An arsenite is an oxyanion of trivalent arsenic, especially the AsO33- anion or protonated derivatives.

Sodium ortho-arsenite, although the name is often shortened to sodium arsenite. It contains the AsO33- ion ortho-arsenite ion. Arsenic has oxidation number +3.

Confusingly there is another compound NaAsO2 sodium meta-arsenite whose name is also shortened to sodium arsenite. This contains a polymeric meta-arsenite ion (AsO2-)n . Again arsenic has oxidation number of +3

1 ppm means 1 mg/l so 1000 ppm means 1000 mg/l or 1gm/l ,

FW of sodium arsenite NaAsO2 is 130 ans As is 75

75 gm of arsenite is present in 130 gm sodium arsenite

1 gm---------------will be---------130/75= 1.73 gm

so if 1.73 gm of sodium arsenite is present in 1 litre of water it makes 1000 ppm solution of arsenite

AsO33- is Arsenite ion

(NH4)3 AsO3