J.R. S. answered • 10/27/19
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A ==> 3B
2.7......0....Initial
-x......+3x..Change
2.7-x...3x..Equilibrium
Kc = 1.28x10-6 = [B]3 / [A] = (3x)3 / 2.7-x
1.28x10-6 (2.7-x) = 27x3
the rest is algebra and solving the cubic equation for x. Then 3x will give you the equilibrium concentration of B
Evey Y.
Update, I used your method but instead just used 2.7 for [A] and I got the answer! Thank you. For those who are curious, the answer is 0.015 = 3x
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10/28/19
J.R. S.
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Yes, all of that is correct. The general rule of thumb is if x is less than 5% of the original concentration, you can neglect it. If 3x = 0.015, then x = 0.005 and that is only 0.19% of 2.7 so can be neglected.
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10/28/19
Evey Y.
In class we learned that because the Kc value is so small we also assume x is small, so we use 2.7 as [A] instead of 2.7-x. Is this correct?10/28/19