Calculate The Ph Of 0.005 M Naoh Solution

Use this interactive calculator to find hydroxide concentration, pOH, and pH for a sodium hydroxide solution. It is optimized for strong base calculations at 25 degrees C and includes a comparison chart to visualize where your answer sits on the pH scale.

NaOH pH Calculator

How to Calculate the pH of 0.005 M NaOH Solution

To calculate the pH of 0.005 M NaOH solution, start with the fact that sodium hydroxide is a strong base. In water, NaOH dissociates essentially completely into sodium ions and hydroxide ions:

NaOH -> Na+ + OH-

Because the dissociation is complete for this type of introductory chemistry calculation, the hydroxide concentration is taken to be the same as the sodium hydroxide concentration. That means a 0.005 M NaOH solution has an [OH-] of 0.005 M. Once you know hydroxide concentration, you can calculate pOH and then pH.

Quick Answer

For 0.005 M NaOH:

• [OH-] = 0.005 M
• pOH = -log10(0.005) = 2.3010
• pH = 14.00 – 2.3010 = 11.6990

Rounded to two decimal places, the pH is 11.70.

Step by Step Formula

Here is the standard sequence used in general chemistry for a strong base like NaOH at 25 degrees C:

1. Write the base dissociation: NaOH -> Na+ + OH-
2. Assign hydroxide concentration: [OH-] = 0.005 M
3. Calculate pOH using the logarithm formula: pOH = -log10[OH-]
4. Substitute the value: pOH = -log10(0.005)
5. Find pOH: pOH = 2.3010
6. Use the relationship at 25 degrees C: pH + pOH = 14.00
7. Calculate pH: pH = 14.00 – 2.3010 = 11.6990

This is why chemistry instructors often teach that the pH of 0.005 M NaOH solution is 11.70 when rounded properly. If your class asks for three decimal places, use 11.699.

Why NaOH Is Treated as a Strong Base

Sodium hydroxide is one of the classic strong bases used in chemistry. It dissociates nearly completely in dilute aqueous solution, so it is not handled the same way as a weak base like ammonia. With weak bases, you usually need an equilibrium constant such as Kb and often an ICE table. With NaOH, that extra equilibrium step is usually unnecessary for standard textbook concentration ranges.

This matters because the pH calculation becomes much simpler. For a strong base, the hydroxide ion concentration comes directly from the stoichiometry of dissociation. Since one formula unit of NaOH produces one hydroxide ion, the mole ratio is 1:1. Therefore:

• 0.100 M NaOH gives about 0.100 M OH-
• 0.010 M NaOH gives about 0.010 M OH-
• 0.005 M NaOH gives about 0.005 M OH-

Detailed Worked Example for 0.005 M NaOH

Let us work through the numbers carefully. First rewrite 0.005 in scientific notation:

0.005 = 5.0 x 10^-3

Now apply the logarithm rule:

pOH = -log10(5.0 x 10^-3)

Using logarithm properties:

log10(5.0 x 10^-3) = log10(5.0) + log10(10^-3)

= 0.6990 – 3 = -2.3010

Therefore:

pOH = 2.3010

Then calculate pH:

pH = 14.00 – 2.3010 = 11.6990

That answer makes chemical sense. A pH above 7 is basic, and 11.7 is strongly basic but still far below the extreme pH values seen with highly concentrated industrial caustic solutions.

Comparison Table: Common NaOH Concentrations and Their pH

The table below shows how pH changes as NaOH concentration changes at 25 degrees C under the strong base assumption. These values are mathematically derived from standard pOH and pH relationships.

NaOH Concentration (M)	[OH-] (M)	pOH	pH
0.100	0.100	1.000	13.000
0.050	0.050	1.301	12.699
0.010	0.010	2.000	12.000
0.005	0.005	2.301	11.699
0.001	0.001	3.000	11.000
0.0001	0.0001	4.000	10.000

What the Number Means in Practical Terms

A pH of 11.70 indicates a distinctly alkaline solution. Neutral water at 25 degrees C has a pH close to 7.00. Every 1 unit change on the pH scale represents a tenfold change in hydrogen ion activity. That means a solution at pH 11.70 is dramatically more basic than neutral water. In educational and laboratory settings, sodium hydroxide solutions at this range are strong enough to require careful handling, eye protection, and appropriate lab procedure.

It is also useful to remember that pH is logarithmic. Many students expect a concentration such as 0.005 M to produce a pH only slightly above 7, but that is not how the scale works. Because pH and pOH use base 10 logarithms, even small molar concentrations can generate substantial changes in acidity or basicity.

Common Mistakes When Calculating pH of NaOH

• Using pH directly from concentration. For bases, you usually calculate pOH first, then convert to pH.
• Forgetting the strong base assumption. NaOH dissociates completely in dilute solution, so [OH-] equals the listed molarity.
• Using the wrong sign in the logarithm. pOH is negative log of hydroxide concentration.
• Mixing up 0.005 and 5 x 10^-3. These are the same value, but scientific notation can help avoid calculator mistakes.
• Forgetting the 25 degrees C relationship. In most introductory chemistry problems, pH + pOH = 14.00.

Comparison Table: Typical pH Ranges of Familiar Substances

The pH scale is easier to interpret when you compare your answer with known materials. The ranges below are commonly cited in educational and government water quality references and show that 0.005 M NaOH falls well into the basic region.

Substance or Reference Point	Typical pH	Interpretation
Battery acid	0 to 1	Strongly acidic
Lemon juice	2	Acidic
Black coffee	5	Mildly acidic
Pure water at 25 degrees C	7	Neutral
Seawater	8.1	Mildly basic
Baking soda solution	8.3 to 9	Basic
0.005 M NaOH solution	11.699	Strongly basic
Household bleach	11 to 13	Strongly basic

When the Simple Formula Is Appropriate

The direct method used here is appropriate for standard classroom chemistry problems, laboratory calculations, and many dilute aqueous systems where NaOH behaves ideally enough to justify the strong base approximation. It is especially useful when:

• You are working at 25 degrees C
• The solution is not extremely concentrated
• You are expected to use textbook pH relationships
• Activity corrections are not part of the assignment

At very high ionic strengths or in advanced physical chemistry, pH calculations can involve activity rather than simple concentration. However, that is beyond what is normally expected for a problem asking for the pH of 0.005 M NaOH.

Why the Answer Is Not Exactly 12

Students often notice that 0.010 M NaOH gives a pH of 12.00 and wonder whether 0.005 M should be close enough to count as 12. The reason it does not is that halving the concentration does not reduce pH by half. Because the scale is logarithmic, reducing [OH-] from 0.010 M to 0.005 M changes pOH from 2.000 to 2.301, which in turn lowers pH from 12.000 to 11.699. That 0.301 shift appears often in chemistry because log10(2) is approximately 0.301 and halving or doubling concentrations creates that change.

Authority Sources for pH and Water Chemistry

If you want to verify the science behind pH, water chemistry, and logarithmic acid base relationships, these sources are useful starting points:

• USGS: pH and Water
• U.S. EPA: pH Background Information
• University level chemistry explanation of pH, pOH, and water autoionization

Final Answer

If you are solving a homework problem or checking a lab setup, the final result is straightforward:

The pH of a 0.005 M NaOH solution is 11.70 at 25 degrees C.

The underlying values are [OH-] = 0.005 M, pOH = 2.301, and pH = 11.699. That makes the solution strongly basic and clearly above neutral on the pH scale.

Educational note: This calculator assumes ideal dilute behavior and complete dissociation of sodium hydroxide. For advanced analytical work, activity effects and temperature dependent equilibria may be considered.