Topscore Test I Chemistry

[Awuah29]

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Heh guys,
Need quick help with those problems from Topscore Test I Chemistry

Question 50
Answer is A
I know the answer is A, but where does the 298 come from?
I applied the free energy change formula, but don?t get the answer.

Question 56
Answer is A
Again I don?t get the answer. I know to write the equil. constant, but don?t get the answer when I calculated it.

Can someone give me the right formulas to those problems or maybe someone has the solution written step by step. Thanks

 

[drat]

If it's not too much hassle, can you post the questions so those of use who uninstalled Topscore on our computers can help you out??

 

[ScottW3]

I just retook topscore 1 and remember your first question. The reaction takes place at 25 C, so that is 298 K.

 

[Awuah29]

When 14.250 moles of PCL5 gas is placed in a 3.00 liter container and comes to equilibrium at a constant temperature, 40 5 of the PCL5 decomposes according to the equation:

PCL5 (g) forms PCl3(g) + Cl2 (g)

What is the value of Kc for this reaction?
The answer is (1.896) 2 / 2.854

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 25 C

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

Absolute Entropies J/Mole K
N2 (g) 192
NO 211
O2 205


The answer here is again A
180.5 ?(298)(25)(1x10-3)

Again where does the 298 come. I know that 25 C is the temp, but the 298?

 

[weiy01]

Im not too sure about the first problem, but I'd like to take a shot at the latter.

To determine free energy, use delta G=(delta H)-(T)(delta S).

Given that delta H=90.25kJ/mole, multiply by 2 since there are 2 moles in the equation. so 180.5kJ/mole.

In this case, T must always be in units of Kelvin, so K=273+25 celcius=298K

Solving for delta S= products - reactant= ((2)(211))-(205+192)= 25 J/molK. (This 25 J/molK has nothing to do with the given temperature 25 Celsius. ) Then, you convert 25 J/molK to kj/molK by multiplying by 10^-3 to keep units consistent.

Plug in your values you get ...

delta G = (180.5 kJ/mole) - (298 K)(25 J/molK)(10^-3 kj/J)

and whoalah! hope this helps 🙂

 

[jheidenr]

For Topscore exam I, #56....

Here are the essentials that you need to know:

Kc=[PCl3][Cl2]/[PCl5]

It says that there wre originally 14.25 moles of PCl5, but 40% decomposed. So 14.25*.4 = 5.7 moles. The remaining moles of PCl5 is now at (14.25-5.7) = 8.55 moles

Now you have the moles for each of the products and reactant, but you still have to convert them into concentrations (this is where I got hung up):

5.7 moles PCl3 / 3 liters = 1.9 M PCl3
5.7 moles Cl2 / 3 liters = 1.9 M Cl23
8.55 moles PCl3 / 3 liters = 2.85 M PCl3

So Kc=[PCl3][Cl2]/[PCl5] = [1.9]^2/2.85

Hope this helps!

 

[Rezdawg]

Ive got a couple questions that were on this test...

I suck at naming anions/cations, acids, etc...

So, one question on this test asked, "Which of the following is named incorrectly?" The answer choices were:
A) HClO ; hypochlorous acid
B) HClO2 ; chloric acid
C) HClO4 ; perchloric acid
D) HNO2 ; Nitrous Acid
E) HNO3 ; Nitric Acid

The answer is B. Is this because B should be named "chlorous acid"?

---------------------------

Another question asked, "Which would not be considered a "chlorate"?"
A) ClO2(-)
B) ClO3(-)
C) ClO4(-)
D) ClO4(2-)
E) All are chlorates

The answer is D. Is it because the Lewis Structure of ClO4(2-) is bogus? Basically, if you are going to have ClO4, it needs to have a charge of minus 1...right?

-----------------------------

Now, my question is, how and where do I learn these? I know the Kaplan book has a page that lists some similar stuff, but even then, I manage to mess up these types of questions. Advice/tips are greatly appreciated.