Titration of mixture of KOH and K2CO3

Chemistry and homework help forum.

Organic Chemistry, Analytical Chemistry, Biochemistry, Physical Chemistry, Computational Chemistry, Theoretical Chemistry, High School Chemistry, Colledge Chemistry and University Chemistry Forum.

Share your chemistry ideas, discuss chemical problems, ask for help with scientific chemistry questions, inspire others by your chemistry vision!

Please feel free to start a scientific chemistry discussion here!

Discuss chemistry homework problems with experts!

Ask for help with chemical questions and help others with your chemistry knowledge!

Moderators: Xen, expert, ChenBeier

Post Reply
User avatar
Neon
Sr. Member
Sr. Member
Posts: 26
Joined: Fri Jun 15, 2018 12:11 pm

Titration of mixture of KOH and K2CO3

Post by Neon »

Weigh 0.7341 g of technical KOH, which also contains K2CO3, quantitatively transfer it to a 250 mL volumetric flask, dissolve and make up to the mark. Prepare two Erlenmeyer flasks, pipette 50 mL of the prepared solution into each, add the indicator phenolphthalein to the first, and methyl orange to the second and titrate each with HCl solution. With the indicator phenolphthalein, the titrant consumption is 26.1 mL, and with methyl orange 33.7 mL. The concentration of the Determine the HCl by weighing out 0.1250 g of the KIO3 primary standard, adding KI, Na2S2O3 and the appropriate indicator, and titrating with the sol. HCl to overflow, consumption is 35.7 mL. Calculate the mass percentage of potassium in the sample.

Please help me with this assignment, I always get too high a score (over 100%).
Thank you!
User avatar
ChenBeier
Distinguished Member
Distinguished Member
Posts: 1500
Joined: Wed Sep 27, 2017 7:25 am
Location: Berlin, Germany

Re: Titration of mixture of KOH and K2CO3

Post by ChenBeier »

First get concentration of HCl

Develop the redoxreaction iodate iodine and the consumed H+.

IO3- + 5 I- + 6 HCl => 3 I2 + 3 H2O + 6 Cl-


With the two first titration you get K2CO3 pH 8.2 and the mixture with KOH pH 4.5 , the difference is KOH

From that calculate mass of K.
User avatar
Neon
Sr. Member
Sr. Member
Posts: 26
Joined: Fri Jun 15, 2018 12:11 pm

Re: Titration of mixture of KOH and K2CO3

Post by Neon »

That's how I calculated it:
c(HCl) = (6*0.1250)/(0.0357*214)= 0.09817 M
n(K2CO3) = 0.0261*0.09817 = 0.002562 mol
V(HCl for KOH) = 33.7 - 26.1 = 7.6 mL
n(KOH) = 0.0076 * 0.09817 = 0.000746 mol
m(K2CO3) = 0.002562 mol *138.21 g/mol * 5 = 1.7705 g

1.7705 g is much more than 0.7341 g. So where did I go wrong?
User avatar
ChenBeier
Distinguished Member
Distinguished Member
Posts: 1500
Joined: Wed Sep 27, 2017 7:25 am
Location: Berlin, Germany

Re: Titration of mixture of KOH and K2CO3

Post by ChenBeier »

It was asked for potassium not for potassium carbonate or hydroxide.
The moles for K2CO3 are wrong

K2CO3 + 2 HCl => 2 KCl + H2O + CO2

For 1 K2CO3 2 HCl needed or 1 HCl correspond to 1/2 K2CO3

n (K2CO3) =0,0261 *0,09817/2 = 0,001281 mol
User avatar
Neon
Sr. Member
Sr. Member
Posts: 26
Joined: Fri Jun 15, 2018 12:11 pm

Re: Titration of mixture of KOH and K2CO3

Post by Neon »

But still, if we consider the aliquot, the mass is too high.
User avatar
ChenBeier
Distinguished Member
Distinguished Member
Posts: 1500
Joined: Wed Sep 27, 2017 7:25 am
Location: Berlin, Germany

