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Please read edit before answering

Background

I am currently studying in 10th grade. This question appeared in my science exam -

Which acid does not form an acidic salt?

  • Phosphoric acid
  • Carbonic acid
  • Hydrochloric acid
  • Sulphuric acid

I understood that all acids other than carbonic acid are strong, and the question is expecting me to answer carbonic acid, as at our level we are only taught

SA + SB -> Neutral salt

WA + SB -> Basic salt

SA + WB -> Acidic salt

My Efforts

Because there was no mention of WA + WB reaction in my book, while this was being taught in class out of curiosity I searched online and found out that to find the nature of salt of WA and WB, it is required to compare their dissociation constants (correct me if I am wrong here)

I felt that there must be a base weaker than carbonic acid so that their reaction would result in an acidic salt. I have tried to find such bases online but because of my lack of understanding of this topic, I am unable to do so.

Question

Can carbonic acid form an acidic salt? If yes, then upon reacting with which base?

Edit: By acidic and basic salts, I mean salts with pH < 7 and pH > 7 respectively. I did not know acidic and basic salts mean something else in chemistry. At our class level we are taught the meaning of acidic and basic salts in this context only

Edit 2: Phosphoric acid is not a strong acid, my bad for writing it as such

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  • $\begingroup$ Phosphoric acid is not strong, all 3 acidic hydrogens are weakly acidic (even is the first one not much weak). Similarly, only the first H in sulfuric acid is strongly acidic, The second one is very strong weak acid. $\endgroup$
    – Poutnik
    Jan 11 at 11:04

2 Answers 2

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Both ammonium and sodium bicarbonate solutions are basic, as the anion $\ce{HCO3-}$ is a stronger base than is the strength of the acid $\ce{NH4+}$, not even talking about $\ce{Na+}$.

Carbonic acid could form a mildly acidic salt in sense of forming acidic solution with a base weaker than $\ce{HCO3-}$, e.g. with pyridine or aniline. But their water solutions would be highly hydrolyzed to the respective base and $\ce{H2CO3(aq)/CO2(aq)}$, as such bases need very acidic pH. Carbon dioxide would be gradually escaping as gas.

$$\ce{CO2(aq) + H2O(l) <=> H2CO3(aq)} \tag{1}$$ $$\ce{H2CO3(aq) + H2O(l) <=> HCO3-(aq) + H3O+(aq)}\tag{2}$$ $$\ce{C5H5N(aq) + H3O(aq) <=> C5H5NH+(aq) + H2O(l)}\tag{3}$$

Very formally, the salt formula can be written as $\ce{(C5H5NH)HCO3}$, as it would in large extent hydrolyze to the left side of reactions (2) , (3) to pyridyne and carbon dioxide.


An acidic salt – aside of pH of its solution – means a salt where not all acidic hydrogens of a multiprotic acid are neutralized. Like

  • $\ce{KHSO4}$ (very acidic solution)
  • $\ce{KH2PO4}$ (acidic solution)
  • $\ce{NaHCO3}$ (mildly basic solution)
  • $\ce{Na2HPO4}$ (basic solution)

As the anion of such partially neutralized acid is both an acid ($\ce{HA- <=> H+ + A^2-}$) and a base ($\ce{HA- + H+ <=> H2A}$), pH of its solution depends on which of them is stronger.

Similarly, basic salts are combination of a salt and a hydroxide, like the bleaching powder is a complex combination of hydrated calcium chloride, hypochlorite and hydroxide.

Wikipedia:

Commercial calcium hypochlorite consists of anhydrous $\ce{Ca(ClO)}$2, dibasic calcium hypochlorite $\ce{Ca3(ClO)2(OH)4}$ (also written as $\ce{Ca(ClO)2·2Ca(OH)2}$), and dibasic calcium chloride $\ce{Ca3Cl2(OH)4}$ (also written as $\ce{CaCl2·2Ca(OH)2}$).

Or there is $\ce{CuCO3 . Cu(OH)2}$ in the mineral malachite.


