Question

Write the formula and names :

1.	Ca	$S{O_4}$	Calcium Sulphate
2.	Mg	Cl
3.	Na	OH
4.	K	$Mn{O_4}$
5.	Ca	$P{O_4}$

Answer 1

When we write the formula, we see the valency of the atom or molecule and then cross multiply their valencies. This gives us the formula of the compound. Suppose if an element X has valency +2 and Y has valency -1, then they will give a compound with molecular formula X${Y_2}$.

Complete answer:
We know that all these compounds are ionic in nature. And ionic compounds are formed by complete donation of electrons. So, the valency of an atom will describe its donation power. We will see the valency of atoms and then by cross multiplying valencies, we will get the formula for compound.
So now let us begin with the first option.
The calcium has valency (+2) and sulphate has valency (-2). So, by cross multiplying, we get $CaS{O_4}$ as the molecular formula.
The second one has Mg which has valency (+2) and chloride has valency (-1). So, the molecular formula is $MgC{l_2}$.
The third option has sodium atom which has valency (-1) and hydroxide ion which also has (-1) as valency. So, the formula is NaOH.
The next one has a potassium metal atom with (+1) valency and $Mn{O_4}$has (-1) valency. So, the molecular formula is $KMn{O_4}$.
The next one is calcium with valence two electrons. The anion is phosphate ion which has (-1) as valency. So, it gives the formula as $Ca{(P{O_4})_2}$.
In summary, the molecular formulas and names of the compounds are-

1.	Ca	$S{O_4}$	$CaS{O_4}$	Calcium Sulphate
2.	Mg	Cl	$MgC{l_2}$	Magnesium chloride
3.	Na	OH	NaOH	Sodium hydroxide
4.	K	$Mn{O_4}$	$KMn{O_4}$	Potassium permanganate
5.	Ca	$P{O_4}$	$Ca{(P{O_4})_2}$	Calcium phosphate

Note:
It must be noted that one should know the basic names of the groups with which the metal is attached. Then, the name can be easily found out. The group ${(C{O_3})^{2 - }}$ is called carbonate. Similarly, the groups with oxygen are called oxides, with nitrogen (-3) are nitrides, ${(P{O_4})^{3 - }}$ are phosphates and with ${(Al{O_2})^ - }$ are aluminates etc.