Question

The formal oxidation numbers of Cr and Cl in the ions $C{r}_2{O}_7^{2-}$ and $Cl{O}_3^{-}$ respectively, are

• A. +6 and +7
• B. +7 and +5
• C. +6 and +5
• D. +8 and +7

Answer 1

The correct option is C +6 and +5

Sum of oxidation number of all atoms in an ion is equal to charge present on ion.
Let oxidation number of Cr in $C{r}_2{O}_7^{2–}$ is x
2x + 7(–2) = –2
2x - 14 = –2
x = +6
Let oxidation number of Cl in $Cl{O}_3^{–}$ is
x + 3(–2) = –1
x = +5