The oxidation number of $As$ in $H_2AsO_4^-$ is:

The oxidation number of carbon in $CH_2O$ is

What is the oxidation state of $Fe$ in $Fe_3O_4$?

In which of the following, the oxidation state of nitrogen is lowest?

An acidified solution of sodium thiosulfate is unstable because in thiosulfate,the two sulfur atoms are in:

Compounds having the highest oxidation state of the central atom act as acids,the lowest ones act as bases,and intermediate oxidation states act as amphoteric. Which one of the following is amphoteric?