Question

formula of rust ,plaster of Paris and common salt

Answer 1

The chemical formula for rust is Fe2O3.nH2O
The chemical formula for plaster of Paris is CaSO4·½H2O
The chemical formula for common salt is NaCl.

Answer 2

common salt is also called sodium chloride. now you can write formula for sodium chloride using criss cross method. 
VALENCY OF SODIUM- 1+
SYMBOL- Na
VALENCY OF CHLORIDE-1-
SYMBOL- Cl
now using criss cross method the formula for sodium chloride becomes NaCl
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The chemical name of PLASTER OF PARIS(POP) IS HEMIHYDRATE OF CALCIUM SULPHATE. NOW WE CAN NOT WRITE CHEMICAL FORMULA OF PLASTER OF PARIS USING CRISS CROSS METHOD. WE CAN ONLY WRITE CHEMICAL FORMULA OF IONIC COMPOUNDS USING CRISS CROSS METHOD. 
OK FIRST LET US WRITE CHEMICAL FORMULA OF CALCIUM SULPHATE USING CRISS CROSS METHOD.
CALCIUM(Ca) 2+
sulphate SO4 2-
chemical formula of calcium sulaphate- CaSO4
now what is hemihydrate. let us see
a crystalline hydrate containing one molecule of water for every two molecules of the compound in question. 
so now 

2 molecules of compound =1 molecule of water
but in this ques, we have only one molecule of compound calcium sulphate
1 molecule of compund=1/2 molecule of water
we have solved this using unitary method
Now the chemical formula of POP is CaSO4.1/2 H20
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The chemical name of rust is ferric oxide.
OK NOW U LEARN A CONCEPT OK,
IRON HAS TWO VALENCIES. IT SHOWS MORE THAN ONE VALENCY. IT SHOWS VARIABLE VALENCY. IT IS A METAL, SO IT WILL SHOW POSITIVE VALENCY. IT SHOWS VALENCY OF 2 + AND 3+.
NOW IF WE SAY ONLY IRON, WE WILL WRITE VALENCY OF 2 +.
(IN THE SAME WAY EVEN COPPER SHOWS VARIABLE VALENCY . IF WE WRITE ONLY COPPER, WE WILL WRITE ITS VALENCY 2+)
NOW IRON CAN BE WRITTEN IN 2 WAYS WITH FERRIC AND FERROUS. FERRIC MEANS GREATER VALENCY. IF FERRIC IS WRITTEN WRITE GREATER VALENCY THAT IS 3+. FERRUM MEANS SMALEER VALENCY.
SO,VALENCY OF FERRIC 3+
SYMBOL= SYmBOL OF IRON= Fe
VALENCY OF OXYGEN 2-
OXYGEN HAS NEGATIVE VALENCY BECAUSE IT IS A NON METAL.
SYMBOL OF OXYGEN IS O
SYMBOL OF OXYGEN ELEMENT IS O
BUT SYMBOL OF OXYGEN MOLECULE OR OXYGEN GAS IS O2
NOW WE WILL WRITE CHEMICAL FORMULA USING CRISS CROSS METHOD.
Fe3+ O2-
VALENCIES WILL INTERCHANGE-

Fe2O3.
NOTE- U CAN FIND SUCH QUES ON GOOGLE VERY EASILY. JUST TYPE CHEMICAL FORMULA OF RUST. IT WILL COME. FORMULA OF POP. IT WILL COME IN BOLD. FORMULA OF COMMON SALT IT WILL COME IN BOLD.
i hope my explanation helps u.