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Topic: Determine pH (Read 1710 times)
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OkPluto
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Determine pH
«
on:
May 31, 2020, 06:37:39 AM »
I'm having trouble with this 2 problems:
Find the pH of the solution obtained by mixing 20 mL of NaOH 0.05 M and 10 mL of HCN 0.1 M
(pKa HCN = 8.96)
Solution is 10.74
I found that there are 1*10^-3 mol of both NaOH and HCN in solution and consequently there are 1*10^-3 mol of NaCN
so i calculate pH = pKa + log_10(1/1) = pkA = 8.96 that is a different from 10.74
Find the pH of the solution obtained by mixing 30 mL of 0.05 M NaOH and 10 mL of 0.1 M HCN
(pKaHCN = 8.96)
Solution is 12.10
I found that there are 1*10^-3 mol NaOH and 1.5*10^-3 mol of HCN in solution and consequently there are 5*10^-4 mol of NaCN
so i calculate pH = pKa + log_10(1.5/0.5) = pkA + log_10(3) = 9.43 that is different from 12.10
Can anyone help me understand where am i wrong?
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Borek
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I am known to be occasionally wrong.
Re: Determine pH
«
Reply #1 on:
May 31, 2020, 07:40:50 AM »
You are making the same mistake in both problems. Henderson-Hasselbalch equation uses concentrations of acid and its conjugate base. What are acid and its conjugate base in this case?
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ChemBuddy
chemical calculators - stoichiometry, pH, concentration, buffer preparation,
titrations.info
OkPluto
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Re: Determine pH
«
Reply #2 on:
May 31, 2020, 08:06:52 AM »
the acid is HCN and its conjugate base is NaCN. I already understood that but i don't know how to solve the problem yet
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AWK
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Re: Determine pH
«
Reply #3 on:
May 31, 2020, 08:26:58 AM »
1 - hydrolysis of the salt
2 - wrong stoichiometry.
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AWK
OkPluto
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Re: Determine pH
«
Reply #4 on:
May 31, 2020, 08:34:31 AM »
Ok, but these comments are not helpful...
There is something that i can't do, underline only where am i wrong will not make me progress in this problem..
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AWK
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Re: Determine pH
«
Reply #5 on:
May 31, 2020, 08:37:33 AM »
Hydrolysis of salt uses other equation. Calculate salt concentration.
Correct stoichiometry will show you the correct way of calculation
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AWK
OkPluto
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Re: Determine pH
«
Reply #6 on:
May 31, 2020, 08:55:34 AM »
Which is that equation? That was the only i saw during lessons
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AWK
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Re: Determine pH
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Reply #7 on:
May 31, 2020, 09:04:03 AM »
https://study.com/academy/answer/calculate-the-oh-concentration-and-ph-of-a-3-8-10-3m-aqueous-solution-of-sodium-cyanide-nacn-finally-calculate-the-cn-concentration-ka-hcn-4-9-10-10-oh-ph-cn.html
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AWK
OkPluto
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Re: Determine pH
«
Reply #8 on:
May 31, 2020, 09:27:45 AM »
I did the first one:
[HCN] = [NaOH] = [NaCN]= 0.033 M
pH = 1/2(14 + 8.96 + log_10(0.033)) = 10.74
But the second one is still wrong
[HCN] = 0.025 M
[NaOH] = 0.0375 M
[HCN] = 0.025 M
pH = 1/2(14 + 8.96 + log_10(0.025)) = 10.68
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AWK
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Re: Determine pH
«
Reply #9 on:
May 31, 2020, 09:32:45 AM »
You have an excess of NaOH. What does it mean?
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AWK
OkPluto
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Re: Determine pH
«
Reply #10 on:
May 31, 2020, 09:37:51 AM »
I think that this excess will make the solution more alkaline, but how can i determine how much?
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AWK
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Re: Determine pH
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Reply #11 on:
May 31, 2020, 09:49:16 AM »
In the presence of a strongman, weaklings do not count.
Treat the solution as pure NaOH with an excess concentration.
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AWK
OkPluto
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Re: Determine pH
«
Reply #12 on:
May 31, 2020, 09:57:42 AM »
Ok i understood.
pH = 14 - (-log_10(0.0125)) = 12.1
One last question:
Where this formula came from?
pH = 1/2(Kw + Ka + log_10(Cs))
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AWK
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Re: Determine pH
«
Reply #13 on:
May 31, 2020, 10:02:59 AM »
pH=-log[H
3
O
+
]=-log(SQRT(K
a
·K
w
/c))
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AWK
OkPluto
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Re: Determine pH
«
Reply #14 on:
May 31, 2020, 12:47:14 PM »
The first exercise that i published is not correct, the solution is 10.74, i read the result wrong. Where is the error?
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