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.

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

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

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”

Related Articles:

Many people talked about teaching Chemistry, I simply LOVE it. I am a passionate Chemistry Coach based in Singapore, Southeast-Asia and aspire to be one of the most dynamic, powerful and humorous speakers in Asia. My 16+ years of coaching experience has equipped me to understand the true reasons why students are not able to perform well in Chemistry, and allow me to structure my teaching methodology to cater to different levels of learners. If you have found this post useful, please share it with your friends. I would really appreciate it! Sean Chua

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

    answer is B


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

    Well Done Adithya.

    Keep up that good work.



  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?


    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”


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

    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!


  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 ?


    sean Reply:

    Hi Anwesha,

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

    Hope this helps. Enjoy learning Chemistry.


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

    why is it called basicity of an acid not acidity?


    sean Reply:

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


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

    The ans is B


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

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


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

    The number of hydrogen atoms in the acid


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

    The answer is c


    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.


  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…


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

    i like dis


    sean Reply:

    Glad you liked it!

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



  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.


  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 ?


  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.


    kachu Reply:

    2 and 1 respectively


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

    The answer is 3, 3 respectively


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

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


    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.

    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

    Sean Chua


  18. […] 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 […]

  19. andy otumala
    May 16, 2014 | 12:58 am

    i chose option B


  20. Kaes
    October 11, 2014 | 8:23 pm

    How do u actually determine the basicity of an acid. I’m a little bit confused about the basicity of H3PO3 n H3PO2..


    Kunal Chand Reply:

    Keep in mind that
    (i)Oxoacids have at least one P==O bond and at least one P–OH bond.
    (ii)Further Oxoacids of Phosphorus in which Oxidation State of Phosphorus is less than +5, they have, in addition to the above mentioned bonds, either P–H or P–P bonds (only one of the two types)
    (iii) As per definition given above by Mr Sean Chua, Hydrogen atom which is replaceable is responsible for basicity of any acid. Remember that H atom attached to P is non replaceable (non-ionisable) due to equal electro-negativity of both the elements.
    Now, Draw the structure of H3PO3 and H3PO2.
    (iv) Since there is only one Phosphorus atom in both the compounds having O.S. of Phosphorus < (+5), both the compounds will have P–H bonds in addition to P==O and P–OH bonds
    (v) H3PO3 will have two P–OH bonds whereas H3PO2 will have only one P–OH bonds.
    (vi) Thus basicity of
    H3PO3 will be 2 and of H3PO2 will be 1.
    (vii) Hope it helps.


    Sean Chua Reply:

    Hi Kunal Chand,

    Good to see your comments here helping out with answering some of the questions posted by other readers.

    Well Done! :)

    Are you a Chemistry teacher or student?

    Sean Chua


    Kunal Chand Reply:

    Thanks for appreciation. I am XII Standard Student from Delhi / India. I am aspiring for IIT. Can I post direct questions to you Sir so that I can get an early clarifications. Thanks once again.

    Sean Chua Reply:

    Hi Kaes,

    Kunal Chand has showed you the way to determine the basicity of an acid.

    Perhaps, you can also look at the structures of H3PO2 and H3PO3 to have a clearer idea.



    Hope you find them useful!

    Enjoy learning Chemistry :)

    Sean Chua


  21. Kunal Chand
    January 7, 2015 | 3:28 pm

    Answer is “B”. I was reading in my school book and think there is something wrong printed over there. It was written in the book that “Basicity of hydrides of Group 15 elements
    decreases in the order NH3 > PH3 > AsH3 > SbH3 > BiH3″.
    I feel that it should have been “Basic Character” in place of “Basicity” as due acidic character / reducing power / basicity will increase down the group due to decrease in bond dissociation enthalpy down the group.


    Sean Chua Reply:

    Hi Kunal Chand.

    Regarding this question i posted, answer is indeed “B” which satisfies the definition of “Basicity of Acids”.

    And i kind of agree with you that “Basicity” is not a good/correct term to discuss about Alkalis/Bases. I don’t like the terms used in many of the textbooks found in Singapore also BUT i guess whoever came out with this concept wanted to introduce less Chemistry terms for students to study. So they used “Basicity” quite generally for both Acids as well as Alkalis/Bases.

    I reckon this is the case in your school book also. Terminologies aside, i think if you are clear about the Chemistry Concepts, then it will be ok for you to do well and enjoy learning Chemistry.

    Sean Chua :)


    Kunal Chand Reply:

    Thanks for the clarification.


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