How can someone figure out which of the bases is stronger between $\ce{HCO3-}$ and Pyridine. And if we have the assertion that pyridinium bicarbonate will be acidic in nature, then is the reason that pyridine is a weaker base than $\ce{HCO3-}$ hence pyridinium ion will be stronger acid than $\ce{HCO3-}$?

The values of acidity/basicity constanst can be found tabulated. The most available ways is getting them from Wikipedia pages for respective compounds, or various chemistry dedicated sited have listed them for many compounds.

If there is an equilibrium

$$\ce{HA(aq) <=> H+(aq) + A-(aq)}$$ or $$\ce{B(aq) + H+(aq) <=> BH+(aq)},$$

then acidity of $\ce{HA}$ or $\ce{BH+}$ and basicity of $\ce{A-}$ or $\ce{B}$ are linked by the equation

$$K_\mathrm{a} \cdot K_\mathrm{b} = K_\mathrm{w},$$ resp. (at $\ce{25 ^{\circ}C}$) $$\mathrm{p}K_\mathrm{a} + \mathrm{p}K_\mathrm{b} = 14$$.

If there are two bases, then the stronger one forms by neutralization a weaker acid. Pyridinium with $\mathrm{p}K_\mathrm{a} = 5.23$ is a stronger acid than CO2(aq) with $\mathrm{p}K_\mathrm{a1}=6.35$, therefore pyridine is a weaker base than $\ce{HCO3-}$.

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  • $\begingroup$ Can you please tell the reaction of Carbonic acid and pyridine? I cannot find it online $\endgroup$ Jan 11 at 10:50
  • $\begingroup$ Thanks for responding to my edits. I have one more query, will the reaction of Carbonic acid and Pyridine be an example of acid-base reaction? I am asking because at my level we only know that acid-base reactions give salt + water, but in the carbnoic acid and pyridine example I cannot understand how water will be produced. $\endgroup$ Jan 11 at 11:10
  • $\begingroup$ Forming water is optional. // Many things that are taught in early classes are highly simplified in expense of distorted knowledge of more complex reality. Like in physics, mechanical work is taught as multiple of force and length $W=F.L$. Later, there is taught $W=\vec F \cdot \vec L=|\vec F|\cdot |\vec L| \cdot \sin{\alpha}$. and yet later is taught $W=\int_L{\vec F \cdot \mathrm{d}\vec L}$ $\endgroup$
    – Poutnik
    Jan 11 at 11:23
  • $\begingroup$ Thanks for answering, I finnally get it $\endgroup$ Jan 11 at 11:25
  • $\begingroup$ The question is poor or even foolish. It relies on a nebulous definition of acid or acidic salt. Most seem to think it refers to the acidity of a solution of a salt. This definition means that the student must know the pH of every solution of every possible salt of the acids. The more astute realize it might mean can the acid form an ion that is still an acid in the Bronsted sense and supply an additional proton [or 2]. The test writer and possibly the class deserve an F. but the student is graded. Bicarbonate, bisulfate, dihydrogenphosphate, etc. are both Bronsted acids and bases. $\endgroup$
    – jimchmst
    Jan 12 at 9:17
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Yes. Carbonic acid $\ce{H2CO3}$ can make an acidic salt, and it does in the compound $\ce{NaHCO3}$ which is sodium hydrogen carbonate, sometimes called sodium bicarbonate. It can react with a base like $\ce{NaOH}$ according to the equation :$$\ce{NaHCO3 + NaOH -> Na2CO3 + H2O}$$ Every acid containing more than one acidic $\ce{H}$ atom in its formula can make acidic salts. Examples : $\ce{H2SO4, H2CO3, H3PO4}$. And of course $\ce{HCl}$ and $\ce{HNO3}$ cannot make acidic salts.

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    $\begingroup$ It needs to be said, though, that the nominally acidic salt NaHCO3 gives a basic solution. $\endgroup$ Jan 11 at 9:39

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