# Gen Chem Questions

#### Dentalstu02

##### Member
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15+ Year Member
I have 3 questions about gen chem from the topscore exams. Hopefully they are not hard for those of you who have gone over topscore already. I missed them on the exam however, please help!!

1) When 14.250 moles of PCl5 gas is placed in a 3.00 liter container and comes to equilibrium at a constant temperature, 40.0% of the PCl5 decomposes according to the equation:

PCl5(g) = PCl3(g) + Cl2(g).

What is the value of Kc for this reaction?

2) Given that the standard enthalpy of formation for NO(g) is 90.25 kJ/Mole, calculate the free energy change for the following reaction at 25oC.

N2(g) + O2(g) = 2 NO(g)

The absolute entropies of N2 is 192, NO is 211, and O2 is 205...

3) BaCO3(s) + HCl(aq) (What is the net ionic equation??)

Any help would be appreciated...thanks!

#### ohgee

##### Senior Member
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Kc which is the same as Keq (i think)

will be [Cl2][PCl3]/[PCl5]

the question says 14.25 mol of PCl5 in 3L solution, so 4.75 M of PCl5 will be reacted.

It says 40% will decompose to PCl3 and Cl2, so 4.75 M of PCl5 will dissociate to 1.9 M of PCl3 and 1.9M of Cl2 (4.75*40%)

But 60% remain as PCl5, so 4.75*60%=2.85M of PCl5

okay, so the Keq will be [1.9][1.9] / [2.85]

#### ohgee

##### Senior Member
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15+ Year Member
for # 2

the free energy equation is
<G = <H- T<S

< is change

okay, well, you got 2M of NO as productso 2*<Hf of NO will be 180.5 (that is your new <H)

Temperature = 273+25 = 298
<S = <S product - <S reactant

therefore, <S = S of 2NO - (S of O2 + S of N2)

so, 2*211- (192+205) = 25 (your <S)

for #3,

i am not sure so i am not going to write anything

hope this helps, good luck studying

#### Dentalstu02

##### Member
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15+ Year Member
Thank You soo much Ohgee, the first one is correct and I figured out what I was doing wrong based on what you said....

#### ohgee

##### Senior Member
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15+ Year Member
something went wrong, so here it is again...

for # 2

the free energy equation is
<G = <H -T<S

< is change

okay, well, you got 2M of NO as product so 2*90.25 = 180.5

Temperature = 273+25 = 298

entropy is the entropy of the product minus the reactants, since your have 2 M of NO you need to multiply it by 2...

therefore, 2*211- (192+205) = 25 (your <S

oh, since the <H is in KJ, you need to convert T<S to KJ as well..
<G = 180.5- (298)(25)(1/1000)

#### ohgee

##### Senior Member
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15+ Year Member
oh well, it doesn't show,
just look into your G-chem book for the question, and remember to convert to the correct units!

good luck

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