5% Rule

[305693278]

Hi, I was doing the HW and on questions 2 and 3 even though Ka was less than 10^-3 the x in the denominator of the Ka calculation cannot be dismissed. I know that if the x is more than 5% of the initial concentration it cannot be dismissed in the denominator, but is there a way to check this without doing all of the calculations to find x first?

[Dominic Pham 3A]

From my understanding, I believe the approximation rule is when you check to see if the Percent Ionization is below 5% in order to deem x in the denominator as negligible. When checking for this, you would have to do the full calculations to be sure.

[Siena Bertacco 1G]

Hi, I believe the 5% rule pertains to percent ionization, and in order to do this, it would be very beneficial to fully complete the calculations without disregarding any variables or pieces of information to help you retrieve a more accurate result.

[Tucker Gruber 2G]

Unfortunately, there is no way to confirm the 5% Rule without going through all of the calculations and then checking back to ensure that the percent ionization is not more than 5%. That said, while we do not have a graphing calculator on exams and need to know how to use the quadratic formula, I would recommend also familiarizing yourself with solving for x by graphing. This is a much shorter method than the quadratic formula and does not require you to disregard x (so no need to worry about the 5% Rule), and in the future in non-exam settings, such as calculating molarities to use in lab settings, you may find it easiest and most efficient to use (in that it is quick but still accurate). Hope that helps!

[Rebecca Liu 2E]

Yes, I think I would go with the rule that if K is less than 10^-3, try the problem with the assumption rule. When you do the entire calculation and get your final answer, then check again that x (percent ionization of acid, for example) is less than 5%. If it's not, I would try doing the calculations again without the assumption rule just to be safe.

[joannar]

Yes as mentioned from previous responses, you would first have to go through the whole calculation to then determine whether the percent ionization is less than 5% !

[Alan Lee 2I]

The 5% rule is just something for you to check to see if your assumption/ approximation of an initial concentration - x is the same as the initial concentration since the K value is very small < 10^-3. If it's below 5% then the approximation is fine, if not then you can't approximate

[Vivek Punn 1E]

I agree with pretty much everything said previously, I'd also add that Professor Lavelle said that most likely for the exam questions this would not be an issue. However it is something that we should all get accustomed to checking. Obviously making this assumption results in an approximation of the true answer, but if the % ionization is too high, the assumption makes this approximation significantly different enough to force us to backpedal a bit and do the quadratic equation ultimately.

[Joyce Lee 1K]

Please correct me if I am wrong, but I believe that in order to find if the 5% rule is valid for a certain reaction you have to compare the percent ionization and check if it is considered a small enough value. Thus, you would have to complete all of the calculations for that.

[Shania Hao 3L]

Yes, I believe you would have to go through the whole calculation in order to use the 5% rule at the end because you are checking the end result of your calculated value to see if the approximation is small enough to be used without having to go through the whole process with the quadratic formula.

[Danielle Paisley 1H]

The 5% is simply there to allow you to check if the approximation of the initial concentration is equal to the initial concentration due to the small value of K (under 10^-3). When it is below 5%, the approximation is appropriate.

[Hayden Kim 1G]

I don't believe there is a way to check 5% without going through the entire calculation but since the approximation is very quick it shouldn't use up too much of your time.

[mieke van daelen 2I]

If Ka is less than 10^-3 it is more likely that you can use the approximation, however you still need to check the percent ionization to see if the approximation was ok to use. There really is no way to make sure before you approximate, so if you are unsure whether or not you can approximate you can always just do the entire problem without an approximation (which is more math, but you only have to do it once). The approximation is meant to cut down on math (when the percent ionization is low enough) you don't have to do it in order to do the problem.

[Nishanth Vemuri 2L]

The 5% rule is only for the ionization of a weak acid/base. If we check the percent ionization of a compound and if it is less than 5%, then the estimation we have done is correct (omitting the x from [for example, 0.10 - x] if K is less than 10^-3) and we can calculate pH from there. However, if the ionization turns out to be greater than 5%, then we must go back and do our calculation without omitting the x from 0.10 - x. This is because x is not negligible in this case, and we must calculate the true ionization.

[Sydney Tanimitsu 2B]

in order to use the calculation that used an approximation then the percent ionization must be lower than 5%, if the percentage is greater then you will need to go back and do the calculation without an approximation

[Daniel Nguyen 2L]

Hello,
the 5% rule definitely skips most calculations, (solving for x using the quadratic formula), only if the x value is less than 5%. This is for determining percent ionization if your x-value does not change significantly for your weak base or acid. Ultimately, doing the whole calculation ensures there is still a change, no matter how small, in order to get more accurate results.

[Reese R 1E]

The 5% rule shortens the calculations in the case of a K value below 10^3 as you can assume (value -x) = value, but you have to solve for x in order to check that the percent ionization is less than 5%.

[Kush Brahmbhatt 2K]

The 5% rule pertains to percent ionization, and in order to determine whether the percent ionization is greater than 5% or not, the full calculation would have to be done.

[Samuel Vansomphone 1H]

There is no true way to make sure before you calculate. The only way to truly confirm it is to do the full calculation.

[506026459]

Although K might've been below 10^-3, always check with the 5% rule to make sure you can use the approximation. You can only find out your percent ionization after going through all the calculations.

[Miyu Tang Dis3I]

I think a general rule of thumb is that you can assume K to be a very very small number if 10^-5 or more. Therefore when it is 10^-4 or less you should find the equilibrium concentration using the quadratic formula. But if not then you can assume it is small enough to disregard the -x in the denominator when finding the equilibrium concentration by solving for x.

[Yasmine_B_1H]

Hello,

Dr. Lavelle shared in class that the best rule to follow is the 5% rule. Once you have completed the ICE table you can plug in the numbers into the percent ionization equation. The change of x over the initial value of the substance, multiplied by 100. If this percentage is less than 5% then it is okay to approximate you K constant value.