IMAT 2015 Q47 [Oxidation Numbers]

Assume that the oxidation numbers are as shown below:

N = –3
H = +1
Cr = +6
O = –2

Which compound formula given below is correct?

A. [ (NH_4)_2CrO_4 ] ^+
B. NH_4Cr_2O_7
C. (NH_4)_2CrO_4
D. [ (NH_4)_3CrO_4 ] ^–
E. (NH_4)_3CrO_4

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When formulating the compound, we can divide it into two separate formulations to compute the final form.

First, we form NH_4, which will have a charge of +1.

Next, we check the charge on CrO_4, which should be -2.

Therefore, in order for these two molecules to react, they must take the form of (NH_4)_2CrO_4, two NH_4^+ ions for one CrO_4^{2-} ion , and our answer is C.