Example pKa and pH Problem
Find [H⁺] for a solution of 0.225 M NaNO2 and 1.0 M HNO2. The Ka value (from a table) of HNO2 is 5.6 e-4. (In table, It is 7.4e-4)
pKa = −log Ka = −log(7.4e−4) = 3.14
pH = pka + log ([A⁻]/[HA])
pH = pKa + log([NO2-]/[HNO2])
pH = 3.14 + log(1/0.225)
pH = 3.14 + 0.648 = 3.788
\([H^+] = 10^{-pH}= 10^{−3.788} = 1.6×10^{−4}\)
Are the above computation correct?https://www.thoughtco.com/the-ph-and-pk ... ng%20acid.
Example of pKa and pH problem.
Moderators: Xen, expert, ChenBeier
- Dhamnekar Winod
- Distinguished Member
- Posts: 257
- Joined: Sat Nov 21, 2020 10:14 am
- Location: Mumbai[Bombay] and Mumbai Suburb,, Maharashtra State,India
Example of pKa and pH problem.
Any science consists of the following process. 1) See 2)Hear 3)Smell,if needed 4)Taste, if needed 5)Think 6)Understand 7)Inference 8)Take decision [Believe or disbelieve, useful or useless, true or false, cause or effect, any other criteria]
- ChenBeier
- Distinguished Member
- Posts: 1563
- Joined: Wed Sep 27, 2017 7:25 am
- Location: Berlin, Germany
Re: Example of pKa and pH problem.
Should be ok
- Dhamnekar Winod
- Distinguished Member
- Posts: 257
- Joined: Sat Nov 21, 2020 10:14 am
- Location: Mumbai[Bombay] and Mumbai Suburb,, Maharashtra State,India
Re: Example of pKa and pH problem.
In the question, the concentration of NaNO2 is 0.225 M and that of HNO2 is 1.0 M.
But in the handerson-Hasselbalch equation, concentration of HNO2 is 0.225 and of NO2⁻ , it is 1.0 M. How is that?
I think the reaction is NaNO2 + HNO2 → NaOH + NO + NO2
But in the handerson-Hasselbalch equation, concentration of HNO2 is 0.225 and of NO2⁻ , it is 1.0 M. How is that?
I think the reaction is NaNO2 + HNO2 → NaOH + NO + NO2
Any science consists of the following process. 1) See 2)Hear 3)Smell,if needed 4)Taste, if needed 5)Think 6)Understand 7)Inference 8)Take decision [Believe or disbelieve, useful or useless, true or false, cause or effect, any other criteria]
- ChenBeier
- Distinguished Member
- Posts: 1563
- Joined: Wed Sep 27, 2017 7:25 am
- Location: Berlin, Germany
Re: Example of pKa and pH problem.
No HNO2 is the acid and NaNO2 is a salt. There is no reaction together. You can use the acid and add some NaOH to neutralize to get the salt.
- Dhamnekar Winod
- Distinguished Member
- Posts: 257
- Joined: Sat Nov 21, 2020 10:14 am
- Location: Mumbai[Bombay] and Mumbai Suburb,, Maharashtra State,India
Re: Example of pKa and pH problem.
I don't understand how does Log(NO2⁻ / HNO2) = Log(1/0.225 )?
Henderson- Hasselbalch equation states pH= pKa + log([conjugate Base ]/[ weak acid])
In this case, NO2⁻ ia a conjugate base and its concentration is unknown. How to find out that? The concentration of HNO2 is 1.0 M. But in ths solution it is 0.225 M. How is that?
Henderson- Hasselbalch equation states pH= pKa + log([conjugate Base ]/[ weak acid])
In this case, NO2⁻ ia a conjugate base and its concentration is unknown. How to find out that? The concentration of HNO2 is 1.0 M. But in ths solution it is 0.225 M. How is that?
Any science consists of the following process. 1) See 2)Hear 3)Smell,if needed 4)Taste, if needed 5)Think 6)Understand 7)Inference 8)Take decision [Believe or disbelieve, useful or useless, true or false, cause or effect, any other criteria]
- ChenBeier
- Distinguished Member
- Posts: 1563
- Joined: Wed Sep 27, 2017 7:25 am
- Location: Berlin, Germany
Re: Example of pKa and pH problem.
Something has to be given pH either a concentration. Otherwise it can not work. You have a acid given 1 M and some salt given 0.225 M then you can calculate the pH.
Or pH is given and concentration of the salt or acid can be calculated.
But I see you mixed concentrations up in the beginning.
Or pH is given and concentration of the salt or acid can be calculated.
But I see you mixed concentrations up in the beginning.
- Dhamnekar Winod
- Distinguished Member
- Posts: 257
- Joined: Sat Nov 21, 2020 10:14 am
- Location: Mumbai[Bombay] and Mumbai Suburb,, Maharashtra State,India
Re: Example of pKa and pH problem.
Whatever is given on https://www.thoughtco.com/the-ph-and-pk ... ng%20acid. , I reproduced here.
Consider the reaction HNO2 + NaOH → NaNO2 + H2O , Now we have 1.0 M of HNO2 and 0.225 M of NO2⁻.
So, pH= 3.13 + log (0.225/1) = 3.13 -0.648 = 2.48 . → [H⁺] = 10^-2.48 =3.31e-3.
Why is this computation wrong?
Consider the reaction HNO2 + NaOH → NaNO2 + H2O , Now we have 1.0 M of HNO2 and 0.225 M of NO2⁻.
So, pH= 3.13 + log (0.225/1) = 3.13 -0.648 = 2.48 . → [H⁺] = 10^-2.48 =3.31e-3.
Why is this computation wrong?
Any science consists of the following process. 1) See 2)Hear 3)Smell,if needed 4)Taste, if needed 5)Think 6)Understand 7)Inference 8)Take decision [Believe or disbelieve, useful or useless, true or false, cause or effect, any other criteria]
- ChenBeier
- Distinguished Member
- Posts: 1563
- Joined: Wed Sep 27, 2017 7:25 am
- Location: Berlin, Germany
Re: Example of pKa and pH problem.
This is correct but in the beginning you mixed up the concentrations.