Potassium hydroxide, KOH
is a strong base and show **great pH values** in aqueous solution. In
this tutorial, we are going to learn *how to calculate pH of different concentration KOH aqueous
solutions*.

*At the end of this tutorial, there is an online calculator which can be used to check your answers (pH values) after you finish your problems.*

*written by: Heshan Nipuna, last update: 03/06/2020*

When calculating pH of KOH solutions, following **theories** and **equations** are important.

**Theory 1:**KOH dissociates completely in the water to K^{+}and OH^{-}ions.**Theory 2:**Due to complete dissociation, initial concentration of KOH is equal to OH^{-}concentration.**Equation 1:**Relationship of pH + pOH = 14**Equation 2:**pH = -log_{10}[H_{3}O^{+}_{(aq)}]**Equation 3:**pOH = -log_{10}[OH^{-}_{(aq)}]

Question #1

In the laboratory, there is a aqueous potassium hydroxide solution which was prepared by a student. In the label of the bottle,
concentration was mentioned as 0.1 mol dm^{-3}. Calculate the theoretical pH value of the KOH solution.

Answer

- Find OH
^{-}concentration - Calculate pOH by OH
^{-}concentration - Calculate pH by pH + pOH = 14

According to the **theory 1**, OH^{-} concentration is 0.1 mol dm^{-3}

Now, you can use the **equation 3** to calculate pOH with known OH^{-} concentration.

pOH = -log_{10}[OH^{-}_{(aq)}]

pOH = -log_{10}[0.1]

pOH = 1

Now, use the **equation 3** to find the pH

pH + pOH = 14

pH + 1 = 14

pH = 13

**pH of 0.1 mol dm ^{-3} KOH solution is 13.**

Question #2

As a student you are curious to know about chemicals and their application in the laboratory. You found the
analytical grade potassium hydroxide bottle in the laboratory. In the bottle, there are potassium hydroxide
pellets. You got some of these pellets and measured the mass. In the balance mass of KOH is shown as 1.4 g.
Now one of your friend came to the lab and ask about what's going on. You explained the stuff you had done and
your friend want to continue the work. So he fill distilled water to a flask upto 250 cm^{3} and say
you to put your measured KOH to it. Then you put them to water and mix the solution very well. You say KOH
solution should be a basic one and should have a pH value greater than 7. So your friend is not sure about that
and ask to prove it. Both of you go to the pH meter and check pH of the solution and it confirms you are correct.
What will the pH of the solution?

K = 39 , O = 16 , H = 1

Answer: pH = 13

In this calculator, we use several methods to calculate pH of KOH solution. You can use one of it according to the data you have.

- When concentration of KOH in mol dm
^{-3}is known - When amount (mol) of KOH and volume of solvent are known.
- When mass of KOH is known and volume of solvent is known.

In our calculators pH of the neutral solution is 7 according to the 25^{0}C.

*Method 1*

Answer

*Method 2*

We assume that after adding KOH, *volume of solute is not changed much*. So you can enter volume of solution as volume of solute.

Answer

*Method 3*

We assume that after adding KOH, *volume of solute is not changed much*. So you can enter volume of solution as volume of solute.

Answer

We can take several KOH solutions and measure their pH values. Here are some KOH solutions with their concentration (mol dm^{-3}).

- 0.1 mol dm
^{-3}: pH = 13 - 0.01 mol dm
^{-3}: pH = 12 - 0.001 mol dm
^{-3}: pH = 11

When concentration of KOH decreases, pH also decreases.

Questions

Ask your question and find the answer free.First step is calculate the pOH of KOH solution. pOH of 1.0 * 10^{-2} mol^{-1} is 2. Then, calculate the pH by using relation of pH + pOH = 14. So pH of the 1.0 * 10^{-2} mol^{-1} KOH is 12.

Because you know the pH value, you can find the pOH. pOH = 14 - pH . pOH of KOH solution is 1.36

Now use the pOH equation.

- pOH = -log
_{10}[OH^{-}_{(aq)}] - 1.36 = -log
_{10}[OH^{-}_{(aq)}] - [OH
^{-}_{(aq)}] = 0.044 mol dm^{-3}

OH^{-} ion is provided by only KOH. Therefore concentration of KOH = 0.044 mol dm^{-3}

If concentration of KOH is 0.1 M, pH value may be 13. If concentation is 0.0001 M, pH value will be 10. Therefore, pH range of KOH solution will be 10-13.

At most cases, this can be true because potassium hydroxide is a strong base. Ammonia give very less OH^{-} ions in aqueous state.

Both Potassium hydroxide and Barium hydroxide are strong bases. Though both have same concentration, their OH^{-} concentration is not same.

Barium hydroxide is a strong base and dissociate like below.

Ba(OH)_{2} → Ba^{2+} + 2OH^{-}

OH^{-} concentration of Barium hydroxide is greater than OH^{-} concentration of KOH. Therefore pH of Barium hydroxide solution is greater than KOH solution.

- pH meter may be not calibrated properly.
- Used KOH has been contaminated with impurities. So there is not KOH as thought.
- Used water may be acidic. It results reacting with KOH and reduce amount of KOH.

OH^{-} concentration of Ba(OH)_{2} solution is greater because when one mol of KOH dissociates, two moles of OH^{-} are given. But, for KOH, one mol of KOH only give one mol of OH^{-}.

When potassium metal is added to water, a strong basic solution, KOH is formed. Due to this strong basic solution, pOH value will be low and pH value will be high.