Re: Titration of mixture of KOH and K2CO3

Post by ChenBeier »

Yes something doesn't fit.
The titration values are wrong or the method to get molarity of HCl.
User avatar
ChenBeier
Distinguished Member
Distinguished Member
Posts: 1500
Joined: Wed Sep 27, 2017 7:25 am
Location: Berlin, Germany

Re: Titration of mixture of KOH and K2CO3

Post by ChenBeier »

Values for KOH and K2CO3 have to be exchangend.
See here an example with sodiumhydroxide and Carbonate

https://www.linkedin.com/pulse/analytic ... oda-kartal
User avatar
ChenBeier
Distinguished Member
Distinguished Member
Posts: 1500
Joined: Wed Sep 27, 2017 7:25 am
Location: Berlin, Germany

Re: Titration of mixture of KOH and K2CO3

Post by ChenBeier »

The solution

p Value is sum of KOH and 1/2 K2CO3 or KHCO3 ( ml HCl consumed Phenolphthalein)
m value is sum of KOH and K2CO3 ( ml HCl consumed Methylorange)

n (KOH) = 2 p - m = 2*(KOH + 1/2 K2CO3) - (KOH + K2CO3) = KOH
n ( K2CO3) = 2*(m-p) = 2*(KOH + K2CO3 - (KOH + 1/2 K2CO3)) = K2CO3

n KOH = 2 * 26,1 - 33,7 = 18,5 ml this times molarity HCl

0,0185 l * 0,09817 mol/l = 0,001816 mol KOH
times molar mass 56 g/mol = 0,101 g
Times 5 = 0,508 g in 250 ml

n(K2CO3) = 2 * (33,7 - 26,1) = 15,2 ml times molarity HCl
0,0152 l * 0,09817 mol/l = 0,00149 mol K2CO3
times molar mass 138 g/mol = 0,205 g
times 5 is 1,025 g in 250 ml

What means in sum its 1,533 g. Its almost double what was dissolved
(0.7341 g).

So one number is not correct.
DanielBot
Newbie
Newbie
Posts: 1
Joined: Mon Feb 05, 2024 6:50 pm
Location: Spain

Re: Titration of mixture of K….

Post by DanielBot »

Can we use alternative titration methods to determine the concentration of potassium in a mixture, such as using an unconventional indicator or a non-aqueous solvent, to enhance accuracy and precision?
User avatar
ChenBeier
Distinguished Member
Distinguished Member
Posts: 1500
Joined: Wed Sep 27, 2017 7:25 am
Location: Berlin, Germany

Re: Titration of mixture of KOH and K2CO3

Post by ChenBeier »

In this case convert all to KCl. pH 7.

Probably the determination of molarity of HCl is also wrong. If it was only the half instead 0.0098 M only 0.0049 M then it would more or less fit.
GeorgeNut
Newbie
Newbie
Posts: 1
Joined: Mon Feb 05, 2024 6:44 am
Location: Russia

Re: Titration of mixture of K...

Post by GeorgeNut »

Can we use titration to determine the concentration of a mixture of potassium (K) isotopes, and if so, what challenges would we face during the process? How would the presence of multiple isotopes affect the accuracy of our results in this unique scenario?
User avatar
ChenBeier
Distinguished Member
Distinguished Member
Posts: 1500
Joined: Wed Sep 27, 2017 7:25 am
Location: Berlin, Germany

Re: Titration of mixture of KOH and K2CO3

Post by ChenBeier »

You cannot determine different Isotops of one element by titration.
If you check for potassium then you will get all Isotopes together. The Isotopes are different by the amount of neutrons in the atom, this is not possible to figure out by chemical analysis. The chemical behaviour is driven by the amount of the electrons in the shell. Potassium has only one in the outer shell, what can be spend to other atoms.
Post Reply