Question

Which of the following is known as glacial phosphoric acid?
\[ A.\;HP{O_3} \\
  B.\;{H_3}P{O_3} \\
  C.\;{H_3}P{O_2} \\
  D.\;{H_3}P{O_4} \\
\]

Answer 1

Hint: The formula of phosphoric acid is \[{H_3}P{O_4}\].It is also called orthophosphoric acid. It is a weak acid which means when it is dissolved in a solution, all the ions do not dissociate completely. When it loses one proton it forms \[{H_2}P{O_4}^ - \]
 and can also form \[HP{O_4}^{2 - }\] and \[P{O_4}^{3 - }\] when it loses two and three protons respectively.

Complete answer:
We have different phosphoric acids like metaphosphoric acid, pyrophosphoric acid, orthophosphoric acid etc among which metaphosphoric acid is also called as glacial phosphoric acid. Its chemical formula is \[HP{O_3}\]and it is said to be an anhydride of phosphoric acid. It is analogous to nitric acid \[HN{O_3}\].

Hence the correct option is A.

Additional information:
Acids are classified as monobasic, dibasic and tribasic acids based on the number of replaceable ions present in it. If there is one replaceable hydrogen ion, it is called monobasic acid and if there are two or three replaceable hydrogen ions, these are called dibasic and tribasic acids respectively. By now you will be able to guess that glacial phosphoric acid is a monobasic acid because when you look at the formula, you will find one hydrogen atom which can be replaced.

Note: \[HP{O_3}\] is widely used in the manufacture of zinc oxyphosphate cement for dentistry. In the same way, we also have glacial acetic acid which is nothing but the 100% pure acetic acid that is free from water. From a chemistry point of view, the word glacial means pure acid.