$3g$ of activated charcial was added to $50 ml$ of acetic acid solution ($0.06N$) and filtred after an hour and it was found to be $0.042 N$ .The amount of acetic acid adsorbed (per gram of charcoal) is

The solution for this question is exactly given as I have written below

No of equivalents before adsorption $=50\times0.06 =3$

No of equivalents after adsorption $=50 \times 0.042=2.1$

No of equivalents left $=0.9/1000 \times 60=54mg$
Acetic acid per gm $= 54mg/3=18mg$

Now I have some doubts regarding the question and it's answer

  1. Why is normality being decreased here?
  2. Why are they taking volume to be same when it generally increases when we add solutes?
  3. What does equivalent mean here? According to Wikipedia, it is said to be "the mass of one equivalent, that is the mass of a given substance which will 1) combine or displace directly or indirectly with 1.008 parts by mass of hydrogen or 8 parts by mass of oxygen or 35.5 parts by mass of chlorine – these values correspond to the atomic weight divided by the usual valence; or 2) supply or react with one mole of hydrogen cations (H+) in an acid–base reaction or 3) supply or react with one mole of electrons (e−) in a redox reaction." The equivalent talked in the question doesn't meet any of this criteria so what is its role here. Can somebody please explain me this?

Number of equivalents and normality are out-of-date concepts in chemistry. For acids, they are just like number of mols and molar concentration (or molarity), with an extra factor indicating how many hydrogens can be effectively ionized in solution. Because acetic acid ($\ce{CH3COOH}$) is monoprotic, you can just exchange the terms by number of mols and molarity and you are good.

Why is normality being decreased here?

Some of the acetic acid adsorbs - meaning it bonds - in the charcoal. So the concentration in solution is decreased. Pay attention to the fact that the charcoal remains solid and does not dissolve.

Why are they taking volume to be same when it generally increases when we add solutes?

As I wrote above, the charcoal stays solid, so it has no effect on the solution volume. The loss of acetid acid in solution might have a slight effect on volume, but that is mostly negligible.

  • $\begingroup$ So as long as charcoal is solid and not dissolved in the solution,the volume will remain same? $\endgroup$
    – Scáthach
    Feb 12 '18 at 6:04
  • $\begingroup$ You should question your own intuition. What happens to the volume of a liquid when you add a solid object into it? $\endgroup$ Feb 12 '18 at 10:36
  • $\begingroup$ If it is floating then no change and if it sinks then the solution will rise but it will still remain same in my opinion $\endgroup$
    – Scáthach
    Feb 12 '18 at 11:11
  • $\begingroup$ The liquid level will rise but the volume will remain the same as you say (like if you add the same volume in a narrower container) so no problem here. The more interesting question would be: what if the solid can dissove in the solution or if it is a liquid that is miscible with your solution, then what happens with the volume? How would you calculate concentration and prepare the solution in practice? $\endgroup$
    – AMM
    Feb 12 '18 at 13:47
  • $\begingroup$ if the solid is dissolvable then some of bonds of the liquid will be replace by the bonds with the solid so volume will change at that case $\endgroup$
    – Scáthach
    Feb 13 '18 at 2:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.