## Homework Question 11.63: a question on assuming

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jennifer_zhou2C
Posts: 22
Joined: Fri Sep 25, 2015 3:00 am

### Homework Question 11.63: a question on assuming

After following the ICE method, I reached an impasse with my calculator when I did the calculation: $x^{2}-(1.28\cdot 10^{-33})x^{2}$
and got 1 (which I should have known seeing as anything 10^{33} will be really close to 0).

Anyways, my question is when is the correct time to practice assuming that x must be close to 0?

Peiliang Zhou 3K
Posts: 21
Joined: Fri Sep 25, 2015 3:00 am

### Re: Homework Question 11.63: a question on assuming

According to my TA, first you have to continue the calculation based on the assumption that x=0. After you get a value for x based on that assumption, find the % dissociation of the chemical species from its initial concentration, and if that dissociation is less than 5%, your assumption is justified. For example, look at the equation: Cl2(g) <---> 2Cl(g) and K=1.2x10-7, Initial [Cl2]=0.0010M

For this equation, if we were asked to find the equilibrium concentration of Cl2, we'd make an ICE chart and put in initial concentration of 0.0010M for Cl2. After making the ICE chart, the equation to find x will then be 1.2x10-7=(2x)2/(0.0010-x). Since K is so small and we want to assume x=0, the new equation will be 1.2x10-7=(2x)2/(0.0010).

From this, we get x=5.5x10-6. Now, we can check if assuming that x is close to 0 was correct or not by calculating the % dissociation of Cl2, which is: %dissociation=Change in Concentration/Initial Concentration x 100%. Since the change of Cl2 was -x, the % dissociation would be: 5.5x10-6/0.0010 x 100%=0.55%. This value is less than 5% and thus it was justified to assume x is close to zero.

Hope this helps!

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