# Ratio of valencies?

## Homework Statement

Copper forms two oxides. For the same amount of copper, twice as much oxygen was used to form the first oxide than to form the second one. What is the ratio of the valencies of copper in the first and second oxides?

(This is not my homework question. I am solving a book on Mole Concept and i am asking questions from this book.)

## The Attempt at a Solution

I am not able to understand how should i start?

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I like Serena
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Hi Pranav-Arora! To understand and solve problems like these, the first step is always to find out what all the words mean.
Usually the solution to the problem becomes self evident then.

In your problem the word "valency" is used.
Do you know what it means?

Do you know which copper oxides they're probably talking about?

Hi Pranav-Arora! To understand and solve problems like these, the first step is always to find out what all the words mean.
Usually the solution to the problem becomes self evident then.

In your problem the word "valency" is used.
Do you know what it means?

Do you know which copper oxides they're probably talking about?
Yes, i know what is valency. Copper exists in two states Cu+2 and Cu+1 i.e. the ratio of the valency is 2:1.
In the book also it is given 2:1 and a hint is also given "Assume that oxides are Cu2Ox and Cu2Oy."
I want to know is there no significance of "twice as much oxygen"?

I like Serena
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Yes, i know what is valency. Copper exists in two states Cu+2 and Cu+1 i.e. the ratio of the valency is 2:1.
In the book also it is given 2:1 and a hint is also given "Assume that oxides are Cu2Ox and Cu2Oy."
I want to know is there no significance of "twice as much oxygen"?
Right! Well, if we would make no assumptions about the copper states (and actually there are more than the 2 you mentioned), we could still give the ratio of the valencies due to the fact it uses "twice as much oxygen".
I think there is nothing more to it than that. ;)

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There is one more hint in the question that we can use "Rule 6".
In the book Rule 6 is:-

"For a compound MxNy, x moles of N = y moles of M"

I want to know how this equation is derived?

I like Serena
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There is one more hint in the question that we can use "Rule 6".
In the book Rule 6 is:-

"For a compound MxNy, x moles of N = y moles of M"

I want to know how this equation is derived?
That is not an equation.
It's like saying apples are equal to pears. ;)

I can only make wild guesses to what they mean.

What I can say is that in a compound like Fe2O3, you have Fe3+ and O2-.
So 1 mole of Fe2O3 consists of 2 moles Fe3+ and 3 moles O2- and their valencies ratio is 3:2.
But I suspect you already knew all that. That is not an equation.
It's like saying apples are equal to pears. ;)

I can only make wild guesses to what they mean.

What I can say is that in a compound like Fe2O3, you have Fe3+ and O2-.
So 1 mole of Fe2O3 consists of 2 moles Fe3+ and 3 moles O2- and their valencies ratio is 3:2.
But I suspect you already knew all that. The rule is "For a compound, MxNy, y*moles of M = x*moles of N"
For instance, we take Fe2O3, according to the rule, 3 * moles of O = 2 * moles of Fe.
I am not able to understand how this comes out?

I like Serena
Homework Helper
The rule is "For a compound, MxNy, y*moles of M = x*moles of N"
For instance, we take Fe2O3, according to the rule, 3 * moles of O = 2 * moles of Fe.
I am not able to understand how this comes out?
I think it's a typo.
Can I assume this is about the definition of a mole?

I think it should say something like:
1 mole of MxNy = x moles of M + y moles of N​

Or as a possible reaction:
x M + y N → MxNy
meaning you need x moles of M and y moles of N to create 1 mole of MxNy

Sorry, can't offer anything else.

Borek
Mentor
Assume Fe2O3 is made of Fe3+ and O2-. Molecules are neutral (in terms of charge). Do you see now where the "rule" comes from?

I think it's a typo.
Can I assume this is about the definition of a mole?

I think it should say something like:
1 mole of MxNy = x moles of M + y moles of N​

Or as a possible reaction:
x M + y N → MxNy
meaning you need x moles of M and y moles of N to create 1 mole of MxNy

Sorry, can't offer anything else.
There is no typo in the rule. My teacher proved it also but i forgot how he did it. Moreover, my teacher is not present currently in my city so i can't ask him also. Assume Fe2O3 is made of Fe3+ and O2-. Molecules are neutral (in terms of charge). Do you see now where the "rule" comes from?
Firstly, Congratulatios on becoming the PF admin. I still didn't get it.

I like Serena
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Assume Fe2O3 is made of Fe3+ and O2-. Molecules are neutral (in terms of charge). Do you see now where the "rule" comes from?
I still didn't get it.
@Borek: I don't get it either.
I can only see that the valency-ratio is the inverse of the mole-ratio....

Oh and sorry for my somewhat slighting comment on chemistry in the "I love physics" thread.
*Hides from Borek*

(Hope you don't mind seeing me here. It's my first post in chemistry! )

Borek
Mentor
Sorry, I obviously switched indexes. You are right - there is something fishy about that rule.

Sorry, I obviously switched indexes. You are right - there is something fishy about that rule.
So please tell me how can i prove it?