The 5% rule for weak acids and bases
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This rule applies when calculating the pH of weak acids and bases. By the way, if you do not know how to calculate the pH of a weak acid or a weak base, please go here for weak acids and here for weak bases.
The key point is that Ka and Kb values are typically known only to an accuracy of ±5%. So it is reasonable to make approximations if they remain within the ±5% range of the true answer. (And remember, the true answer is not within our grasp. Another way to view the 5% is that the Ka and Kb values have a built-in error and that the true answer remains hidden within that ±5% error.)
So, the 5% Rule follows. I'll state it for acids and comment on the parallel expression for bases further down.
If the expression
(x / [HA]o) (100)
is less than or equal to 5%, we consider the approximation valid.
Now, for those of you that are totally confused, my recommendation is to go back and study the Ka and Kb links provided above.Here's a brief description of the above expression:
'x' is the value being calculated in weak acid (or a weak base) calculation. 'x'' is the [H+], the value that will give us our pH[HA]o is the original starting concentration of whatever weak acid was used in the problem. This value is generally provided in the problem.
The approximation stems from the term '[HA]o − x.' The '− x' portion is dropped as part of the problem solution, thus resulting in an approximate answer.
By the way, what is being calculated just above is called the 'percent dissociation.'
For bases, the x represents [OH¯] and the orginal starting concentration would be represented [B]o. In the Kb calculations, the term '[B]o − x' would have the '− x' dropped.
The 5% Rule for bases is:
If the expression
(x / [B]o) (100)
is less than or equal to 5%, we consider the approximation valid.
In calculating the pH of a weak acid or a weak base, use the approximation method first (the one where you drop the '− x').
Then apply the 5% rule.
If you exceed 5%, then you would need to carry out a calculation that does not drop the '− x.' This would result in quadratic equation, which would be solvable. It would just be tedious. Here is an example of a problem where 5% is exceeded:
The Ka of HF is 6.46 x 10¯4. Calculate the pH of 0.0100 M solution of HF.
If you solve this by the approximate method, you will exceed 5% by quite a bit (it's 25.4%). To obtain as accurate an answer as possible, the quadratic method must be used.
What could happen in your classroom is that your teacher may mention using the quadratic and possibly show one example. Then, everything after that would use the approximate method. This is what the ChemTeam did when he was in the classroom.
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FAQs
The 5% approximation rule is a guideline used in chemistry to simplify calculations involving weak acids and bases. It states that if the ionization of a weak acid or base is less than 5%, then the concentration of the un-ionized species can be approximated as equal to the initial concentration.
What is the 5% rule equation? ›
Re: 5% rule
So you find your x value through the approximation method then divide by your initial amount of weak acid or base and multiply by 100. If the number calculated is greater than 5 then the quadratic formula should be used to solve for x. (x/[HA]) x 100 = some percent.
What is the 5% rule the amount of acid that reacts is assumed to be? ›
What is the 5% rule? The amount of acid that reacts can be assumed negligible in the subtraction from the original amount of acid if no more than 5% of the acid reacts.
How to determine weak acid and weak base? ›
Re: Tips and Trick to identify weak acids/bases
We can also compare the Ka/Kb or pKa/pKb values. Weaker acids/bases have small Ka/Kb values and large pKa/pKb values. Most organic acids are weak acids. Also, the N atom with a lone pair is used to identify weak bases, not weak acids.
What is the 5 percent rule for acid bases? ›
The 5% approximation rule is a guideline used in chemistry to simplify calculations involving weak acids and bases. It states that if the ionization of a weak acid or base is less than 5%, then the concentration of the un-ionized species can be approximated as equal to the initial concentration.
What is the 5% rule in science? ›
The 5% rule tests whether you can use your approximation. If the percent is more than 5% then you must redo the problem with the given small number. If it is less than 5%, then the approximation is valid.
What is the 5% rule? ›
It dates back to 1943 and states that commissions, markups, and markdowns of more than 5% are prohibited on standard trades, including over-the-counter and stock exchange listings, cash sales, and riskless transactions. Financial Industry Regulatory Authority (FINRA).
What are the rules for acid bases? ›
The general rules suggest that the stronger of a pair of acids must form the weaker of a pair of conjugate bases. The fact that HCl is a stronger acid than the H3O+ ion implies that the Cl- ion is a weaker base than water.
What is the acid-base balance rule? ›
To maintain homeostasis, the human body employs many physiological adaptations. One of these is maintaining an acid-base balance. In the absence of pathological states, the pH of the human body ranges between 7.35 to 7.45, with the average at 7.40.
What is the 5% rule in AP Stats? ›
The traditional cutoff for a small p-value is 0.05, which means that there is only a 5% chance of obtaining the observed results, or something more extreme, if the null hypothesis is true. If the p-value is below this threshold, it is considered statistically significant and you can reject the null hypothesis.
In calculating the pH of a weak acid or a weak base, use the approximation method first (the one where you drop the '− x'). Then apply the 5% rule. If you exceed 5%, then you would need to carry out a calculation that does not drop the '− x. ' This would result in quadratic equation, which would be solvable.
What percent right is a 5 on AP Chem? ›
While the exact percentage needed for a 5 can vary each year, you can estimate that you'll generally need to score around 65% to 75% of the total possible points to earn a 5 on the AP Chemistry exam.
What is the rule of pH in a weak acid? ›
Key Takeaways: pH of a Weak Acid
The pH equation is still the same (pH = -log[H+]), but you need to use the acid dissociation constant (Ka) to find [H+]. There are two main methods of solving for hydrogen ion concentration. One involves the quadratic equation.
What is the indicator for weak acids and weak bases? ›
Methyl orange, Methyl red are used for the titration of strong acid and strong base or strong acid and weak base while Bromothymol blue is used for weak acid and weak base titration indicator.
What are weak acids and bases? ›
A weak acid is one that does not dissociate completely in solution while a weak base is a chemical base that does not ionize fully in an aqueous solution. Weak Acids.
What is the 5% rule in physics? ›
The 5% error rule = the absolute value of the y intercept / highest y value *100. If above 5% you keep the y intercept. If below 5 % you can cancel the y intercept.
What is the rule of 5 in psychology? ›
According to Robbins, when you count backwards from five, you're using a form of metacognition to override the patterns that get encoded in your basal ganglia and draw your focus to your pre-frontal cortex, the part of your brain that gives you control over your thoughts and actions.
What is the 5 by 5 rule example? ›
If your social media feed tends to pick up a lot of inspirational quotes and motivational creeds, you may have seen the 5-by-5 rule before: “If it won't matter in five years, don't spend five minutes worrying about it.” While it's usually meant to apply to your personal life, it's also sound professional advice.
What is the rule of 5 in economics? ›
“A 5% increase in price received, a 5% decrease in costs, and a 5% increase in yield will often produce more than a 100% increase in net returns. The effect is cumulative, multiplicative and compounding.”
