topscore DAT gen chem test 1

[xtra333]

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I have a question on the following topscore DAT gen chem question. The question is from topscore test 1 number 56 on the most recent edition. when 14.25 moles of PCl5 is placed in a 3 liter container and comes to equilibrium at constant T, 40 percent of the PCl5 decomposes according to the equation:

• PCl5(g)<=> PCl3 (g) + Cl2(g)
• The question asks me to find K(c)
• I have a problem with their answer to the question. I interpreted the 40 percent of the PCl5 as the amount in equilibrium in the ICE chart, so the concentration of PCl5 at equilibrium is 1.896 M. But in the answers the 1.896 mol/L is apparently the change part of the ICE part, so the answers said k(c)=2.854^2/1.896
• K(c)=1.896^2/2.854
• But I believe the K(c) expression I posted above is correct.

The question depends on your interpretation of the 40 percent then, I suppose?

 

[xtra333]

Bump I guess. Can anyone answer as to why their answer is correct?

 

[ROCOCO8]

... i am getting the same answer as you. i wonder why no one else has asked this question before.

 

[Neil45]

BUMP. I'm stuck on this problem as well.

 

[ttpharm]

Initial [PCl5] = 14.25 mol / 3 L = 4.75 M

40% of this decomposes, and for every molecule of PCl5 that decomposes you get one molecule of PCl3 and one molecule of Cl2.

4.75 M x 40% = 1.9 M reacted

This gives 1.9 M PCl3 and 1.9 M Cl2 and leaves 2.85 M PCl5 (= 4.75 M - 1.9 M).

Kc = [1.9][1.9]/[2.85]

 

[ttpharm]

BTW, you might have an older version because mine has the right answer which is what we posted.