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# How to deduce the molecular formula of a compound? watch

1. In questions such as:

"In the extraction of iron, carbon monoxide reacts with iron(III) oxide. Write an equation for this reaction."

"Write an equation for the reaction of bromine with cold aqueous sodium hydroxide."

How are we supposed to know the molecular formula of iron oxide, for example, and of the products that are formed?
Is it something we just need to learn or is there a way to deduce it somehow (or am I missing something here)?
This doesn't apply to these questions specifically, just "Write an equation" questions in general.

Many thanks.
2. (Original post by Productivity)
In questions such as:

"In the extraction of iron, carbon monoxide reacts with iron(III) oxide. Write an equation for this reaction."

"Write an equation for the reaction of bromine with cold aqueous sodium hydroxide."

How are we supposed to know the molecular formula of iron oxide, for example, and of the products that are formed?
Is it something we just need to learn or is there a way to deduce it somehow (or am I missing something here)?
This doesn't apply to these questions specifically, just "Write an equation" questions in general.

Many thanks.
Iron (III) oxide: the (III) bit indicates that Fe has an oxidation state of +3. You know (well you have to remember/memorise that oxygen has an oxidation state of -2).

Remember the charges have to cancel out/be the same since there's no overall charge given

As Fe has an oxidation state of +3, and Oxygen -2, you need 3 oxygens and 2 iron for both of the charges to be equal (+6 -6 = 0) so therefore Iron (III) oxide = Fe2O3
3. (Original post by KINGYusuf)
Iron (III) oxide: the (III) bit indicates that Fe has an oxidation state of +3. You know (well you have to remember/memorise that oxygen has an oxidation state of -2).

Remember the charges have to cancel out/be the same since there's no overall charge given

As Fe has an oxidation state of +3, and Oxygen -2, you need 3 oxygens and 2 iron for both of the charges to be equal (+6 -6 = 0) so therefore Iron (III) oxide = Fe2O3
Oh, that was much easier than I anticipated!
Thanks for the explanation, it's quite worrying that I didn't know this, with 8 weeks left until exams
4. For the second reaction, I would suspect that it is as specific example on your exam board because the equation for sodium hydroxide and bromine is :
.
5. (Original post by B_9710)
For the second reaction, I would suspect that it is as specific example on your exam board because the equation for sodium hydroxide and bromine is :
.
Ah, yes.
I just checked in the textbook and there is the equation for that there, I must have skipped over it without realising
Thank you!

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