How to calculate pOH from molarity?, pOH from hydroxide (OH-) ion concentration
pOH stands for the power of hydroxide ions. It determines the basicity of an aqueous solution in terms of the concentration of OH– ions present in it. Molarity is one way of measuring the concentration of OH– ions.
If we know the molar concentration or molarity of an aqueous solution, we can find its pOH. But how? Come along and find out for yourself in this article.
It is calculated by taking the negative logarithm of the concentration of OH– ions in an aqueous solution, as shown in equation (i).
pOH = -log10 [OH–]……….Equation (i)
As a base is primarily defined as a chemical substance that liberates OH– ions in water, thus pOH determines the basicity or alkalinity of an aqueous solution.
The greater the hydroxide ion concentration [OH–], the higher the basicity of an aqueous solution, thus lowering its pOH value.
In this way, pOH is inversely related to the basicity of an aqueous solution. Contrarily, it is directly related to acidity, which means that with increasing acidity, pOH increases and vice versa.
On a scale of 0-14, acidic solutions have a pOH above 7, while basic solutions have a pOH below 7. A neutral solution such as water has pOH = pH = 7.
pH is related to pOH by the formula given in equation (ii).
pH + pOH = 14………Equation (ii)
Equation (ii) shows that aqueous solutions having high pOH values possess low pH values and vice versa.
For instance, the pOH of 0.1 M NaOH solution is 1 while its pH is 13. It is thus a strongly basic solution. In contrast, the pOH of a strongly acidic solution such as 0.1 M H2SO4 is 13.3, while its pH is 0.7.
What is molarity?
The hydroxide ion concentration of an aqueous solution can be expressed in terms of molarity.
Molarity refers to the moles of solute dissolved per litre of the solution.
Thus if the concentration of hydroxide ions [OH–] is expressed in moles per litre, then it is known as its molar concentration or molarity. Unit of molarity = mol/L or M.
NaOH dissociates to release Na+ and OH– ions in water.
As per the balanced chemical equation, 1 OH– ion is produced per NaOH molecule. Thus 0.1 M NaOH solution represents 0.1 mol/L OH– ions liberated in the aqueous solution.
What is the relationship between pOH and molarity?
The pOH of an aqueous solution is inversely related to the molarity of OH– ions present in it.
A strong base readily ionizes in water to liberate a large number of OH– ions. As the concentration of OH– ions increase, the pOH of the aqueous solution decreases as per equation (i).
How to find pOH from molarity (concentration of OH– ions)?
If the molar concentration of OH– ions present in an aqueous solution is known, then we can find its pOH easily by substituting the given value into equation (i).
For example, as we discussed already, 0.1 M NaOH solution represents 0.1 mol/L OH– ions released in water, so now let’s find its pOH by applying equation (i).
pOH = -log10 [OH–]……….Equation (i)
∴ pOH = -log10 (0.1) = 1
Thus the pOH of 0.1 M NaOH solution is 1.
You can practice further; how to determine pOH from molarity or pOH from hydroxide (OH–) ion concentration with the help of the solved examples given below. So happy learning!
Solved examples for finding pOH from molarity
Example # 1: What is the pOH of a 0.05 M aqueous solution of a strong base B?
A strong base B completely ionizes in water to produce BH+ and OH– ions.
As the molarity is given in the question statement thus, molarity = [OH–]= 0.05 M.
We can substitute the given value into equation (i) and find pOH as follows:
pOH = -log10 [OH–]……….Equation (i)
∴ pOH = -log10 (0.05) = 1.30
Result: The pOH of solution B is 1.30.
A low pOH value reaffirms that it is a strongly basic aqueous solution.
Example # 2: What is the pOH of a strongly basic solution X if its molarity is 0.02 mol/L?
As the molar concentration of hydroxide (OH–) ions is given in the question statement so we can substitute the given value into equation (i) and find pOH as follows:
pOH = -log10 [OH–]……….Equation (i)
∴ pOH = -log10 (0.02) = 1.70
Result: The pOH of the strongly basic solution X is 1.70.
Example # 3: What is the pOH of a 0.02 M ammonia solution if its Kb value is 1.8 x 10-5?
A unique aspect of this question is that ammonia (NH3) is a weak basethat only partially dissociates in water to release a limited number of OH– ions, as shown below.
Thus [NH3] initial is not equal to [OH–] in this case. Rather we can determine [OH–] released as the above reaction reaches equilibrium using the Kb value given in the question statement.
The base dissociation constant (Kb) expression for the above reaction is:
At this stage, we can either solve the above equation by assuming that as ammonia is a weak base, so the value of x will be quite small.
In turn, it will not have much effect on [NH3] thus, 0.02-x can be replaced with 0.02 only.
