Calculate pH of 0.005 M NaOH
Use this premium chemistry calculator to find pOH, pH, hydroxide concentration, and solution classification for sodium hydroxide solutions. The default example is 0.005 M NaOH, a common strong base problem in general chemistry.
Results
Enter or keep the default value of 0.005 M NaOH and click Calculate pH.
How to calculate pH of 0.005 M NaOH
To calculate the pH of 0.005 M sodium hydroxide, start with the fact that NaOH is a strong base. In introductory and most intermediate chemistry contexts, a strong base is assumed to dissociate completely in water. That means every formula unit of sodium hydroxide contributes one hydroxide ion, so the hydroxide concentration is numerically equal to the molarity of the NaOH solution. For a 0.005 M NaOH solution, you can therefore set [OH-] = 0.005.
Once you know the hydroxide concentration, compute the pOH using the base-10 logarithm formula pOH = -log10[OH-]. Substituting 0.005 gives pOH = -log10(0.005) = 2.3010 approximately. At 25°C, aqueous acid-base calculations typically use the relationship pH + pOH = 14. Therefore, pH = 14 – 2.3010 = 11.6990, which is usually rounded to 11.70.
This result confirms that 0.005 M NaOH is clearly basic. It is not merely slightly above neutral. A pH close to 11.7 means the hydroxide concentration is significantly higher than in pure water, where the hydroxide concentration at 25°C is only 1.0 × 10^-7 M. Understanding that difference is important in both classroom chemistry and practical settings such as cleaning chemistry, titration preparation, and laboratory safety training.
Quick answer
- Given concentration: 0.005 M NaOH
- Strong base assumption: [OH-] = 0.005 M
- pOH: 2.3010
- pH at 25°C: 11.6990
- Rounded pH: 11.70
Why sodium hydroxide is treated as a strong base
Sodium hydroxide is one of the classic strong bases taught in general chemistry alongside potassium hydroxide and the more soluble hydroxides of certain group 1 and group 2 metals. In water, sodium hydroxide dissociates essentially completely according to the equation:
NaOH(aq) → Na+(aq) + OH-(aq)
That complete dissociation matters because it simplifies the problem dramatically. For strong bases that release one hydroxide ion per formula unit, the molar concentration of the base equals the concentration of hydroxide ions. Since NaOH provides one OH- ion, a 0.005 M solution yields 0.005 M hydroxide. You do not need an ICE table, and you do not need to solve an equilibrium expression as you would for a weak base like ammonia.
Step-by-step method for calculating the pH of 0.005 M NaOH
- Identify the compound. NaOH is sodium hydroxide, a strong base.
- Write the dissociation. It dissociates fully into Na+ and OH-.
- Set hydroxide concentration. Since one OH- comes from each NaOH, [OH-] = 0.005 M.
- Find pOH. Apply pOH = -log10(0.005).
- Calculate numerical pOH. pOH ≈ 2.3010.
- Use the 25°C relationship. pH = 14 – 2.3010.
- State the final pH. pH ≈ 11.6990 ≈ 11.70.
Detailed explanation of the logarithm step
Students often get stuck at the logarithm stage, especially when the concentration is written in decimal form rather than scientific notation. Here is the same number rewritten:
0.005 = 5.0 × 10^-3
So:
pOH = -log10(5.0 × 10^-3)
Using logarithm properties:
log10(5.0 × 10^-3) = log10(5.0) + log10(10^-3)
= 0.6990 – 3 = -2.3010
Applying the negative sign from the pOH formula gives pOH = 2.3010. This is why the final pH becomes 11.6990. Working through the logarithm manually can help you estimate whether a calculator result is reasonable. For example, because 0.005 is between 10^-2 and 10^-3, the pOH must be between 2 and 3. That makes the final pH between 11 and 12, which matches the exact answer.
