oxidation state

[MoooShuuu]

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sorry dumb question:

when I do the oxidation state forAl in Al(OH)4- I get -3 and the answer is +3. What am I doing wrong?

 

[Contach]

sorry dumb question:

when I do the oxidation state forAl in Al(OH)4- I get -3 and the answer is +3. What am I doing wrong?

This is the way I used to do it:

[Al (OH)4 ]-

Thus the sum of Al + 4(OH-) = -1
if we are looking for Al, we substitute it for x
x + 4(OH-) = -1
since we know OH- has an overall oxidation state of -1, we can plug this in
x + 4(-1) = -1
x - 4 = - 1
now you must ADD + 4 to each side
x - 4 + 4 = -1 + 4
x = +3

If you cant do something, take it back to the basics and work through it....

 

[tejapriya]

This is the way I used to do it:

[Al (OH)4 ]-

Thus the sum of Al + 4(OH-) = -1
if we are looking for Al, we substitute it for x
x + 4(OH-) = -1
since we know OH- has an overall oxidation state of -1, we can plug this in
x + 4(-1) = -1
x - 4 = - 1
now you must ADD + 4 to each side
x - 4 + 4 = -1 + 4
x = +3

If you cant do something, take it back to the basics and work through it....

 

[tejapriya]

we should not consider oxidation state of overall (OH) as -1. We should calculate separetely .
[Al(OH)4]-
let us consider oxidation state of Al is x
the oxidation state of O is -2 so for O4= 4(-2)=-8
the oxidation state of H is +1 so for H4 = 4(+1)=4
add and equate this to -1
x-8+4=-1
x-4=-1
x=+3

 

[DAT Destroyer]

Hope this helps.

Dr Romano