Calculate pH of 0.005M NaOH
Use this interactive chemistry calculator to find the pH, pOH, and hydroxide ion concentration for sodium hydroxide solutions. The default setup is 0.005 M NaOH at 25 C, which is the classic textbook example for a strong base.
Strong Base pH Calculator
Enter the molarity, choose a hydroxide compound, and set temperature to estimate pH using the appropriate pKw value.
Results and Visual Breakdown
The chart compares concentration, hydroxide ion level, pOH, and pH for the selected strong base solution.
Computed Output
How to Calculate the pH of 0.005M NaOH
To calculate the pH of 0.005M NaOH, you start with the fact that sodium hydroxide is a strong base. In introductory and most practical general chemistry problems, strong bases are assumed to dissociate completely in water. That means every mole of NaOH releases one mole of hydroxide ions, OH-. Because the concentration is 0.005 M, the hydroxide concentration is also 0.005 M. From there, you calculate pOH using the base ten logarithm and then convert pOH to pH.
So, the pH of 0.005M NaOH at 25 C is approximately 11.70. That is the standard answer you will see in chemistry classes, labs, homework sets, and exam solutions. The reason the answer is above 7 is that NaOH is basic, not acidic. The reason it is not closer to 14 is that 0.005 M is dilute compared with very concentrated hydroxide solutions.
Why NaOH Is Easy to Analyze
Sodium hydroxide is one of the most common examples used to teach pH calculations because it behaves simply in water. Unlike weak bases, it does not require an equilibrium table in normal classroom calculations. There is no need to solve for a small dissociation fraction or use a base dissociation constant. For NaOH, the stoichiometry gives you the hydroxide concentration directly.
- NaOH is a strong base.
- It dissociates essentially completely in dilute aqueous solution.
- Each mole of NaOH produces one mole of OH-.
- That makes the conversion from molarity to hydroxide concentration straightforward.
Students sometimes overcomplicate this problem by trying to use an ICE table or by calculating hydrogen ion concentration first. You do not need to do that here. The simplest and cleanest route is to calculate OH-, then pOH, then pH.
Step by Step Method
- Write the dissociation equation: NaOH -> Na+ + OH-
- Identify the molarity: 0.005 M
- Assign hydroxide concentration: [OH-] = 0.005 M
- Calculate pOH: pOH = -log(0.005) = 2.301
- Use the pH relation at 25 C: pH + pOH = 14.00
- Solve for pH: pH = 14.00 – 2.301 = 11.699
If you round to two decimal places, the final answer is 11.70. If your instructor asks for three decimal places, report 11.699. If your concentration had only one significant figure, then your final result might be rounded differently, but in many educational contexts 11.70 is accepted.
Common Mistakes When Solving 0.005M NaOH Problems
Even though this is a simple strong base calculation, there are several common mistakes:
- Forgetting to calculate pOH first. pH is not equal to negative log of the NaOH concentration directly. Because NaOH supplies OH-, you calculate pOH first.
- Using 7 instead of 14. The relationship at 25 C is pH + pOH = 14, not 7.
- Dropping the negative sign in the logarithm formula. pOH = -log[OH-].
- Misreading 0.005 as 5 x 10^-2. It is actually 5 x 10^-3.
- Ignoring temperature. At temperatures other than 25 C, pKw changes, so pH + pOH may not equal exactly 14.00.
What the Number 0.005M Means
The concentration 0.005 M means there are 0.005 moles of NaOH per liter of solution. Because NaOH is a strong base and dissociates into one sodium ion and one hydroxide ion, the hydroxide ion concentration is also 0.005 M. That direct one to one relationship is the heart of this problem.
It helps to rewrite the concentration in scientific notation:
Now the logarithm becomes easier to conceptualize. Since log(5.0 x 10^-3) = log(5.0) – 3, the pOH becomes:
Comparison Table: pH of Several NaOH Concentrations at 25 C
The table below shows how the pH changes with concentration for sodium hydroxide solutions at 25 C. These are textbook style values based on complete dissociation and pKw = 14.00.
