Question

Calculate the oxidation number of Cr in $CrO_4^{2-}$ ion and $K_2Cr_2O_7$ respectively

• A. +4 and +6
• B. +3 and +2
• C. +6 and +6
• D. +8 and +2

Answer 1

Answer: (C)

+6 and +6

Answer 2

For the chromate ion, $\mathrm{CrO_4^{2-}}$:

$$x + 4(-2) = -2 \quad \Rightarrow \quad x - 8 = -2 \quad \Rightarrow \quad x = +6.$$

For the dichromate compound, $\mathrm{K_2Cr_2O_7}$:

$$2(+1) + 2x + 7(-2) = 0 \quad \Rightarrow \quad 2 + 2x - 14 = 0 \quad \Rightarrow \quad 2x - 12 = 0 \quad \Rightarrow \quad x = +6.$$

Core Explanation:

• In $\mathrm{CrO_4^{2-}}$, set $x + 4(-2) = -2$ to get $x=+6$.
• In $\mathrm{K_2Cr_2O_7}$, balance $2(+1) + 2x + 7(-2) = 0$ to find $x=+6$.