Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: 350chev on October 10, 2009, 06:44:09 PM

Title: Problems with mass % and calculations.
Post by: 350chev on October 10, 2009, 06:44:09 PM
c) Calulcate the mass of KClO3 that contains 0.386 g oxygen.
.621g
d) If a KClO3/KCl mixture weighing 1.67 g loses 0.386 g upon heating, what is the % KClO3 in the mixture?
  %


Ok so I got C wrong after 5 tries. Obviously I am doing something way wrong. This is relevant to my other question in another thread. I was hoping someone could walk me through the mass percent calculations because I have been sitting here for about an hour and have no idea how to even start them properly.

What I WANT to do is find how much oxygen left the mixture for D because that is what separates and leaves but It is asking for KCLO3 which I cant compute.
Title: Re: Problems with mass % and calculations.
Post by: UG on October 10, 2009, 07:00:11 PM
For c, how many moles of O is there in 0.386 grams?
Title: Re: Problems with mass % and calculations.
Post by: 350chev on October 10, 2009, 08:44:46 PM
Quote from: UG on October 10, 2009, 07:00:11 PM
For c, how many moles of O is there in 0.386 grams?

Well if 1 mole of O is 16g. So .386/16=.024125?
Title: Re: Problems with mass % and calculations.
Post by: UG on October 10, 2009, 10:45:10 PM
Yep, good. And how many moles of K and Cl is there in KClO3 for every one mole of oxygen? That was worded a bit awkwardly  :-\
Title: Re: Problems with mass % and calculations.
Post by: 350chev on October 10, 2009, 10:52:47 PM
Quote from: UG on October 10, 2009, 10:45:10 PM
Yep, good. And how many moles of K and Cl is there in KClO3 for every one mole of oxygen? That was worded a bit awkwardly  :-\

Ok.

K= .00987212

Cl= .01088858

Thats 1 mole in .386.

How would I find it in terms of 1 mole of oxygen?
Title: Re: Problems with mass % and calculations.
Post by: UG on October 10, 2009, 10:57:46 PM
I don't know what you were doing above but the main principle here is that 3 moles of O reacts with 1 mole of K and 1 mole of Cl to produce 1 mole of KClO3 right? Are we clear with this point?
Sooo....following on from this principle, 0.024125 moles of oxygen will react with 0.024125/3 moles of K and 0.024125/3 moles of Cl to produce 0.024125/3 moles of KClO3. Are we fine with this?
From here you just multiply the number of moles by the molar mass of KClO3 to get the mass.
Hope this helps.  :)
Title: Re: Problems with mass % and calculations.
Post by: 350chev on October 10, 2009, 11:37:11 PM
Quote from: UG on October 10, 2009, 10:57:46 PM
I don't know what you were doing above but the main principle here is that 3 moles of O reacts with 1 mole of K and 1 mole of Cl to produce 1 mole of KClO3 right? Are we clear with this point?
Sooo....following on from this principle, 0.024125 moles of oxygen will react with 0.024125/3 moles of K and 0.024125/3 moles of Cl to produce 0.024125/3 moles of KClO3. Are we fine with this?
From here you just multiply the number of moles by the molar mass of KClO3 to get the mass.
Hope this helps.  :)

Ok so Cl and K had .00804167 moles in KClO3

So the molar mass which is (.00804167)2+.024125? I multiplied by the number of moles. Where is this number of moles?

Its helping some I just wish there was a standard formula for these kinds of problems. I was hoping to get D out of the way but understanding C is also important. Since I already missed C I REALLY need to get D for the overall pre lab grade haha
Title: Re: Problems with mass % and calculations.
Post by: UG on October 10, 2009, 11:51:31 PM
No, for the molar mass, you gotta look on your periodic table. The atomic weight is what you are looking for.
Add up these values (ie, 1 K + 1 Cl + 3 O) and this your molar mass, then multiply this by the moles
Title: Re: Problems with mass % and calculations.
Post by: 350chev on October 10, 2009, 11:56:03 PM
Quote from: UG on October 10, 2009, 11:51:31 PM
No, for the molar mass, you gotta look on your periodic table. The atomic weight is what you are looking for.
Add up these values (ie, 1 K + 1 Cl + 3 O) and this your molar mass, then multiply this by the moles


But my moles vary?

So my molar mass is 122.55 for KClO3. What moles would I multiply this by?

Also for D I have the % of oxygen at 39.167697 and the difference being 1.284 since .386 oxygen dissolved. I just cant figure out how the calculation for the % KClO3 in the original mixture.
Title: Re: Problems with mass % and calculations.
Post by: UG on October 11, 2009, 12:02:40 AM
Multiply 122.55 by the number of moles of KClO3
Remember earlier we got 0.024125 moles of oxygen will react with 0.024125/3 moles of K and 0.024125/3 moles of Cl to produce 0.024125/3 moles of KClO3, you've just worked this number out, .00804167 so multiply the two together
Title: Re: Problems with mass % and calculations.
Post by: 350chev on October 11, 2009, 12:29:35 AM
Quote from: UG on October 11, 2009, 12:02:40 AM
Multiply 122.55 by the number of moles of KClO3
Remember earlier we got 0.024125 moles of oxygen will react with 0.024125/3 moles of K and 0.024125/3 moles of Cl to produce 0.024125/3 moles of KClO3, you've just worked this number out, .00804167 so multiply the two together

So 122.55 x 5 = 612.75

then

612.75 x .00804167 = 4.927

So that would have been C?
Title: Re: Problems with mass % and calculations.
Post by: UG on October 11, 2009, 12:44:30 AM
Where did the 5 come from?
You will have 122.55 x 0.00804167 and get something like 0.986. This is the answer to c, the mass of KClO3 which contains 0.386 g oxygen is 0.986 grams.