   Question Answers

# The oxidation state of Cr in ${{K_2}{Cr_2}{O_7}}$ is:A) +4B) +3C) +6D) +5

Hint: Oxidation state is also known as oxidation number and it is defined as the total number of electrons present in an atom after gaining and losing in chemical reactions. $Cr$ is the element named as chromium and atomic number is 24.

${K_2}C{r_2}{O_7}$is named as potassium dichromate, which contains the dichromate ions and where chromium is in the hexavalent form. Now we see the charge carry by each element is:
Potassium carries +1 charge on it and oxygen carries -2 charge on it.
For calculating the oxidation number there are some rules:
1. The oxidation number of free elements is always zero.
2. The oxidation number of H is +1, but it is -1 when combined with less electronegative elements.
3. The oxidation number of a monatomic ion equals the charge of the ion.
4. The oxidation number of group 1 elements in a compound of +1.
5. The oxidation number of group 2 elements in a compound of +2.
6. The oxidation number of group 17 elements in a compound of -1.
Now, we calculate oxidation state of chromium
2( + 1) + 2x + 7( - 2) = 0
= 2 + 2x - 14 = 0
= 2x - 12 = 0
= 2x = 12
x = + 6

Hence, we see that the oxidation state or oxidation number of chromium is +6.

Note: For finding the oxidation state or oxidation number we had known with the no of molecules used in the chemical reaction of the elements and by multiplying molecule number with their charge we easily find the oxidation state.