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# What is the oxidation state of chromium in the dichromate ion, ##"Cr"_2"O"_7^(2-)##?

+6

Problem: What is the oxidation number (O.N.) of chromium (Cr) in the dichromate ion ##"Cr"_2"O"_7"^(2-)##?

Let the O.N. of Cr be ##x##. The O.N. of oxygen is -2.

The sum of the oxidation states in an ion must add up to the charge on the ion. The dichromate ion has a charge of ##"2"^-##, as indicated by its formula, ##"Cr"_2"O"_7"^(2-)##.

##2*x## + ##7*##(-2) = (-2)

##2*x## - 14 = -2

Add 14 to both the sides;

##2*x## -14 +14 = -2 +14

##2*x## = 12

Divide both sides by 2 ;

##"2x"/2"## =##"12"/"2" ##

##x = 6##

The oxidation number of chromium in the dichromate ion is ##+6##.

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