June 2009-44
Given the balanced equation representing a reaction:
Fe_{2}O_{3}+
2Al--> Al_{2}O_{3}+
2Fe
During this reaction, the oxidation number of Fe changes
from
(1) +2 to 0 as electrons are transferred
(2) +2 to 0 as protons are transferred
(3) +3 to 0 as electrons are transferred
(4) +3 to 0 as protons are transferred

Jan 2009-44
What is the oxidation number of sulfur in Na_{2}S_{2}O_{3}?
(1) -1 (3) +6
(2) +2 (4) +4

More Practice

What is the oxidation number of .
. .

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1) N in NO_{3}¯

The O is -2 and three of them
makes -6. Since -1 is left over, the N must be
+5

2) C in CO_{3}^{2}¯

The O is -2
and three of them makes -6. Since -2 is left
over, the C must be +4

3) Cr in CrO_{4}^{2}¯

The O is -2
and four of them makes -8. Since -2 is left
over, the Cr must be +6

4) Cr in Cr_{2}O_{7}^{2}¯

The O is -2
and seven of them makes -14. Since -2 is left
over, the two Cr must be +12. Each Cr is
+6.

5) Fe in Fe_{2}O_{3}

The O is -2
and three of them makes -6. Each Fe must then be
+3