| NaOH Concentration (M) | [OH-] (M) | pOH | pH at 25 C | Interpretation |
|---|---|---|---|---|
| 0.0001 | 0.0001 | 4.000 | 10.000 | Mildly basic |
| 0.001 | 0.001 | 3.000 | 11.000 | Clearly basic |
| 0.005 | 0.005 | 2.301 | 11.699 | Moderately basic |
| 0.01 | 0.01 | 2.000 | 12.000 | Strongly basic |
| 0.10 | 0.10 | 1.000 | 13.000 | Very basic |
How Temperature Changes the Result
Many quick online explanations assume 25 C, but in more advanced chemistry the ion product of water changes with temperature. That means pKw is not always exactly 14.00. As temperature rises, pKw usually decreases, and the neutral pH shifts too. For a strong base such as NaOH, the hydroxide concentration from dissociation still comes from stoichiometry, but the final pH number changes because the pH to pOH relationship changes with pKw.
This is why scientific calculators and lab software often allow a temperature selection. For standard general chemistry, however, unless otherwise stated, assume 25 C.
| Temperature | Approximate pKw of Water | pOH for 0.005 M NaOH | Calculated pH | Neutral pH Context |
|---|---|---|---|---|
| 20 C | 14.16 | 2.301 | 11.859 | Neutral is above 7.00 |
| 25 C | 14.00 | 2.301 | 11.699 | Neutral is 7.00 |
| 30 C | 13.82 | 2.301 | 11.519 | Neutral is below 7.00 |
| 37 C | 13.60 | 2.301 | 11.299 | Warmer water lowers neutral pH |
Why the Strong Base Assumption Works Here
For sodium hydroxide in ordinary aqueous chemistry, complete dissociation is the standard assumption. In other words, if you dissolve 0.005 moles per liter of NaOH, you treat the hydroxide concentration as 0.005 moles per liter. This is appropriate for educational calculations and many practical laboratory estimates. At much higher concentrations, activity effects can cause measured pH to deviate from the idealized textbook result, but those nonideal corrections are beyond the scope of a basic pH problem.
For most learners, the strongest takeaway is that molarity becomes hydroxide concentration directly for a one hydroxide strong base such as NaOH, KOH, or LiOH. That is different from weak bases like ammonia, where only a fraction of the dissolved base generates OH-.
NaOH Compared with Other Bases
The same calculation method applies to other strong monohydroxide bases:
- KOH: 0.005 M KOH also gives [OH-] = 0.005 M and pH about 11.70 at 25 C.
- LiOH: 0.005 M LiOH behaves similarly in basic textbook calculations.
- Ba(OH)2: 0.005 M barium hydroxide gives [OH-] = 0.010 M because each formula unit contributes two hydroxides, so the pH is higher.
This is why stoichiometry matters. Strong base calculations are not only about whether a substance dissociates completely, but also about how many hydroxide ions are released per formula unit.
Worked Example with Scientific Notation
Here is the same problem in a format useful for homework or lab reports:
- Given: 0.005 M NaOH
- Since NaOH is a strong base, [OH-] = 0.005 M = 5.0 x 10^-3 M
- pOH = -log(5.0 x 10^-3) = 2.301
- At 25 C, pH = 14.00 – 2.301 = 11.699
- Final answer: pH = 11.70
When You Might Need a More Advanced Approach
There are situations where a simple concentration based pH calculation does not tell the full story. For example, in concentrated solutions, highly ionic mixtures, or formal analytical chemistry work, activity coefficients may matter. In environmental and biological settings, temperature and buffering can also alter interpretation. Still, for the direct question “calculate pH of 0.005M NaOH,” the accepted chemistry answer remains the strong base approximation shown above.
Practical Interpretation of pH 11.70
A pH near 11.70 indicates a solution that is definitely basic and potentially irritating or corrosive depending on exposure, volume, and context. Sodium hydroxide is widely used in laboratories, industrial cleaning, soap making, and pH adjustment, but it requires careful handling. Even relatively dilute solutions can irritate skin and eyes. In real lab work, gloves, splash protection, and proper dilution technique are important.
Authoritative References and Further Reading
For additional background on pH, water chemistry, and sodium hydroxide safety, review these reputable sources:
Final Answer
If you need the short version, here it is: the pH of 0.005M NaOH at 25 C is 11.699, usually rounded to 11.70. The calculation works because NaOH is a strong base, so its hydroxide ion concentration equals its molarity. Then you compute pOH and convert to pH using pH + pOH = 14.00 at 25 C.