[350chev]

Well I just started Chemistry again in college, but my memory isnt holding up too well after 2 years of doing nothing chemistry related. It is a fairly basic question which has me stumped. We use web assign so those of you who are familiar with it know how horrid and demotivating those red X's can be.

Anyway here is the question.

Upon strong heating, Cu2(CO3)(OH)2 or Cu3(CO3)2(OH)2 decomposes to leave CuO(s) in the heating crucible. Upon heating of 1.102 g of sample, 0.792 g remains. Calculate the %Cu in the sample.

Well what I am getting at is that you need %Cu/CuO X 1.102. Or %Cu/.792 X 1.102.

However that is wrong and I cannot find another way around it. Hopefully someone can point me in the right direction. I am not here for an answer as I am for the procedure.

Thanks guys!

[Borek]

You need mass of copper in the sample - that will be the same as mass of copper in CuO.

[350chev]

You need mass of copper in the sample - that will be the same as mass of copper in CuO.

So to find the mass of copper in the remaining sample I have to.

1. Know that .792 is my total mass of CuO

2. Then know that in CuO Cu=79%~ and O=21%~

3. Calculate 79% of .792 for the answer? So my answer would be .62568g?

[Borek]

Not yet, but you are close. What you have is a copper mass - you should calculate what percentage of 1.102g it was.

Note: it is not 'about 79%' - you should calculate it much more precisely using exact molar masses.

[350chev]

Not yet, but you are close. What you have is a copper mass - you should calculate what percentage of 1.102g it was.

Note: it is not 'about 79%' - you should calculate it much more precisely using exact molar masses.

Right I just kept it short for post reference. So my calculation would be: .79 X 1.102 = .870 which would be what it was in 1.102?

[Borek]

No.

What is mass percentage definition?

What is mass of copper in CuO?

[350chev]

No.

What is mass percentage definition?

What is mass of copper in CuO?

The mass percentage is the elements percent of taken mass in a compound.

The mass of copper in CuO is 63.55.

The copper mass percent would be (63.55/(63.55+16))X100.

[Borek]

I am not asking about percentages, I am asking about exact mass of copper in the 0.792 g of CuO.

Once you know that - does it differ from the mass of copper in the original sample?

And this will be the moment to apply - correctly - mass pecrentage definition.

[350chev]

I am not asking about percentages, I am asking about exact mass of copper in the 0.792 g of CuO.

Once you know that - does it differ from the mass of copper in the original sample?

And this will be the moment to apply - correctly - mass pecrentage definition.

Ok so my original sample of Cu2(CO3)(OH)2 has a total of 221.1

Cu2
O5
H
C
=
221.1

Do I calculate the (OH)2 as just 16+1 or 32+2?

And CuO as 79.55.

So my original sample had 1.102 and I found that it had .6334 Cu

My remaining sample had .6327 Cu

Seems like there was too little of a difference...

Or is it supposed to be the same and I just had rounding error? If so what is the next step?

[Borek]

Do I calculate the (OH)2 as just 16+1 or 32+2?

Either 32+2 or 2(16+1)

So my original sample had 1.102 and I found that it had .6334 Cu

No. You don't know what was formula of the compound in the original sample, it was one of two given - either Cu₂(CO₃)(OH)₂ or Cu₃(CO₃)₂(OH)₂. No reason to select one and use it for calculations at this stage.

My remaining sample had .6327 Cu

Yes.

You started with 1.102 g of SOME substance, and you now know it contained 0.6327 g of copper. Apply definition of mass percentage to these numbers.

[350chev]

Do I calculate the (OH)2 as just 16+1 or 32+2?

Either 32+2 or 2(16+1)

So my original sample had 1.102 and I found that it had .6334 Cu

No. You don't know what was formula of the compound in the original sample, it was one of two given - either Cu₂(CO₃)(OH)₂ or Cu₃(CO₃)₂(OH)₂. No reason to select one and use it for calculations at this stage.

My remaining sample had .6327 Cu

Yes.

You started with 1.102 g of SOME substance, and you now know it contained 0.6327 g of copper. Apply definition of mass percentage to these numbers.

So my original sample of something had .6327g of copper. I thought I had applied that definition to obtain .6327? I thought at this point I just needed to find the difference...

[Borek]

Calculate the %Cu in the sample.

All copper in the sample made its way to the copper oxide.

[350chev]

Calculate the %Cu in the sample.

All copper in the sample made its way to the copper oxide.

Well if everything made it over then Cu in CuO is 79.88% would it be the same for the beginning of the process?

[350chev]

Calculate the %Cu in the sample.

All copper in the sample made its way to the copper oxide.

Well if everything made it over then Cu in CuO is 79.88% would it be the same for the beginning of the process?

Well I finally got it... It was a simple two step process... Mass of Cu in sample = product of CuO x % Cu in CuO then to get the % of Cu in sample = mass of cu in sample/mass of reactant x 100...

I did miss the sig fig though I cant stand those.

[Borek]

0.6334g / 1.102g * 100%

You were circling around from the very begining.