Determining oxidation number

sarahtang4B
Posts: 132
Joined: Fri Sep 28, 2018 12:27 am

Determining oxidation number

on homework question 9C1 on 7th edition, how do we determine oxidation number of a metal in a complex ion such as [Co(CN)5 (OH2)]^-2 and [Co(NH3)5 (SO4)]+

Christine Chow 4G
Posts: 43
Joined: Fri Sep 28, 2018 12:28 am

Re: Determining oxidation number

In [Co(CN)5 (OH2)]^-2, the oxidation number for Co is +3 because OH2 has a charge of 0 and CN has a charge of -1.
In order for the final charge to be -2, Co has to be +3. -5 +3 = -2
In [Co(NH3)5(SO4)]+ the oxidation number for Co is 3+ because (NH3)5 has a charge of 0 and (SO4) has a charge of 2-
In order for the final charge to be +1, Co has to be +3. -2 +3 = +1

KarlaArevalo2F
Posts: 67
Joined: Tue Nov 14, 2017 3:01 am
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Re: Determining oxidation number

Is there an easy way to the oxidation number for ions like sulfate? This is the first time I take chem in my life (HS didnt offer it) so I feel behind in knowing oxidation numbers.

Aurbal Popal
Posts: 63
Joined: Fri Sep 28, 2018 12:27 am

Re: Determining oxidation number

KarlaArevalo4D wrote:Is there an easy way to the oxidation number for ions like sulfate? This is the first time I take chem in my life (HS didnt offer it) so I feel behind in knowing oxidation numbers.

Group 1 has an oxidation number of +1
Group 2: +2
Group 13 (with Boron): +3
Group 14: +4/-4
Group 15: -3
Group 16: -2
Group 17: -1
Group 18:0

I would recommend looking up "oxidation number periodic table" to get a better picture of the above.

I typically think about it like this: group one elements have one electron, and to get an octet, it will be easier for them to remove that one electron, so it has an oxidation number of +1. Something from group 15, like nitrogen, have five electrons, and will typically want three electrons to complete its octet, so it will have an oxidation number of -3. That is how I remember oxidation numbers.

Hydrogen can either be +1/-1, and oxygen is usually -2, but can be -1 in peroxides.
Metals are typically harder to find out, but if you do the other elements first, you can figure it out by making sure all the charges cancel out.

Hope this helps

KarlaArevalo2F
Posts: 67
Joined: Tue Nov 14, 2017 3:01 am
Been upvoted: 1 time

Re: Determining oxidation number

Aurbal Popal wrote:
KarlaArevalo4D wrote:Is there an easy way to the oxidation number for ions like sulfate? This is the first time I take chem in my life (HS didnt offer it) so I feel behind in knowing oxidation numbers.

Group 1 has an oxidation number of +1
Group 2: +2
Group 13 (with Boron): +3
Group 14: +4/-4
Group 15: -3
Group 16: -2
Group 17: -1
Group 18:0

I would recommend looking up "oxidation number periodic table" to get a better picture of the above.

I typically think about it like this: group one elements have one electron, and to get an octet, it will be easier for them to remove that one electron, so it has an oxidation number of +1. Something from group 15, like nitrogen, have five electrons, and will typically want three electrons to complete its octet, so it will have an oxidation number of -3. That is how I remember oxidation numbers.

Hydrogen can either be +1/-1, and oxygen is usually -2, but can be -1 in peroxides.
Metals are typically harder to find out, but if you do the other elements first, you can figure it out by making sure all the charges cancel out.

Hope this helps

This is SOOO helpful! thank you !

becca_vandyke_4b
Posts: 30
Joined: Fri Sep 28, 2018 12:29 am

Re: Determining oxidation number

Memorize the oxidation numbers of the polyatomic ions like SO4 and use previous knowledge of ions on the periodic table's charge like Cl being -1 and then you deduce the charge of the transition metal since they can differ.