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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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

38 Responses to O Level Chemistry – Basicity of Acids

  1. I’m a little bit confused on the meaning “REPLACEABLE HYDROGEN ATOM”, cause in the case of ethanoic acid, CH3COOH there are four hydrogen atoms per molecule, but it is a monobasic acid. how can I identify it?

  2. sir,
    I think in Sulphuric acid Hydrogen atoms are replaced as there is no chance for direct dissociation like HCl.
    Is this right?
    need confirmation…….

  3. 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.

  4. 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.

    Cheers!
    Sean Chua 🙂

  5. 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

  6. 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.

  7. 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.

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

  9. 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

  10. 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. 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 ?

  12. 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.

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

  14. 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!

  15. 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”

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