Calculate the pH of the Following Solution: 0.085 M LiOH
Use this premium calculator to find pOH, pH, hydroxide concentration, and solution classification for lithium hydroxide. The default example is set to 0.085 M LiOH, a strong base that dissociates essentially completely in water.
Calculated Results
Hydroxide ion [OH⁻]
0.085 M
pOH
1.071
pH
12.929
For 0.085 M LiOH at 25°C, the solution is strongly basic because LiOH dissociates to produce approximately 0.085 M OH⁻.
How to Calculate the pH of 0.085 M LiOH
To calculate the pH of a 0.085 M lithium hydroxide (LiOH) solution, you use the fact that LiOH is a strong base. In standard general chemistry problems, strong bases are treated as substances that dissociate essentially completely in water. That means each formula unit of LiOH releases one hydroxide ion, OH⁻, into solution. Because the molar concentration of LiOH is 0.085 M and the stoichiometric ratio between LiOH and OH⁻ is 1:1, the hydroxide ion concentration is also 0.085 M.
Once you know the hydroxide concentration, you can calculate pOH using the logarithmic relation:
pOH = -log[OH⁻]
Substituting the concentration:
pOH = -log(0.085) = 1.071
Then use the common 25°C relationship between pH and pOH:
pH + pOH = 14.00
So:
pH = 14.00 – 1.071 = 12.929
Final answer: The pH of a 0.085 M LiOH solution is 12.929, which is usually reported as 12.93.
Why LiOH Is Treated as a Strong Base
Lithium hydroxide belongs to the family of alkali metal hydroxides. In introductory and most intermediate chemistry calculations, these compounds are considered strong electrolytes in aqueous solution. That means they separate into ions to a very high extent:
LiOH(aq) → Li⁺(aq) + OH⁻(aq)
Because one mole of LiOH yields one mole of OH⁻, the hydroxide concentration is numerically equal to the LiOH concentration if the solution is dilute and there are no competing equilibria or activity corrections being applied. This is why the calculation is direct and simpler than it would be for a weak base such as ammonia.
Students often wonder whether lithium hydroxide behaves differently from sodium hydroxide or potassium hydroxide in pH problems. In practical textbook calculations of this type, the answer is no. If the concentration is given and the question asks for pH, the workflow is the same: determine OH⁻ concentration from dissociation, compute pOH, and then convert pOH to pH.
Step-by-Step Solution Method
- Identify the substance: LiOH is a strong base.
- Write dissociation: LiOH → Li⁺ + OH⁻.
- Determine hydroxide concentration: [OH⁻] = 0.085 M.
- Calculate pOH: pOH = -log(0.085) = 1.071.
- Find pH: pH = 14.00 – 1.071 = 12.929.
- Report with appropriate significant figures: pH ≈ 12.93.
Short Exam-Style Answer
LiOH is a strong base, so it dissociates completely. Therefore, [OH⁻] = 0.085 M. Then:
pOH = -log(0.085) = 1.07
pH = 14.00 – 1.07 = 12.93
Answer: 12.93
Common Mistakes When Solving This Problem
- Using pH = -log(0.085) directly: That would be wrong because 0.085 M is the concentration of a base, not an acid. For a base, calculate pOH first.
- Forgetting the 1:1 dissociation: LiOH releases one hydroxide ion per formula unit, so [OH⁻] equals the base concentration.
- Rounding too early: If you round pOH too soon, your pH can shift slightly. Keep extra digits until the end.
- Mixing up strong and weak bases: LiOH is not treated like NH₃. No base dissociation constant calculation is needed in a standard problem.
- Ignoring temperature assumptions: The relation pH + pOH = 14.00 is standard at 25°C. In advanced work, pKw changes slightly with temperature.
Comparison Table: pH of Common Strong Bases at 0.085 M
The exact pH approach depends on how many hydroxide ions each base releases. Here is a comparison using standard 25°C assumptions.
| Compound | Base Type | Stoichiometric OH⁻ Released | [OH⁻] at 0.085 M | pOH | pH |
|---|---|---|---|---|---|
| LiOH | Strong monoprotic base | 1 | 0.085 M | 1.071 | 12.929 |
| NaOH | Strong monoprotic base | 1 | 0.085 M | 1.071 | 12.929 |
| KOH | Strong monoprotic base | 1 | 0.085 M | 1.071 | 12.929 |
| Ba(OH)₂ | Strong dibasic base | 2 | 0.170 M | 0.770 | 13.230 |
This comparison shows an important concept: the identity of the strong base matters less than the number of hydroxide ions produced per formula unit. LiOH, NaOH, and KOH all give one hydroxide ion each, so solutions of equal molarity have essentially the same pH under basic textbook conditions.
