Chemistry, 18.07.2019 10:20 journeyburks07

Chromate is a polyatomic ion with a charge of negative two. it contains chromium and oxygen in a one-to-four ratio. what is its formula?

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$CrO_{4} ^{-2}$

Because the ratio of the chromium to oxygen should be 1 is to 4, the first formula that comes to mind is $CrO_{4} ^{-2}$ , with 1 chromium atom and 4 oxygen atoms.

The most common oxidation states of chromium are +6, +3, and +2. Less common are the oxidation states +5, +4, and +1 states, which are present in a few stable compounds.

The oxide ion has a charge of -2. With the chromate formula being $CrO_{4} ^{-2}$ , the chromium oxidation state (x) is calculated to be

x + 4*(-2) = -2
x = -2+8
x = +6

The chromium oxidation state is +6, which is a known common oxidation state of chromium.

Testing other 1:4 ratios of Cr and O by multiplying the ratio with different factors would give,

$Cr_{2}O_{8} ^{-2}$
$Cr_{3}O_{12} ^{-2}$

and so on.

Determining the oxidation state (x) of chromium for the two possible chromate ion formulas,

1.   $Cr_{2}O_{8} ^{-2}$

2x + 8*(-2) = -2
2x = -2+16
2x  = +14
   x  = +7

2.   $Cr_{3}O_{12} ^{-2}$

3x + 12*(-2) = -2
  3x = -2+24
3x  = +22
   x  = +7.3

As observed, using chromate ion formulas in which the 1:4 ratio is multiplied by factors greater than 1, the calculated oxidation states of chromium are not considered to be possible. Therefore, the only possible formula for the chromate ion with a chromium to oxygen ratio of 1:4, and an ionic charge of -2 is $Cr O_{4}$ .
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Chromate is a polyatomic ion with a charge of negative two. it contains chromium and oxygen in a one...