[Khyati]

1)All strong acids in the aqueous solution appear almost equally strong ,i.e, they ionise completely and their relative strength cannot be compared. This phenomenon is known as levelling effect. What's the cause of this ?

2)Flourine is the most electronegative element in the periodic table, still HF is considered to be the strong acid and it has no basic properties at all ( I read this in one book). But I have a doubt since it is electronegative shouldnt it have the tendency of attracting the proton towards itself. And why is it so strong that even the strongest acid such as nitric acid also behave like base when treated with HF ?

[Thujone]

1)All strong acids in the aqueous solution appear almost equally strong ,i.e, they ionise completely and their relative strength cannot be compared. This phenomenon is known as levelling effect. What's the cause of this ?

2)Flourine is the most electronegative element in the periodic table, still HF is considered to be the strong acid and it has no basic properties at all ( I read this in one book). But I have a doubt since it is electronegative shouldnt it have the tendency of attracting the proton towards itself. And why is it so strong that even the strongest acid such as nitric acid also behave like base when treated with HF ?

  Are you sure HF is a strong acid?

[Borek]

1)All strong acids in the aqueous solution appear almost equally strong ,i.e, they ionise completely and their relative strength cannot be compared. This phenomenon is known as levelling effect. What's the cause of this ?

Think about solvent properties.

[aeacfm]

2)Flourine is the most electronegative element in the periodic table, still HF is considered to be the strong acid and it has no basic properties at all ( I read this in one book). 

 i read in a book (1000 solved problems - inorganic- ) that perchloric acid is the strongest acid ever and writes like that HClO4 > H3O+ > other acids

also HI >HBr>HCl>HF in acidity due to Hudrogen - halogen bond length , strength

i dont know may be iam wrong

[Khyati]

I didn't said that HF is the strongest acid.

I agree that perchloric acid is the strongest acid, but I compared HF with nitric acid and in the book it was written that when HF is reacted with nitric acid nitric acid accepts the H⁺ ion donated by HF. This implies that HF acts as a proton donor here and is stronger than nitric acid. Even I was amazed after reading it because I think nitric acid is stronger than HF and it should act as a proton donor when reacted with HF

I think its written wrong there and I have read that paragraph 10 times just to that I am not missing something, but it was written the same there that I have quoted in my question.

[Khyati]

1)All strong acids in the aqueous solution appear almost equally strong ,i.e, they ionise completely and their relative strength cannot be compared. This phenomenon is known as levelling effect. What's the cause of this ?

2)Flourine is the most electronegative element in the periodic table, still HF is considered to be the strong acid and it has no basic properties at all ( I read this in one book). But I have a doubt since it is electronegative shouldnt it have the tendency of attracting the proton towards itself. And why is it so strong that even the strongest acid such as nitric acid also behave like base when treated with HF ?

  Are you sure HF is a strong acid?

I think that HF is not a strong acid, that is why I had written since it is electronegative shouldnt it have the tendency of attracting the proton towards itself it implies I think its a base fairly strong one.

[aeacfm]

see pKa values in this table

http://evans.harvard.edu/pdf/evans_pKa_table.pdf

The larger the value of pKa, the smaller the extent of dissociation. A weak acid has a pKa value in the approximate range −2 to 12 in water. Acids with a pKa value of less than about −2 are said to be strong acids; a strong acid is almost completely dissociated in aqueous solution, to the extent that the concentration of the undissociated acid becomes undetectable.

[aeacfm]

see the top of page 2 of this link

http://chemweb.calpoly.edu/dgragson/Teaching/chem129/129Workbooks/Ch18book_ans.pdf
it says that F^- is stronger base than NO₃^- so HNO₃ gives H⁺ to F^- and not the reverse .

[Khyati]

see the top of page 2 of this link

http://chemweb.calpoly.edu/dgragson/Teaching/chem129/129Workbooks/Ch18book_ans.pdf
it says that F^- is stronger base than NO₃^- so HNO₃ gives H⁺ to F^- and not the reverse .

Thanks a lot for helping me, now my doubt is completely solved.

[Borek]

A weak acid has a pKa value in the approximate range −2 to 12 in water. Acids with a pKa value of less than about −2 are said to be strong acids;

I wouldn't call acids with pKa between -2 and 1 weak ones (you have just classified nitric acid as a weak one). But this is kind of a gray area, there is no one, universal definition of what weak/strong acid is.

[opti384]

I wouldn't call acids with pKa between -2 and 1 weak ones (you have just classified nitric acid as a weak one). But this is kind of a gray area, there is no one, universal definition of what weak/strong acid is.

Agreed. There is no absolute definition stating which is a weak acid or a strong acid. Rather, being weak or strong is relative. It's same with solubility--even though a particular is insoluble, it does not mean that nothing dissolved, but it means relatively small amounts are dissolved.

[Khyati]

Yeah even I agree that there is no universal definition for what is strong acid and what is weak acid.

The strength of acid and bases depends on the roles of the solvent used. The greater  the tendency of the solvent to accept proton, the more will be the strength of the acid giving proton.

For example, acids like HCl, H₂SO₄ etc dissociate completely in water (solvent) as water accepts all their protons. Thus these acids are said to have equal acid strength (leveling effect). But the same acid when take in acetic acid(solvent), dissociate partially and act as weak acids.

Again acetic acid in water is a weak acid, whereas in the solvent ammonia it is a strong acid.