2013 DAT Destroyer G Chem #235

[Tajlrak]

In the answer key part of the way to get to the answer is 2x=1, x=0.5 which is what is confusing me.

N2+3H2 -><- 2NH3
8 moles of N2 and 8 moles of H2 are placed in a 2 liter flask and allowed to come to equilibrium. At equilibrium, 2 miles of NH3 are formed. Calc Keq.

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[BrutalViking]

Sometimes Destroyer over-complicates the solution. Here's how I would solve/explain:
Look at this formula and understand it N2+3H2-->2NH3. The question states that at equilibrium, we only have 2 moles of NH3.
The number of moles of nitrogen at equilibrium is (8-1) 7. Because one mole of N2 makes two moles of NH3. Thus, molarity of N2 is 7/2 (moles/liters)=3.5M
The number of moles of hydrogen at equilibrium is (8-3) is 5.Because 3 moles of H2 are required to form the two moles of NH3. Thus the molarity is 5/2=2.5M
Finally, the molarity of the product (NH3) is 2/2=1M

Now set up the Keq formula:
Keq=[product]/[reactant]; remember that the coefficents of the reactions (in this problem) are the powers for each concentration
Keq=[1]^2/[3.5]*[2.5]^3

 

[Tajlrak]

Sometimes Destroyer over-complicates the solution. Here's how I would solve/explain:
Look at this formula and understand it N2+3H2-->2NH3. The question states that at equilibrium, we only have 2 moles of NH3.
The number of moles of nitrogen at equilibrium is (8-1) 7. Because one mole of N2 makes two moles of NH3. Thus, molarity of N2 is 7/2 (moles/liters)=3.5M
The number of moles of hydrogen at equilibrium is (8-3) is 5.Because 3 moles of H2 are required to form the two moles of NH3. Thus the molarity is 5/2=2.5M
Finally, the molarity of the product (NH3) is 2/2=1M

Now set up the Keq formula:
Keq=[product]/[reactant]; remember that the coefficents of the reactions (in this problem) are the powers for each concentration
Keq=[1]^2/[3.5]*[2.5]^3

Thanks for the help. Ya I dont think I saw this example in Chads material(someone correct me if I wrong). Sometimes I like to understand my questions more than one way. In that case you dont happen to know what the 2x=1 in the solution meant do you?

 

[BrutalViking]

The way they solved it in destroyer was by using the ICE chart. X in their solution represented change in concentration. What they did was set up +2x (change in NH3 concentration) equal to the molarity of NH3 at equilibrium. Thus: 2x=1 orrrr 1x=0.5. Notice they were able to do this because they already knew the molarity for NH3 at equilibrium. That's the way I understood it. Could be wrong. Could be right.

 

[Tajlrak]

The way they solved it in destroyer was by using the ICE chart. X in their solution represented change in concentration. What they did was set up +2x (change in NH3 concentration) equal to the molarity of NH3 at equilibrium. Thus: 2x=1 orrrr 1x=0.5. Notice they were able to do this because they already knew the molarity for NH3 at equilibrium. That's the way I understood it. Could be wrong. Could be right.

See that is what I figured as well, its just I never used the ICE chart like that which is what threw me off.

 

[BrutalViking]

Live and learn I guess. Good luck with the rest!

 

[Tajlrak]

Live and learn I guess. Good luck with the rest!

Likewise!

 

[Tajlrak]

The way they solved it in destroyer was by using the ICE chart. X in their solution represented change in concentration. What they did was set up +2x (change in NH3 concentration) equal to the molarity of NH3 at equilibrium. Thus: 2x=1 orrrr 1x=0.5. Notice they were able to do this because they already knew the molarity for NH3 at equilibrium. That's the way I understood it. Could be wrong. Could be right.

And they divided everything by 2 because its a 2 liter flask? Or bc they made 2 moles of NH3 like you said? Bc in the answer key you can see it says 2/2=1M so that is from the 2 liter flask?

 

[BrutalViking]

Yes. You are trying to find molarity so: M=moles/liter
So the 2 is for the liters.