Ch. 3 - Acids and BasesWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins
Johnny Betancourt

A conjugate acid is the protonated version of a molecule. In other words, it has an extra hydrogen.

Give the conjugate acid for each compound below:

In this post, we're going to cover what a conjugate acid is and then apply that to a very common kind of problem that you might find on your exam.

Give the conjugate acid for each compound belowGive the conjugate acid for each compound below

When talking about conjugate acids and conjugate bases, we’re usually referring to Bronsted-Lowry acids and bases. Those are proton donors and acceptors, respectively.

Conjugate Acid of Hydrogen Sulfate Ion: 

So, that means that if we want to find a conjugate acid, all we need to do is add a proton! Let’s try that out first with  (hydrogen sulfate ion) and then move on to the other two. 

HSO4– Hydrogen sulfate structureHSO4– Hydrogen sulfate structure

Check it out! I’ve drawn this molecule in the three main ways you’ll see it drawn in Orgo. If you need a review of drawing Lewis structures, check out these videos. Notice that on the left is the molecule provided to us and on the right is its conjugate acid: sulfuric acid. All I did is add a hydrogen. Notice that we actually no longer have a negative charge; that’s a consequence of one of the anionic oxygen’s lone pairs being used to form a bond to the hydrogen. Let’s take a quick look at the formal charge of the oxygen on the right. 

HSO4- Hydrogen sulfate Lewis structureHSO4- Hydrogen sulfate Lewis structure

This equation, Group # – (sticks + dots) = Formal Charge, is very helpful! Sticks = bonds, and dots = lone-pair electrons. Notice how adding the hydrogen to the molecule sets the formal charge on the oxygen to zero. This leads us to a major lesson: If you protonate a molecule, it’ll become more positive; if you deprotonate a molecule, it’ll become less positive (or more negative).

Basically, all you have to do is add a hydrogen!

Conjugate Acid of Carbonate Ion: 

So now let’s take a look at CO32– (carbonate). Since all we’re doing is adding a hydrogen, we could just write it out as CO3H. Let’s draw that out in Lewis structure. 

CO32–-Carbonate-Lewis-structureCO32– Carbonate Lewis structure

Conjugate Acid of Ammonia:

Just like the hydrogen sulfate ion above, sulfate becomes less negative when a hydrogen is added to it. This isn’t so bad, right? Let’s go ahead and tackle the last one: NH(ammonia). Even though it’s neutral, it can absolutely have another bond to hydrogen. How? It’s got a lone pair that it can use to deprotonate another molecule. Let’s take a look at those Lewis structures. 

NH3 Ammonia Lewis structureNHAmmonia Lewis structure

More Examples:

So, what about other molecules? Let’s take on four others:

  • H2O
  • HPO42–
  • S2-
  • CH3COO

H2O Water Lewis structure H2O Water Lewis structure 

HPO42–-Hydrogen-phosphate-Lewis-structureHPO42– Hydrogen phosphate Lewis structure

S2– Sulfide Lewis structure

S2– Sulfide Lewis structure

CH3COO–-Acetate-Lewis-structureCH3COO– Acetate Lewis structure

So now you know how to give the conjugate acid for each compound! I have awesome videos explaining acid-base reaction mechanisms and how to find the direction of equilibrium that I recommend you check out. Good luck!

Johnny Betancourt

Johnny got his start tutoring Organic in 2006 when he was a Teaching Assistant. He graduated in Chemistry from FIU and finished up his UF Doctor of Pharmacy last year. He now enjoys helping thousands of students crush mechanisms, while moonlighting as a clinical pharmacist on weekends.