Which one of the following is a tribasic acid?
- (a)Hydrochloric acid
- (b)Nitric acid
- (c)Sulphuric acid
- (d)Phosphoric acid
Correct — D, phosphoric acid. Basicity counts how many hydrogen ions one molecule of an acid can give up to a base, and orthophosphoric acid, H₃PO₄, has three hydrogen atoms bonded through oxygen, all three of which are ionisable. It therefore neutralises three moles of sodium hydroxide per mole and forms three distinct salts in succession — sodium dihydrogen phosphate, disodium hydrogen phosphate and trisodium phosphate — which is exactly what 'tribasic' means. None of the other three acids in the list can shed more than two.
- (a)Hydrochloric acid — HCl carries a single hydrogen and gives a single hydrogen ion, so it is monobasic. It is a very strong acid, but strength and basicity are different ideas — strength is how completely the acid ionises, basicity is how many protons the molecule has to give.
- (b)Nitric acid — HNO₃ also has just one hydrogen and is monobasic. Its formula looks bulkier than that of hydrochloric acid because of the three oxygens, but oxygens do not add replaceable hydrogen; only the one hydrogen attached through oxygen is ionisable.
- (c)Sulphuric acid — H₂SO₄ is dibasic, not tribasic. It releases its two protons in two stages and forms two salts — sodium hydrogen sulphate and sodium sulphate. This is the closest wrong answer, because a student who remembers only that sulphuric acid is 'the strongest' may reach for it.
The basicity of an acid is the number of replaceable hydrogen ions per molecule. Hydrochloric, nitric and acetic acids are monobasic; sulphuric, carbonic and oxalic acids are dibasic; orthophosphoric acid and citric acid are tribasic. The count is not simply the number of hydrogen atoms written in the formula — it is the number attached in a way that lets them ionise, which in oxoacids means the hydrogens bonded to oxygen. Phosphorous acid, H₃PO₃, is the classic counter-example: it shows three hydrogens but only two of them sit on oxygen, so it is dibasic.
Two habits get this question wrong. The first is confusing strength with basicity — hydrochloric and nitric acids are strong and still monobasic, while phosphoric acid is only moderately strong and yet tribasic. The second is counting hydrogens straight off the formula without asking where they are attached; acetic acid, CH₃COOH, has four hydrogens in the formula but only the carboxyl one comes off, so it is monobasic. Here the arithmetic happens to be safe: H₃PO₄ genuinely has all three hydrogens on oxygen. Remember the salt series as the proof — a tribasic acid must be able to form three different sodium salts, and phosphoric acid does.
- Basicity is the number of replaceable hydrogen ions one molecule of the acid can furnish.
- Orthophosphoric acid, H₃PO₄, is tribasic and forms NaH₂PO₄, Na₂HPO₄ and Na₃PO₄ in turn.
- Sulphuric acid is dibasic; hydrochloric, nitric and acetic acids are monobasic.
- Phosphorous acid, H₃PO₃, is dibasic despite showing three hydrogens, because only two are bonded to oxygen.
Only phosphoric acid reaches three, so it alone answers to 'tribasic'.
- Equating a strong acid with a high basicity. Hydrochloric acid is very strong and only monobasic.
- Counting every hydrogen shown in the formula. In acetic acid only one of the four ionises.
- Assuming H₃PO₃ must be tribasic because it starts with three hydrogens; it is dibasic.
NDA asks for the basicity of a named acid, or gives four acids and asks which is mono-, di- or tribasic.
No directly related past PYQ was found.
- practice — not a real PYQ
The basicity of acetic acid, CH₃COOH, is
- (a)one
- (b)two
- (c)three
- (d)four
Answer(a) one — only the hydrogen of the carboxyl group is replaceable; the three on the methyl carbon do not ionise.
- practice — not a real PYQ
How many moles of sodium hydroxide are needed to neutralise one mole of orthophosphoric acid completely?
- (a)One
- (b)Two
- (c)Three
- (d)Four
Answer(c) Three — the acid is tribasic, so complete neutralisation gives trisodium phosphate.