However, the more accurate method is to solve the above equation by using the quadratic formula as follows:
Hence [NH4+] equilibrium = [OH–]equilibrium = 5.9 x 10-4 M.
Now that the molarity of OH– is known, we can easily find the pOH of the given ammonia solution using equation (i)
pOH = -log10 [OH–]……….Equation (i)
∴ pOH = -log10 (5.9 x 10-4) = 3.23
Result: The pOH of a 0.02 M ammonia solution is 3.23.
Example # 4: Which of the following options gives us the correct pOH of a 6 x 10-7 M aqueous solution of HCl?
A) 8.9 B) 6.2 C) 5.2 D) 7.8 E) 10.6
Answer: Option D (pOH = 7.8) gives us the correct answer.
Explanation: The molarity of HCl is given in the question statement. HCl is a strong acid that completely ionizes in water. 1 H+ ion is released per HCl molecule, as shown below.
[HCl] = [H+] = 6 x 10-7 M so we can use the equation, pH = -log10 [H+] = -log10 (6 x 10-7) = 6.22.
Now that we know the pH of the given solution, we can easily find its pOH by applying equation (ii) as follows:
pH + pOH = 14………Equation (ii)
6.22 + pOH = 14
pOH = 14 – 6.22 = 7.78
7.78 rounded off to one decimal place is 7.8 thus, the pOH of 6 x 10-7 M aqueous solution of HCl is 7.8.
pOH represents the negative logarithm of hydroxide (OH–) ion concentration. It determines the basicity of an aqueous solution.
The more basic an aqueous solution is; the more OH– ions are present in it; thus, the pOH of the solution decreases as per the following formula:
pOH = -log10 [OH–]
What is molarity?
Molarity is a way of measuring the concentration of a solution. It is defined as the moles of solute dissolved per litre of the solution.
How is pOH related to the molar concentration of OH– ions present in an aqueous solution?
pOH is inversely related to the molar concentration of OH– ions present in an aqueous solution.
Greater the molarity of OH– ions, the smaller its pOH value as per the equation pOH = -log10 [OH–], the higher the basicity of the aqueous solution.
How to find the pOH of an aqueous solution from [OH–]?
The pOH can be easily determined by substituting the given value of [OH–] into the formula: pOH = -log10 [OH–].
How to find the pOH of an aqueous solution from [H+]?
The pH of a solution can be determined by taking the negative logarithm of [H+].
pH = -log10 [H+]
Once the pH is determined, we can easily find pOH by applying the following formula:
pOH = 14 – pH
What is the relationship between pH and pOH?
The pH of a solution is inversely related to its pOH.
Strongly acidic solutions have low pH but high pOH values. However, strongly basic solutions have low pOH but a high pH value.
The sum of pH and pOH of an aqueous solution is always equal to 14.
pH + pOH = 14
Summary
pOH stands for the power of hydroxide ions. It determines the basicity of an aqueous solution. It is calculated by taking the negative logarithm of hydroxide ion concentration [OH–].
pOH = -log10 [OH–].
Molarity is one way of representing the concentration of an aqueous solution. It is defined as the moles of solute dissolved per litre of the solution. The units of molarity are mol/L, mol/dm3 or simply M.
pOH of a basic solution is inversely related to the molarity of OH– ions present in it.
The higher the [OH–], the lower the pOH value and vice versa.
Strongly basic solutions have pOH values below 7, while strongly acidic solutions have pOH values above 7.
If the molar concentration of hydroxide (OH–) ions or molarity in an aqueous solution is known, we can find its pOH by directly applying the formulapOH = -log10 [OH–].
Enhanced Chemistry for Enhanced Science Learning. (n.d.). Introduction to pH. eCampusOntario. https://ecampusontario.pressbooks.pub/enhancedchemistry/chapter/intro-ph/
Vedantu. (2021). Calculate the pH and pOH of a solution which is 0.1 M in HCl. Vedantu Innovations Pvt. Ltd. https://www.vedantu.com/question-answer/calculate-the-ph-and-poh-of-a-solution-which-class-11-chemistry-cbse-604edf93c4b1400cb39ca646
Ammara Waheed is a highly qualified and experienced chemist, whose passion for Chemistry is evident in her writing. With a Bachelor of Science (Hons.) and Master of Philosophy (M. Phil) in Physical and Analytical Chemistry from Government College University (GCU) Lahore, Pakistan, with a hands-on laboratory experience in the Pakistan Council of Scientific and Industrial Research (PCSIR), Ammara has a solid educational foundation in her field. She comes from a distinguished research background and she documents her research endeavors for reputable journals such as Wiley and Elsevier. Her deep knowledge and expertise in the field of Chemistry make her a trusted and reliable authority in her profession. Let's connect - https://www.researchgate.net/profile/Ammara-Waheed
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