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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: OkPluto on May 31, 2020, 06:37:39 AM

Title: Determine pH
Post by: OkPluto 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?

Title: Re: Determine pH
Post by: Borek 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?

Title: Re: Determine pH
Post by: OkPluto 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

Title: Re: Determine pH
Post by: AWK on May 31, 2020, 08:26:58 AM

1 - hydrolysis of the salt
2 - wrong stoichiometry.

Title: Re: Determine pH
Post by: OkPluto 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..

Title: Re: Determine pH
Post by: AWK 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

Title: Re: Determine pH
Post by: OkPluto on May 31, 2020, 08:55:34 AM

Which is that equation? That was the only i saw during lessons

Title: Re: Determine pH
Post by: AWK 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

Title: Re: Determine pH
Post by: OkPluto 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

Title: Re: Determine pH
Post by: AWK on May 31, 2020, 09:32:45 AM

You have an excess of NaOH. What does it mean?

Title: Re: Determine pH
Post by: OkPluto 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?

Title: Re: Determine pH
Post by: AWK 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.

Title: Re: Determine pH
Post by: OkPluto 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))

Title: Re: Determine pH
Post by: AWK on May 31, 2020, 10:02:59 AM

pH=-log[H₃O⁺]=-log(SQRT(K_a·K_w/c))

Title: Re: Determine pH
Post by: OkPluto 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?

Title: Re: Determine pH
Post by: AWK on May 31, 2020, 01:01:27 PM

The correct answer is 10.74.