Calculate pH of 0.005 M NaOH
Use this premium calculator to find the pH, pOH, and hydroxide concentration of a sodium hydroxide solution. For dilute aqueous NaOH, the usual assumption is complete dissociation, so the hydroxide concentration is approximately equal to the NaOH molarity.
NaOH pH Calculator
Enter the concentration and choose the display settings. The default example is 0.005 M NaOH at 25 C.
How to calculate the pH of 0.005 M NaOH
If you need to calculate the pH of 0.005 M NaOH, the chemistry is straightforward because sodium hydroxide is a strong base. In water, NaOH dissociates essentially completely into sodium ions and hydroxide ions:
[OH–] ≈ 0.005 M
Once you know the hydroxide concentration, you can find pOH from the base-10 logarithm:
At 25 C, pH and pOH are related by the standard water ion product expression:
pH = 14.00 – 2.3010 = 11.699
So, the pH of 0.005 M NaOH is approximately 11.70. That answer is the one typically expected in general chemistry, analytical chemistry, and most online homework systems. Because NaOH is a strong base, there is no need to set up a complicated equilibrium table in the usual introductory treatment. The key idea is that every mole of sodium hydroxide contributes one mole of hydroxide ions in dilute solution.
Quick answer
- Given concentration: 0.005 M NaOH
- Hydroxide concentration: 0.005 M
- pOH: 2.301
- pH: 11.699
- Rounded pH: 11.70
Step by step method
Students often remember the final answer but forget the logic that gets them there. Here is the cleanest procedure to use on a quiz, lab report, or exam when you are asked to calculate the pH of 0.005 M NaOH.
- Identify NaOH as a strong base. Strong bases dissociate essentially 100 percent in dilute aqueous solution. This means the hydroxide ion concentration comes directly from the formula concentration.
- Write the dissociation. NaOH splits into Na+ and OH–. Because the stoichiometric ratio is 1:1, a 0.005 M NaOH solution gives about 0.005 M OH–.
- Calculate pOH. Use pOH = -log[OH–]. Taking the negative log of 0.005 gives 2.3010.
- Convert pOH to pH. For standard problems at 25 C, use pH = 14.00 – pOH. This gives 11.699.
- Round appropriately. A common classroom answer is pH = 11.70.
This method works not only for 0.005 M NaOH but also for any other dilute concentration of a strong monohydroxide base such as KOH, assuming ideal introductory conditions. The only thing that changes is the hydroxide concentration term inside the logarithm.
Why NaOH makes this calculation easier than weak bases
The reason this problem is so simple is that sodium hydroxide is not a weak base. Weak bases like ammonia only partially react with water, so their pH must be found using an equilibrium constant and an ICE table. NaOH is different. In most general chemistry settings, it is treated as fully dissociated. That means the hydroxide concentration is known immediately from the analytical concentration.
Compare that with a weak base calculation. For ammonia, you would need a Kb value, set up the equilibrium expression, and solve for the hydroxide concentration generated by reaction with water. For 0.005 M NaOH, none of that is necessary. The concentration of hydroxide is already present because the solute itself produces OH– directly.
Common point of confusion: does lowercase m mean molal or molar?
The phrase “calculate pH of 0.005 m NaOH” can sometimes create notation confusion. In formal chemistry, uppercase M means molarity, while lowercase m means molality. Most classroom pH problems and web searches use lowercase casually, even when they really mean molarity. The standard answer people expect for this query is based on 0.005 M NaOH.
If the concentration were truly 0.005 molal, the exact hydroxide activity would depend slightly on solution density and nonideal behavior. However, at such a dilute concentration, the numerical difference between 0.005 m and 0.005 M in water is very small for most educational purposes. In other words, the pH still comes out very close to 11.70 under typical assumptions.
Comparison table: NaOH concentration vs pH at 25 C
The table below shows how rapidly pH changes as sodium hydroxide concentration changes. These values are computed using the same strong base approach used in this calculator.
| NaOH concentration (M) | [OH–] (M) | pOH | pH | 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 | Your target example |
| 0.010 | 0.010 | 2.000 | 12.000 | Typical textbook strong base case |
| 0.100 | 0.100 | 1.000 | 13.000 | Strongly basic |
Second comparison table: pH scale reference values
It also helps to place 0.005 M NaOH on the larger pH scale. A pH of about 11.70 is strongly basic compared with neutral water at pH 7.00.
| Solution type | Approximate pH | [H3O+] (M) | [OH–] (M) | Comments |
|---|---|---|---|---|
| Strong acid example | 2.0 | 1.0 × 10-2 | 1.0 × 10-12 | Highly acidic |
| Pure water at 25 C | 7.0 | 1.0 × 10-7 | 1.0 × 10-7 | Neutral benchmark |
| 0.005 M NaOH | 11.699 | 2.0 × 10-12 | 5.0 × 10-3 | Strongly basic aqueous solution |
| 0.1 M NaOH | 13.0 | 1.0 × 10-13 | 1.0 × 10-1 | Much more basic |
Important assumptions behind the answer
When a chemistry teacher asks for the pH of 0.005 M NaOH, the expected solution relies on a few standard assumptions. Understanding those assumptions makes you a more accurate problem solver and helps you know when a shortcut is valid.
- Complete dissociation: NaOH is treated as fully dissociated in water.
- 25 C conditions: The relation pH + pOH = 14.00 assumes Kw = 1.0 × 10-14.
- Dilute solution behavior: Activity effects are ignored, which is standard in introductory calculations.
- One hydroxide per formula unit: NaOH contributes one OH– ion per formula unit.
In more advanced physical chemistry or high ionic strength situations, chemists may use activities rather than concentrations, and the exact pH can shift slightly. But for general use, education, and quick lab estimation, 11.70 is the correct and accepted result.
Common mistakes students make
Even though this is a basic strong base calculation, there are a few errors that appear again and again. Avoiding them can save points on tests and prevent inaccurate lab work.
- Using pH = -log[OH–]. That formula gives pOH, not pH.
- Forgetting the 1:1 dissociation. For NaOH, the hydroxide concentration equals the NaOH concentration.
- Rounding too early. Keep at least four decimal places in the pOH value before the final subtraction.
- Mixing up M and mM. A value entered as 0.005 mM would be 1000 times smaller than 0.005 M.
- Ignoring temperature in advanced contexts. The pH + pOH = 14 shortcut is specifically tied to standard conditions often assumed in general chemistry.
Real world meaning of a pH around 11.7
A solution with pH 11.7 is distinctly basic and can be chemically aggressive compared with neutral water. Sodium hydroxide solutions are commonly used in cleaning, titration work, industrial neutralization, and laboratory preparation. Even when relatively dilute, basic solutions can irritate skin and eyes, so proper handling matters. This is one reason pH calculations are not just academic exercises. They connect directly to safety, environmental monitoring, and process control.
Environmental and regulatory science also pays close attention to pH because aquatic systems, industrial discharges, and treated water all depend on maintaining ranges compatible with materials, biology, and human use. Strong bases like NaOH are frequently used to adjust pH upward in controlled settings, which makes knowing how concentration relates to pH especially useful.
Authoritative references for pH and water chemistry
If you want to go beyond the quick calculation and review trusted explanations of pH, water chemistry, and alkaline solutions, these references are excellent places to start:
Final answer
To calculate the pH of 0.005 M NaOH, assume complete dissociation so that [OH–] = 0.005 M. Then calculate pOH = -log(0.005) = 2.301, and finally use pH = 14.00 – 2.301 = 11.699. Rounded to two decimal places, the pH is 11.70.