[johnnyjohn993123]

CaCO₃(s) CaO(s) +  CO₂(g)

Kp for this reation is 1.16 at 800°C. A 5.00 L vessel containing 10.0g of CaCO₃ was evacuated to remove the air, sealed, and then heated to 800C. Ignoring the volume occupied by the solid, what will be the overall mass percent of carbon in the solid once equilibrium is reached?

Attempt

Kp= 1.16
P =1.16 atm ; PV=nRT

n= 0.0659 mol CO₂ which is equal to 2.90 CO₂

10.0- 2.90

remaining solid weight : 7.1 g


Stoich

2.90 g   *   1mol CO2/ 44g  CO2  *   1 mol CaCO3 /1mol CO2  * 100.09 g CaCO₃/1 mol CaCO3

6.60g of CaCO₃  will be consumed.


7.1-6.60 = 0.5 g of CaO-- Is this logical right so far XD

6.60 g 1mol CaCO₃ / 100.09g  *  1 mol C/1mol CaCO₃  *  12.01g/1mol C

=0.79 g of C

Percent : 0.79/ 6.60 x 100 = 12.0%

Some source says it was 5% only pls help

[mjc123]

7.1-6.60 = 0.5 g of CaO-- Is this logical right so far XD

No. 6.6 g is the CaCO₃ consumed, not that left, which is what you should subtract from 7.1 to find CaO.
It is probably easier to work in moles until the end.
How many moles of CaCO₃ do you start with?
How many moles of CO₂ are produced?
How many moles of CaCO₃ are converted to CaO?
How many moles of CaCO₃ and CaO are now in the solid?
How many g of Ca, C and O? What is the percentage of C?

[johnnyjohn993123]

7.1-6.60 = 0.5 g of CaO-- Is this logical right so far XD

No. 6.6 g is the CaCO₃ consumed, not that left, which is what you should subtract from 7.1 to find CaO.
It is probably easier to work in moles until the end.
How many moles of CaCO₃ do you start with?
How many moles of CO₂ are produced?
How many moles of CaCO₃ are converted to CaO?
How many moles of CaCO₃ and CaO are now in the solid?
How many g of Ca, C and O? What is the percentage of C?

1.  0.099mol of CaCO3
2. AT equilibrium there are 0.0659mol CO2
3. 6.60 g will consumed right ? so 0.0659 mol CaCO₃
4. there will be 3. 70g of CaO produced right ? so 0.0660mol of CaO and there is 0.5 g of CaCO3 so there is 5.0X10^-3mol of CaCO3
5.i dont know how then ?
5.

[mjc123]

there is 0.5 g of CaCO3 so there is 5.0X10-3mol of CaCO3

How do you get that? You started with 0.099 mol, and consumed 0.0659 mol, so how much is left?
How many moles of Ca in 0.066 mol CaO + x mol CaCO₃? How many g Ca?
Same for O
Same for C
What is the mass percentage of C?

[johnnyjohn993123]

there is 0.5 g of CaCO3 so there is 5.0X10-3mol of CaCO3

How do you get that? You started with 0.099 mol, and consumed 0.0659 mol, so how much is left?
How many moles of Ca in 0.066 mol CaO + x mol CaCO₃? How many g Ca?
Same for O
Same for C
What is the mass percentage of C?

well since there will be 2.90 g of CO2 formation , therefore 6.60g of CaCO3 will be consumed. If there are 6.60g of CaCO3 consumption there will be 3.70g of CaO produced( doing my stoich) and that would give me 0.5g of CaCO3 left  ( 7.1 -6.60).  NOt only CO2 is produced in the decomposition of CaCO3 but as well as CaO , and that would affect the mass of CaCO3 at equilibrium.

as for Carbon masss percent:  0.5g of CaCO3 would give me  0.060g of Carbon which is 12% Carbon coming from CaCO3. 

Is this concept of mine even right? If not what part did I miss?

[mjc123]

Nonononononononono. You start with 10g, you consume 6.6g, so you have 3.4g left. You have 7.1g solid left, so you have 7.1 - 3.4 = 3.7g CaO. (You got that right earlier on, I don't see why you're getting it wrong now.)
How much C is there in 3.4g CaCO₃? (I'll tell you for nothing there isn't any in CaO.)
What is the % C in the total solid (that's what you're asked for, not just in CaCO₃)

[johnnyjohn993123]

Nonononononononono. You start with 10g, you consume 6.6g, so you have 3.4g left. You have 7.1g solid left, so you have 7.1 - 3.4 = 3.7g CaO. (You got that right earlier on, I don't see why you're getting it wrong now.)
How much C is there in 3.4g CaCO₃? (I'll tell you for nothing there isn't any in CaO.)
What is the % C in the total solid (that's what you're asked for, not just in CaCO₃)

3.4g CaCO3 /100.09g CaCO3 * 12.01g C/1mole C = 0.4080g  C (just some shortcut)

0.4080g/3.4g * 100 = 12%
 I still get 12% is this wrong?

[mjc123]

I repeat: What is the % C in the total solid (that's what you're asked for, not just in CaCO₃)
What is the total solid mass?
What is 0.408g as a percentage of this?

[johnnyjohn993123]

I repeat: What is the % C in the total solid (that's what you're asked for, not just in CaCO₃)
What is the total solid mass?
What is 0.408g as a percentage of this?

I dont get it. Why would you compute for  % C for the solid when CaO has no C in it? I dont see how can stoich can help with that one.

[mjc123]

Short answer - because the question asks you to. Why it does, who knows? But determining the amount of a particular element in a mass of solid that may contain several compounds is something that analytical chemists often have to do. E.g. a mining company might want to know how much metal they can recover from a sample containing the metal ore and other stuff.
Why would they botther with the rigmarole of the scenario if they just wanted the percentage of C in CaCO₃? That's a constant, it's always the same whatever the scenario.
I don't see the difficulty in working out 0.408g as a percentage of 7.1g. Do you?

[johnnyjohn993123]

OMG thanks ! <3