## Solving Cubics [ENDORSED]

Michelle Fu 1H
Posts: 35
Joined: Fri Sep 28, 2018 12:19 am

### Solving Cubics

I know in the module Dr. Lavelle solved ICE tables involving cubics using the fact the change in X is small because the K value is small. However, what if the K value was large? How would we go about solving the cubic then? Are we expected to know how to for the exam?

Chem_Mod
Posts: 17949
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 406 times

### Re: Solving Cubics  [ENDORSED]

For Chem 14B simplifying the cubic equation when K is small is part of the syllabus.

Solving cubic equations is not part of the syllabus. Therefore examples with large K and solving cubic equations will not occur in Chem 14B.

See my detailed Outlines.

Below is an example simplifying a cubic equation (which can be done whenever K < 10-3).
When K is smaller than 10-3 then X is much smaller than the initial concentration, [I], and therefore: [I] - X is approximately equal to [I].

Note: X is not zero.

But X is much smaller than [I] and therefore the difference is approximately the initial value.

For example, someone with \$1 million gives \$1,000 to someone with no money. The \$1,000 is meaningful (not zero) to the person receiving it.
But initial - change is essentially the initial (\$1 million - \$1,000 is approximately \$1 million).

In the above example the equilibrium constant is really small.