Comparison table: NaOH concentration vs pH at 25°C
The table below compares several sodium hydroxide concentrations and the resulting pOH and pH values under the standard assumption of complete dissociation at 25°C. These values are mathematically derived from the standard formulas used in general chemistry.
| NaOH Concentration (M) | [OH-] (M) | pOH | pH | Interpretation |
|---|---|---|---|---|
| 0.0001 | 0.0001 | 4.0000 | 10.0000 | Moderately basic |
| 0.001 | 0.001 | 3.0000 | 11.0000 | Clearly basic |
| 0.005 | 0.005 | 2.3010 | 11.6990 | Strongly basic for classroom scale |
| 0.01 | 0.01 | 2.0000 | 12.0000 | Strong base solution |
| 0.1 | 0.1 | 1.0000 | 13.0000 | Very strongly basic |
How 0.005 M NaOH compares with neutral water and acidic solutions
It is useful to compare the 0.005 M NaOH result with familiar reference points. Pure water at 25°C is neutral at pH 7. A solution with pH 11.70 is not just a little more basic than water. Because the pH scale is logarithmic, each one-unit change corresponds to a tenfold change in hydrogen ion concentration. That means a solution with pH 11.70 is orders of magnitude different from neutral water.
Likewise, if you compare 0.005 M NaOH to a weakly acidic solution around pH 5, the difference is dramatic. The pH gap is about 6.7 units, indicating a hydrogen ion concentration difference of roughly 10^6.7, or about five million times. This logarithmic behavior is why pH calculations matter so much in chemistry, biology, environmental science, and industrial process control.
Comparison table: reference pH ranges and practical context
| Reference System | Typical pH | Relative to 0.005 M NaOH | Practical Meaning |
|---|---|---|---|
| Pure water at 25°C | 7.0 | 0.005 M NaOH is 4.7 pH units higher | Much more basic than neutral water |
| Blood, normal physiological range | 7.35 to 7.45 | 0.005 M NaOH is over 4.2 pH units higher | Far too basic for biological compatibility |
| Typical household ammonia solution range | 11 to 12 | Comparable basicity range | Strongly basic handling precautions are needed |
| 0.1 M HCl | 1.0 | 0.005 M NaOH is 10.7 pH units higher | Extremely different acid-base environments |
Common mistakes when solving this problem
- Using pH instead of pOH first. For bases, begin with hydroxide concentration and calculate pOH before converting to pH.
- Forgetting NaOH is a strong base. If you treat it like a weak base, you overcomplicate the problem and may get the wrong answer.
- Mistyping the logarithm. Be sure to enter -log(0.005), not log(0.005).
- Dropping the negative sign incorrectly. The logarithm of a number less than 1 is negative, but pOH must be positive here because of the formula.
- Rounding too early. Keep more digits in pOH and round only at the final pH step.
- Ignoring temperature assumptions. The common sum of 14 is standard for 25°C calculations.
Why the answer is not exactly 12
Some learners assume that any small concentration of a strong base should give a pH near 12, but pH is logarithmic. A solution with 0.01 M hydroxide has pOH 2 and pH 12. Since 0.005 M is half of 0.01 M, the pOH is slightly larger than 2, specifically 2.3010, and the pH is slightly lower than 12, specifically 11.6990. This is a subtle but important example of how halving a concentration does not simply subtract 0.5 pH units. Instead, it changes the logarithmic value by log10(2) ≈ 0.3010.
Real-world relevance of a 0.005 M NaOH solution
Although 0.005 M NaOH is not as concentrated as stock laboratory sodium hydroxide solutions, it is still meaningfully basic. In practical use, sodium hydroxide solutions can appear in cleaning formulations, pH adjustment steps, standardization experiments, and titrations. Even dilute hydroxide solutions should be handled with care because they can irritate skin and eyes. The exact risk depends on concentration, exposure time, and application context, but in any lab setting you should follow your institution’s chemical hygiene plan.
From an educational perspective, this concentration is ideal because it shows students how strong bases behave mathematically without requiring advanced equilibrium treatment. It also gives a pH value that is not a whole number, making it a useful practice problem for logarithms, sig figs, and conceptual interpretation.
Authoritative chemistry references
If you want to verify the underlying concepts with high-authority educational and government sources, review the following materials:
- LibreTexts Chemistry for strong acid and strong base pH calculation tutorials.
- U.S. Environmental Protection Agency for pH fundamentals and water chemistry context.
- NCBI Bookshelf (.gov) for sodium hydroxide toxicology and safety context.
Final takeaway
To calculate the pH of 0.005 M NaOH, assume complete dissociation because sodium hydroxide is a strong base. That gives [OH-] = 0.005 M. Then calculate pOH = -log10(0.005) = 2.3010. Finally, at 25°C, use pH = 14 – 2.3010 = 11.6990. Rounded appropriately, the answer is pH = 11.70. If you are checking homework, preparing for an exam, or validating a simple lab calculation, this is the standard correct result.