Reference Data: pH Scale Benchmarks
A pH of about 12.93 places the LiOH solution firmly in the strongly basic region. The table below gives context for interpreting the number.
| pH Range | Classification | Typical Example | Interpretation |
|---|---|---|---|
| 0 to 3 | Strongly acidic | Strong acid solutions | High hydronium concentration |
| 4 to 6 | Weakly acidic | Dilute acidic beverages | Mildly acidic region |
| 7 | Neutral | Pure water at 25°C | [H⁺] = [OH⁻] |
| 8 to 10 | Weakly basic | Bicarbonate solutions | Moderate alkalinity |
| 11 to 14 | Strongly basic | Alkali metal hydroxides | High hydroxide concentration |
What the 0.085 M Concentration Really Means
The concentration 0.085 M means there are 0.085 moles of LiOH in every liter of solution. Since LiOH dissociates into Li⁺ and OH⁻, that same liter contains about 0.085 moles of hydroxide ions. pH and pOH are logarithmic, so even a concentration that looks modest can correspond to a strongly basic solution.
This is one reason chemistry students need to be comfortable with logarithms. The pH scale compresses a huge range of ion concentrations into a manageable numerical scale. A change of 1 pH unit corresponds to a tenfold change in hydrogen ion concentration. For bases, the same logarithmic reasoning applies through pOH and hydroxide concentration.
Deeper Concept: Why We Calculate pOH First for Bases
By definition, pH is based on hydrogen ion concentration and pOH is based on hydroxide ion concentration. Since a lithium hydroxide problem directly gives or implies hydroxide concentration, pOH is the natural first step. After that, pH follows from the water ion-product relationship at 25°C:
Kw = [H⁺][OH⁻] = 1.0 × 10-14
Taking the negative logarithm of both sides gives:
pH + pOH = 14.00
In high-level analytical chemistry, you may sometimes use activities rather than concentrations, especially for more concentrated or non-ideal solutions. But for a standard educational problem involving 0.085 M LiOH, concentration-based calculation is the expected method.
Worked Comparison With a Weak Base
It helps to compare LiOH with a weak base such as ammonia, NH₃. If you had a 0.085 M ammonia solution, you could not simply set [OH⁻] equal to 0.085 M because ammonia only partially reacts with water. Instead, you would need the base dissociation constant, Kb, and solve an equilibrium expression. With LiOH, no ICE table is required in the standard approach. That is the main reason this problem is relatively straightforward.
Strong Base Workflow
- Use stoichiometry directly.
- Set [OH⁻] equal to the strong base contribution.
- Compute pOH and then pH.
Weak Base Workflow
- Write the equilibrium reaction with water.
- Use Kb and an ICE table.
- Solve for equilibrium [OH⁻].
- Find pOH and then pH.
Authority Sources for pH, Water Chemistry, and Base Calculations
For readers who want to verify pH conventions and chemical fundamentals, these authoritative educational and government sources are useful:
- U.S. Geological Survey (USGS): pH and Water
- LibreTexts Chemistry, hosted by higher-education institutions
- U.S. Environmental Protection Agency (EPA): pH Overview
Practical Interpretation of the Result
A pH of about 12.93 indicates a highly alkaline solution. In practical settings, solutions in this range can be corrosive or irritating depending on concentration and exposure. In laboratory work, standard safety precautions apply: eye protection, gloves, proper labeling, and careful handling. The chemistry answer itself is simple, but the practical meaning is important because strong bases can react with tissues, some metals, and acidic materials.
Even though 0.085 M is far below the concentration of many stock laboratory hydroxide solutions, the resulting pH is still very high because hydroxide ions are present at a significant concentration. This illustrates how strongly basic alkali hydroxides are in water.
Final Summary
To solve “calculate the pH of the following solution: 0.085 M LiOH”, remember that lithium hydroxide is a strong base and dissociates completely:
LiOH → Li⁺ + OH⁻
That gives:
- [OH⁻] = 0.085 M
- pOH = -log(0.085) = 1.071
- pH = 14.00 – 1.071 = 12.929
The reported answer is therefore pH = 12.93. If you are practicing for homework, quizzes, AP Chemistry, or general chemistry exams, this is a classic strong-base calculation and a useful model for any group 1 metal hydroxide concentration problem.