Monoprotic or diprotic?

Question

How can one tell if an acid is monoprotic or diprotic?

For example: H2SO4. Is it diprotic? (because of the 2 after H).

And somewhat related, how do you know how many OH ions a base produces? Eg: Ca(OH)2. Does it produce 2 because of the 2 after OH?

Thanks a lot! Any help, brief or not, is appreciated.

Answer 1

The answers you have gotten are mostly right.   As with many things in chemistry,  it isn't always that simple.

The answer is that it is the number of active or labile hydrogens that determine if a compound is monoprotic or diprotic.

For example,  there is an organic compound  C2H4O2  acetic acid (the active ingredient in vinegar.)   One of the hydrogens is bonded to oxygen and is active.  The other three hydrogens are bonded to carbon and are not active.   So it is monoprotic.

Another example is adipic acid.  The formula is C3H4O4.   It is diprotic.

If your teacher is fair,  he or she will not give you a compound like this as the only way to know is either to look in detail at the chemical structure.   If your teacher is not fair,  then you need to have memorized the common acids of this type.

Figure that your teacher will be fair.

Answer 2

OK, here it is brief - YES.

Now for not so brief ........

A monoprotic acid is an acid containing one ionizable hydrogen atom per molecule.

A diprotic acid is one such as H2SO4 (sulfuric acid) the molecular structure of which contains two hydrogen atoms capable of dissociation in water. (The term dibasic is also used, which is a bit confusing)

Likewise, you are right about Ca (OH)2  

Answer 3

The number of hydrogens indicates it.  Monoprotic-1, diprotic-2, triprotic-3.  The subscript under the OH ions in a base formula shows how many are needed to balance the formula, because it's ionic, it's needs to be balanced out.  The number of OH ions can be calculated through balancing equations.