## Calculations using ice box

Katie 1E
Posts: 21
Joined: Fri Sep 29, 2017 7:04 am
Been upvoted: 2 times

### Calculations using ice box

When you're using ice box and you are solving for the value of x and you get to the point where you have initial concentration minus x in the denominator of the equilibrium equation, how do you know when you can ignore the x? In what cases is the x negligible and how do you know? Sorry if this was confusing

Julia Meno 1D
Posts: 28
Joined: Fri Sep 29, 2017 7:04 am

### Re: Calculations using ice box

Usually if the equilibrium constant K is less than 10-3, the addition or subtraction of x to/from your initial value is negligible. But keep in mind that you can only "ignore" x when it is being added to or subtracted from a constant. In other words, if K < 10-3 and it is equal to (x)(x)/(1.00 - x), I can ignore the x in the denominator but not the x values in the numerator.

Ishan Saha 1L
Posts: 60
Joined: Fri Sep 29, 2017 7:03 am

### Re: Calculations using ice box

You can also use the 5% rule, which states that if the value of x is less than or equal to 5% of the initial, then you can assume the equilibrium amount to be approximately the same value.

diangelosoriano
Posts: 49
Joined: Fri Sep 29, 2017 7:07 am

### Re: Calculations using ice box

Is there a time to use the 5% approximation versus using the value of K to determine if x is negligible?

Nha Dang 2I
Posts: 53
Joined: Fri Sep 29, 2017 7:07 am

### Re: Calculations using ice box

Usually just seeing if K < 10-3 is sufficient, but if you want to be extra certain, you can use the 5% rule. I think either works.