O Level Chemistry – Basicity of Acids


In the last post, we have covered Chemical Definitions of ACIDs. Today we are going to cover on the “Basicity of ACIDs” – this part is where lots of students have misconception in their schools.

Do note that “BASICITY of ACIDs” is associated with the acids itself, and not on BASES. Many students always argued that Basicity comes directly from the word BASE itself and should only be associated with BASE as “BASICITY of BASEs”. I want to highlight over here that this is incorrect! Best way to persuade you would be checking out the Chemical Definition of “BASICITY”.

Definition of Basicity of An Acid:
Basicity of an acid refers to the number of replaceable hydrogen atoms in one molecule of the acid

Lets look closer on the 3 common types of Basicity of an acid.

Monobasic
Definition: 1 molecule produce 1 H+ ion upon dissociation
Example: HCl, HNO3
Dissociation Equation: HCl(aq) –> H+(aq) + Cl-(aq)

Dibasic
Definition: 1 molecule produce 2 H+ ion upon dissociation
Example: H2SO4
Dissociation Equation: Figure it out yourself!!

Tribasic
Definition: 1 molecule produce 3 H+ ion upon dissociation
Example: H3PO4
Dissociation Equation: H3PO4(aq) –> 3H+(aq) + PO4 3-(aq)

Learnt something from the above already? Good! Now, let’s try out an exam-based questions that are associated with it.

Quick Check 1:
Sulphuric acid is a dibasic acid because:
A. each molecule contain 2 hydroxide ions
B. each molecule can produce 2 hydrogen ions
C. one mole of the acid contains two atoms of hydrogen that can be replaced by a metal
D. sulphuric acid can only be neutralized by 2 bases
E. sulphuric acid has the usual properties of an acid, but is also a dehydrating agent

PS: Do leave your suggested answers in the “Comments Section” directly below this post.

PPS: This post belongs to a series of blogposts that is associated with Secrets of “Acids, Bases & Salts and Qualitative Analysis revelaed”

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26 Responses to O Level Chemistry – Basicity of Acids
  1. Adithya
    September 10, 2008 | 9:22 pm

    answer is B

    Reply

  2. sean
    September 15, 2008 | 9:40 pm

    Well Done Adithya.

    Keep up that good work.

    Cheers
    Sean

    Reply

  3. Joe
    April 7, 2009 | 7:53 pm

    why isnt the answer C? True, the answer is B, but what is the difference between B and C?

    Reply

    sean Reply:

    Thanks Joe for the participation.

    The question is asking you about the definition of “Dibasic Acid” which is the answer for (B).

    You need to know this definition well for GCE O Levels and ICGSE syllabus.

    Hope this information clarify your doubts.

    Let me know if you need further clarification.

    To Your Academic Success in Chemistry.
    Sean Chua
    Master Trainer & Author
    WINNERS Education Group

    “Experience Learning with A Difference”

    Reply

  4. jane
    March 12, 2011 | 11:40 pm

    HI!
    i have a question.when determining the basicity of the acid,why we have to divide number of moles of a metal hydroxide by no of moles of the acid??i dont understand.hope you can help me .thank you!

    Reply

  5. anwesha
    May 2, 2011 | 12:37 pm

    sir how to find the basicity of an acid in order to find equivalent weight of acids ?

    Reply

    sean Reply:

    Hi Anwesha,

    Try this out: http://easycalculation.com/chemistry/weightofacid.php

    Hope this helps. Enjoy learning Chemistry.

    Reply

  6. reeta
    July 3, 2011 | 8:35 pm

    sir
    why is it called basicity of an acid not acidity?

    Reply

    sean Reply:

    Because it refers to how many moles of the acid that is required to react with the base.

    Reply

  7. Sarbah Emmanuel
    October 14, 2011 | 2:57 pm

    The ans is B

    Reply

  8. abdul
    October 15, 2011 | 4:55 pm

    sir how do we identify basicity of an acid in an equation. Thx

    Reply

  9. Jonsavvy
    March 15, 2012 | 3:36 pm

    The number of hydrogen atoms in the acid

    Reply

  10. Joseph
    August 22, 2012 | 12:56 am

    The answer is c

    Reply

    Muffin Reply:

    I don’t think it’s C, since it says one MOLE of acid (which means 6.02 X 10^23 particles) contains two replaceable Hydrogen ATOMS (should be ions, since ionization occurs. Metal cation replaces cation not atom). Hence I think it’s B.

    Reply

  11. O.S.D
    December 25, 2012 | 6:34 pm

    Why cnt its b C,I agree its B,but C also agree wit B…

    Reply

  12. olagunju jeremiah
    January 17, 2013 | 4:33 am

    i like dis

    Reply

    sean Reply:

    Glad you liked it!

    Keep learning! Feel free to share it with your friends also :)

    Sean

    Reply

  13. Irfan
    February 20, 2013 | 7:45 pm

    perfect answer is B because each molecule can produce 2 hydrogen ions.Option C is not correct because it says that two atoms of hydrogen are replaced by a metal.These are not replaced by metal but they are replaced by nonmetal.

    Reply

  14. Shobhitanshu
    March 1, 2013 | 12:53 am

    Why it is said that Hydrogen ions in an Acid can only be replace by only bases and not by metals especially in defining the basicity of acids ?

    Reply

  15. Satyabrata
    March 15, 2013 | 1:34 am

    It is right i know.Then what is the basicity of H3PO3 and H3PO2? Reply as fast as possible, if u have the answer.

    Reply

    kachu Reply:

    2 and 1 respectively

    Reply

  16. Ibrahim Dahir
    May 3, 2013 | 1:06 am

    The answer is 3, 3 respectively

    Reply

  17. olasco don
    May 7, 2013 | 12:53 am

    if the basicity number of H is 7 what do we cal it.help me pls

    Reply

  18. Muffin
    December 22, 2013 | 1:13 am

    What is the difference between “diprotic” and “dibasic”?

    Reply

    sean Reply:

    Hi Muffin,

    From my knowledge and research, there is no difference between dibasic acid and diprotic acid?
    They both mean the same thing which is acid molecules with 2 protons: e.g. H2SO4 and H2S for example.

    The two anions (SO42- and S2-) are both bases (this is covered under Bronsted Lowry Acids & Bases in GCE Advanced-Level i.e. A-Level and equivalent syllabus. GCE O-Level syllabus does not need to know the concepts).

    So both H2SO4 and H2S is dibasic (= forming two bases).

    Since two protons are released, so H2SO4 and H2S is diprotic (= forming two protons).

    It seems like “diprotic” is a more modern Chemistry term used for “dibasic” these days.

    Note:
    If you need to learn Bronsted Lowry Acids & Bases, you can go to my A-Level H2 Chemistry blog to learn Advanced Chemistry.
    Go to: http://www.ALevelH2Chemistry.com

    Cheers!
    Sean Chua

    Reply

  19. […] If you are new to the concepts on Acids, do check out my blog post on 22 August 2008 where i shared on the definition of “Basicity” with common […]

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