Have you ever heard of lactose intolerance? It's estimated that 65% of the world's population suffers from this condition, which makes it hard for them to digest milk products. The disaccharide lactose found in milk is broken down by the enzyme lactase into glucose and galactose. But some people lack this enzyme, so their bodies can't break down the lactose. This causes the lactose to undergo a process called bacterial fermentation, leading to nausea, abdominal cramps, and diarrhea. The activity of lactase is better at pH 6, so changing the pH has a big effect on lactase activity. But what exactly is pH and how does it relate to pOH? Read on to find out!

The main difference between pH and pOH is that pH is a measure of hydrogen ions, while pOH is a measure of hydroxide ions. The pH and pOH of a neutral solution at 25°C are both 7.00. The relation between the hydronium and hydroxide ion concentrations expressed as p-functions is easily derived from the Kw expression. The pKw, or the pH plus the pOH, is 14.00 at 25°C.

The pH scale is used to measure the acidity or basicity of a solution. The higher the pH value, the more basic the solution and the lower the concentration of hydrogen ions. The pOH scale is used to measure the acidity or basicity of a solution in terms of hydroxide ions. The pH and pOH of a strong acid or base can be calculated by subtracting or adding the corresponding pKa or pKb value to the pH or pOH of the solution.

Difference between pH and pOH

Let's start with the basics - what are acids and bases? There are different ways to define them, but we'll stick to the most common ones. Acids are substances that release hydrogen ions (H+) in water. Bases, on the other hand, are substances that release hydroxide ions (OH-) in water. 

Now, let's talk about pH and pOH. These are measures of the acidity and basicity of a solution. pH measures the concentration of hydrogen ions in a solution, while pOH measures the concentration of hydroxide ions. The pH scale ranges from 0 to 14, with values below 7 indicating an acidic solution, values of 7 being neutral, and values above 7 being basic. The pOH scale works in a similar way, but in reverse. The sum of pH and pOH is always 14.

Acid rain is an example of how pH can affect our environment. When pollutants in the air mix with rainwater, it makes the rain more acidic. This can harm plants and animals, and even corrode statues made of limestone.

Remember, the lower the pH, the more acidic the solution. The higher the pH, the more basic the solution. And if you know either the pH or pOH, you can easily calculate the other value using the equation pH + pOH = 14.

‍

‍

If the pH of a solution is 4.6, calculate the pOH. We know that the sum of pH and pOH should be equal to 14. So, we can rearrange the equation above to solve for pOH.

pH and pOH Formula

Let's put all this theory into practice with some examples!

Example 1: Find the pH of a solution with an [H+] ion concentration of 0.000001 M.

To find the pH, we just need to plug the [H+] ion concentration into the formula pH = -log[H+]. So:

pH = -log(0.000001)
pH = 6

Therefore, the pH of the solution is 6.

Example 2: Find the pOH of a solution containing an [H+] ion concentration of 0.00001 M.

To find the pOH, we need to first find the pH using the formula pH = -log[H+]. So:

pH = -log(0.00001)
pH = 5

Now, we can use the formula pH + pOH = 14 to solve for pOH:

pOH = 14 - pH
pOH = 14 - 5
pOH = 9

Therefore, the pOH of the solution is 9.

Example 3: Calculate the [OH-] ion concentration of a solution that has a pH of 3.3.

First, we need to find the [H+] ion concentration using the formula pH = -log[H+]. So:

3.3 = -log[H+]
[H+] = 5.01 x 10^-4 M

Now, we can use the formula [H+][OH-] = 1x10^-14 to find the [OH-] ion concentration:

[H+][OH-] = 1x10^-14
(5.01 x 10^-4)[OH-] = 1x10^-14
[OH-] = 1.996 x 10^-11 M

Therefore, the [OH-] ion concentration of the solution is 1.996 x 10^-11 M. 

pOH to pH

Let's continue with more examples to practice converting between pH and pOH and calculating pH for strong acids.

Example 1: Convert pOH to pH for a solution with a pOH of 2.5.

To convert pOH to pH, we can use the formula pH + pOH = 14. So:

pH + 2.5 = 14
pH = 11.5

Therefore, the pH of the solution is 11.5.

Example 2: Find the pH of a substance containing an [OH-] concentration of 3.2 x 10^-7 M.

Step 1: Use [OH-] to calculate pOH.

pOH = -log[OH-]
pOH = -log(3.2 x 10^-7)
pOH = 6.5

Step 2: Use the calculated pOH to solve for pH.

pH + 6.5 = 14
pH = 7.5

Therefore, the pH of the substance is 7.5.

Example 3: Calculate the pH of a 0.023 M HCl solution.

Since HCl is a strong acid, we can assume that the concentration of H+ ions is equal to the concentration of HCl. So:

pH = -log[H+]
pH = -log(0.023)
pH = 1.64

Therefore, the pH of the solution is 1.64.

As for the question of whether a solution can have a negative pH, the answer is yes, as we saw in the examples of strong acids with very high [H+] ion concentrations. However, it's important to note that pH is a logarithmic scale, so a decrease of 1 in pH actually represents a tenfold increase in [H+] ion concentration. A solution with a pH of -1 is extremely acidic and potentially dangerous.

pH of Strong Bases

The pH of a 0.023 M Ca(OH)2 solution can be calculated using the following equation:

pH = -log[OH-]

Since Ca(OH)2 dissociates into two OH- ions, we need to multiply the concentration of the Ca(OH)2 solution by 2 to get the concentration of OH- ions:

[OH-] = 2 x 0.023 M = 0.046 M

Now, we can calculate the pH of the solution:

pH = -log[0.046] = 1.33

Therefore, the pH of a 0.023 M Ca(OH)2 solution is 1.33. ¹

Sources:

1. Malone, L. J., Dolter, T. O., & Gentemann, S. (2013). Basic concepts of Chemistry (8th ed.). Hoboken, NJ: John Wiley & Sons.

pH and pOH

How to calculate the pH of a strong base?

To calculate the pH of a strong base, we can use the following formulas:pOH = -log [OH-]pH + pOH = 14For example, if you are given the [OH-] ion concentration of a solution, you can use it to find pOH and then solve for pH. 

How do pH and pOH relate to the strength of an acid or base?

The lower the pH value, the more acidic a solution is, the higher the H+ concentration and the lower the OH- concentration. The lower the pOH value, the more basic a solution is, the higher the OH- concentration and the lower the H+ concentration.

What is the pH of a strong acid and strong base?

Strong acids have very low pH values, whereas strong bases have very high pH values.

How do you find the pOH of a strong acid?

To calculate the pH of a strong acid, we can use the following formulas:pH = -log [H-]pH + pOH = 14So, f you are given the [H-] ion concentration of a solution, you can use it to find pH.

What is the pOH if the pH is 11?

In a solution, the sum of the pH and pOH values equals 14. So, a solution containing a pH value of 11 has a pOH of 3 because 14 - 11 = 3.

‍