Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: xstrae on June 15, 2006, 12:04:15 PM

I came across this question in a book. can somoene help me out?
A metal oxide is reduced by heating it in a stream of hydrogen. It is found that after complete reduction, 3.15g of the oxide has yielded 1.05g of the metal. Which of the following is true?
(A): The atomic weight of the metal is 8
(B): The atomic weight of the metal is 4
(C): The equivalent weight of the metal is 4.
(D): The equivalent weight of the metal is 8.
Thanks! :)

Show what you did.

ok here goes.
I first looked at the choices. since A and B say atomic weight is 4 and 8 then the valency of the metal must be 2. According to this logic i framed the equation :
MO + H2 > M + H2O (M is the metal and x is the atomic mass)
3.15 1.05
x + 16 x
Using unitary method, i got x = 8.40
Therefore, option A must be the correct answer.
But the right answer is C.. can u please explain how?

How do you know oxide formula is MO? It could be any M_{x}O_{y}.

ya thats where the flaw is. I assumed its MO because atomic number 4 and 8 suggest that valency is plus or minus 2. How else can u do it?

Try to do it for "general" oxide M_{x}O_{y}  you will see that mass of M must be multpilication of some weight  this weight is just an equivalent.

hey borek.. isnt equivalent weight of a compound also defined as the number of parts by weight of a compound that combines with or displaces directly or indirectly 8 parts by weight of oxygen?
so in this case the metal's weight is 1.05g and since the metal oxide weights 3.15 grams, the weight of oxygen in the metal oxide is equal to 2.10g.
1.05 g of the metal > 2.10 g of oxygen
'x' g of metal > 8g of Oxygen
therefore x= 4.
is that correct? thanks for your help

OK, although there are much more general ways of finding answer, not based of very specific definition (which is not always true BTW  it fails for peroxides and superoxides).
http://en.wikipedia.org/wiki/Equivalent_weight

ok got it. thanks for your *delete me*