Homework Help: Chemistry

Posted by Val on Monday, March 8, 2010 at 11:16pm.

Consider the gas-phase equilibrium A ⇌ B. In a series of experiments, different initial
amounts of A and B are mixed together, and the mixture in each case is allowed to come to
equilibrium. Which one of these experiments would yield values for the amounts of A and B
present at equilibrium different from all the other experiments?
a. 3.0 moles A, 4.5 moles B
b. 4.5 moles A, 3.0 moles B
c. 1.5 moles A, 4.5 moles B
d. 7.5 moles A, no B
e. 0.5 moles A, 7.0 moles B

The answer is given as C....but I have absolutely no idea what is going on here....
No one has answered this question yet.

Chemistry - DrBob222, Tuesday, March 9, 2010 at 12:02am
I will do one of these and let you finish.
A........B
4.75....3.75 for experiment a.

If we start with A = 3.0 and B = 4.5, then we change by x. So A must increase to 3.0 + x and B must decrease by 4.5 - x. Set those equal to obtain
3.0+x = 4.5-x, solve for x and I obtained 0.75. Therefore, at equilibrium A will be 3.0 + 0.75 = 3.75 and B will be 4.5 - 0.75 = 3.75

If you go through the series like that, solving for x in each and put the equilibrium values in a table such as the one I started at the top, you will find that the equilibrium concns are 3.75 for A and 3.75 for B EXCEPT for C where they are 3.00 and 3.00. I must be honest and tell you that I've never seen a problem like this one before and I've been at this chemistry business for more than 65 years. Good luck to you.

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