Question

What is the chemical formula of calcium hydroxide?

Answer 1

Hint :Calcium hydroxide, a colourless crystal or white powder, is actually an inorganic compound that is also known by several other names like hydrated lime, slack lime, caustic lime, builders' lime, and pickling lime. It is manufactured commercially by the reaction of lime with water.

Complete Step By Step Answer:
 As the name suggests, the given compound calcium hydroxide comprises a metal calcium and a group of non-metals i.e. hydroxide. We know that any compound containing a metal and a non-metal is considered to be an ionic compound. In ionic compounds, we have to take into account the charges of each of the elements present in the compound. Now let us identify the formula for the given compound i.e. calcium hydroxide. Calcium hydroxide possesses a polyatomic ion (i.e. group of nonmetals (hydroxide in this case) after the metal) so we'll use a table of names for the common polyatomic ions, along with the Periodic Table. Rules to write formula of a ternary ionic compound are listed below:
1. Using the periodic table, write down the symbol for the metal along with its charge.
In the present case, the symbol of metal Calcium is $ Ca $ having a charge +2.
2. Using the Common ion table, identify the symbol as well as charge of the polyatomic ion.
In the present case, polyatomic ion hydroxide is written as $ OH $ having a charge -1.
3. Check if the charges are balanced
In the present case it becomes $ C{a^{2 + }}O{H^{ - 1}} $ . The charges are not balanced as summation of charges is non-zero i.e. $ ( + 2) + ( - 1) = + 1 \ne 0 $
4. If unbalanced charges are there, add subscripts such that net charge for the compound is zero.
In the present case, we will follow the criss-cross method and add subscript ‘1’ to $ Ca $ and subscript ‘2’ to $ OH $ so it becomes $ {(C{a^{2 + }})_1}{(O{H^{ - 1}})_2} $ and now, we will check the net charge of the compound which will now be zero i.e. $ (1 \times ( + 2)) + (2 \times ( - 1)) = 0 $ .
Hence, the chemical formula for calcium hydroxide is $ Ca{(OH)_2} $ .

Note :
While writing the chemical formula of an ionic compound, never write the subscript '1'. And it should be noted that we can also have two polyatomic ions like $ N{H_4}N{O_3} $ in the same compound. In this case, we have to find and write both of the names as identified from the Common Ion Table.