The problem on my test was to balance a reaction using the half reaction method. The formula is H3AsO4 + Zn –> AsH3 + Zn+2 . I put the oxidation number of H as 1 because it is bonded exclusively to a metal (As). So that makes the oxidation number of As +3 correct? The teacher put it wrong and corrected it to an oxidation number of 3 for As. Who is right?
3 Answers

If you remember which is more electronegative, then it is easy. Since As is more electronegative than H, thereforre we cannot put ON of H as negative and of As as positive. However, if you do not remember which is more electronegative, then read the following:
H has oxidation number 1 when it combines with metal provided the metal is not a transition element. As is a transition element.It can have a range of oxidation numbers. Therefore its oxidation number should be calculated from the oxidation numbers of other elements in a compound.
In H3AsO4, put oxidation number of H = +1, of O = 2. Then you will find that the O.N. of As = +5
In AsH3, put O.N. of H=+1. Then O.N. of As = 3
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SO4 is a polyatomic ion, meaning entire all of those atoms have a cost at the same time as you positioned them jointly. SO4 has a cost of 2 and you have have been given 3 of them, so the traditional fee is 6. even inspite of the undeniable fact that the total fee of the molecule Fe2(SO4)3 is 0, meaning that the two irons (Fe) cancel it out with an excellent 6 fee. So if 2 irons have a confident 6 cost, then one ought to have an excellent 3 fee. this suggests it incredibly is iron(III) (i.E., the oxidation quantity is 3). So the compound may be referred to as ferric sulfate.

According to one electronegativity scale, the electronegativity of As is 2.18 and H is 2.20. The more electronegative element is assigned the negative oxidation number.
See the electronegativity scale at: http://en.wikipedia.org/wiki/Electronegativity