Confusion about acid strength (1 Viewer)

[lolmb]

could someone point out the flaw in this and correct me please bc im really confused.

ok so a strong acid (take HCl) is always strong regardless of it's molarity. so i can get 10mol/L HCl and call it strong and then get 0.0001 mol/L HCl and call it strong.

so when i talk about degree of ionisation = [H3O+]/[acid] x100% why does that depend on the molarity of the acid itself when i just said that acid strength does not relate to concentration of the solute species.

pls help thank you

 

[Librah]

could someone point out the flaw in this and correct me please bc im really confused.

ok so a strong acid (take HCl) is always strong regardless of it's molarity. so i can get 10mol/L HCl and call it strong and then get 0.0001 mol/L HCl and call it strong.

so when i talk about degree of ionisation = [H3O+]/[acid] x100% why does that depend on the molarity of the acid itself when i just said that acid strength does not relate to concentration of the solute species.

pls help thank you

When you say 10mol/L HCL, that refers the concentration of molecular (unionised) acid. If at a very high concentration, say 20 mol/L of HCL, you would say it is very concentrated, however barely any acid molecules have ionised and therefore the [H30+] is extremely low. Your equation only really refers to WEAK acids since [H3O+]/[acid]=1 (approximately) for strong acids like HCL since they ionise almost completely in a solution. You can explain your question for weak acids, by using simple equilibrium theory. HA+H20<--->h30+ +A-. Where HA is a weak acid (we'll just say it's monoprotic for simplifcity.) To sum it up, an acid at lower concentration (more diluted) will have a greater % ionisation but when more concentrated( less dilute) it will have a lower % ionisation.

 

[lolmb]

ok you say that 'If at a very high concentration, say 20 mol/L of HCL, you would say it is very concentrated, however it is barely ionised and therefore the [H30+] is extremely low.'

i thought since HCl is a strong acid, it always ionises completely?

 

[enigma_1]

ok you say that 'If at a very high concentration, say 20 mol/L of HCL, you would say it is very concentrated, however it is barely ionised and therefore the [H30+] is extremely low.'

i thought since HCl is a strong acid, it always ionises completely?

Oh wait yeah HCl ionises completely no matter how concentrated it is.

Degree of ionisation is 100% for strong acids like HCl.

 

[Librah]

ok you say that 'If at a very high concentration, say 20 mol/L of HCL, you would say it is very concentrated, however it is barely ionised and therefore the [H30+] is extremely low.'

i thought since HCl is a strong acid, it always ionises completely?

Concentration of "acid" is different from saying concentration of h30+. Molecular acid needs to first ionise to provide H30+ ions in a solution. H20+HX--->H30+ + X-, for ionisation of monoprotic acid. I edited/ reworded it a bit. "If at a very high concentration, say 20 mol/L of HCL, you would say it is very concentrated, however barely any acid molecules have ionised and therefore the [H30+] is extremely low"

Also for bolded part, i did mention this.

 

[lolmb]

wait. yunli in your example of 20mol/L HCl you say barely any acid molecules have ionised. Why is that if HCl is a strong acid which completely ionises in solution?

 

[Librah]

wait. yunli in your example of 20mol/L HCl you say barely any acid molecules have ionised. Why is that if HCl is a strong acid which completely ionises in solution?

Means the molecules of HCl have not yet donated a proton to form H30+ with water. What your kind of asking is, if i have 100000000 strong acid molecules, how come i can't match them up with 5 molecules of water?, it just won't fit.

Generally what you'll have in a solution with 20 mol/L strong acid, is only a few ionised acid molecules, and then a whole bunch of excess non-ionised acid molecules, as all the water molecules available have already been protonated to form H30+. When we say a strong acid completely ionises, it means it will "react" with all (almost all) of the water molecules if they are available. If all water molecules are "used up", the strong acid molecules will just be in excess and floating around in the solution.

Note: This explanation used Bronsted Lowry Theory.

 

[lolmb]

ahh i see.

thanks for all of your help! greatly appreciated