There are two broad classes of protonic acid.

• OXO ACIDS: The acidic protons are bound to oxygen, which in turn is bonded to a central atom. Some examples:

  Acid anhydride	Molecular formula	Structural formula
  Hydrogen sulphate	H2SO4	S(O)2(OH)2
  Hydrogen nitrate	HNO3	N(O)2(OH)

• BINARY ACIDS: The acidic protons are bound to the central atom. An example: Hydrogen chloride (HCl)

OXO ACID ANHYDRIDES

*Note: To obtain the name of the acid from the anhydride "ate" becomes "ic" and "ite" becomes "ous". Examples, hydrogen nitrate gives nitric acid and hydrogen nitrite gives nitrous acid. However the respectively salts formed are known as nitrate and nitrite.

For phosphorus and sulphur, phosphate becomes phosphuric and phosphite becomes phosphurous, sulphate becomes sulphuric and sulphite becomes sulphurous. However the names of the salts are still phosphate, phosphite, sulphate and sulphite respectively.

An empirical method to "predict" whether an oxo protonic acid is a strong (or weak) acid is to consider the "charge" on the centre atom. Let us illustrate this with sulphuric acid, S(O)₂(OH)₂, phosphoric acid [P(O)(OH)₃] and hypochlorous acid Cl(OH)

The lone oxygen (O) attached to the sulphur atom is seen as siphoning electron to itself causing the sulphur to be slightly positive. This is then transmitted along the S-O-H chain to the hydrogen atom at the end, pushing the proton out so that the sulphur can have its electron. So the acidity is "judged" by the number of (O) versus (OH). For sulphuric acid it will work out to be (4 oxygen - 2 replaceable proton) = 2. For phosphorous acid it will be (4 oxygen - 3 replaceable proton) = 1. For hypoclorous acid it will be (1 oxygen - 1 replaceable proton) = 0.

It was noticed that

• For the value of 0, pKa is about 8 to 9
• For the value of 1, pKa is about 1 to 4
• Strong oxo acids have a value above 2.

BINARY ACID ANHYDRIDES

SOME COMMERCIALLY IMPORTANT PROTONIC ACIDS

HYDROCHLORIC ACID is prepared by passing hydrogen chloride gas into water until it is saturated. This is sold as fuming hydrochloric acid of about 37% strength. That means about 37 grams of hydrogen chloride in 100 ml of solution. When you open the bottle expect fumes of hydrogen chloride to be released. Do take the necessary precaution and conduct the operation in a fume hood.

NITRIC ACID is prepared by dissolving hydrogen nitrate (otherwise known as anhydrous nitric acid) in water. Hydrogen nitrate is a colourless liquid with a freezing point of   - 41.59�C and a boiling point of 84.1⁰C. Nitric acid is not as strong an acid as hydrochloric acid or sulphuric acid. As an acid it work best at low concentration of about 0.1M.

SULPHURIC ACID is prepared commercially by dissolving a certain amount of hydrogen sulphate in water, before passing in SO₃ gas. This will be sold as fuming sulphuric acid (with a concentration of about 30%).

Hydrogen sulphate (commercially also known as anhydrous sulphuric acid) is a colourless liquid freezing at 10.37�C. It has great affinity for water. So if it touches your skin it will absorb whatever water that is in your skin, causing a "burnt". Bottles containing hydrogen sulphate should carry a "corrosive" label. If you accidentally spilled some hydrogen sulphate onto your skin, wash it with plenty of running water. As a matter of fact, it any acids come into contact with your skin wash it with plenty of running water and seek medical attention immediately if a large area (or vital part) involved.

CARBONIC ACID Is present in aerated drinks.

PHOSPHORIC ACID is present in Coca Cola along with carbonic acid.