration of C6H5NH2 (aniline).
4. Recheck your calculation steps.
Review your calculations step by step to identify any potential errors, such as arithmetic mistakes, missing negative signs, or incorrect substitution of values.
I hope these suggestions help you identify the mistake in your calculation and find the correct stoichiometric concentration of aniline (C6H5NH2) to obtain an aqueous solution with a pH of 9.05.