Calculate Oh- And Ph For 1.5 X 10-3M Sroh 2

Chemistry Calculator

Use this interactive calculator to find hydroxide concentration, pOH, and pH for aqueous strontium hydroxide. The default setup matches the classic problem: 1.5 x 10-3 M Sr(OH)2 at 25 degrees Celsius.

Base Type

Strong

OH- per Unit

2

Default Molarity

1.5e-3 M

Expected pH

11.48

How to calculate OH- and pH for 1.5 x 10-3 M Sr(OH)2

To calculate OH- and pH for 1.5 x 10-3 M Sr(OH)2, you use the fact that strontium hydroxide is treated as a strong base in typical general chemistry problems. That means it dissociates essentially completely in dilute aqueous solution. The balanced dissociation equation is:

Sr(OH)2(aq) -> Sr2+(aq) + 2OH-(aq)

This equation is the key idea. One formula unit of strontium hydroxide releases two hydroxide ions. Therefore, the hydroxide concentration is not the same as the formal concentration of the base. It is double the listed molarity. For a solution that is 1.5 x 10-3 M in Sr(OH)2, the hydroxide ion concentration becomes:

[OH-] = 2 x 1.5 x 10-3 = 3.0 x 10-3 M

Once you know [OH-], you can find pOH using the negative logarithm:

pOH = -log(3.0 x 10-3) = 2.52

At 25 degrees Celsius, pH and pOH add to 14.00, so:

pH = 14.00 – 2.52 = 11.48

So the final answer for the standard problem is:

• [OH-] = 3.0 x 10-3 M
• pOH = 2.52
• pH = 11.48

Why Sr(OH)2 changes the calculation compared with NaOH

Students often memorize that strong bases give hydroxide directly, but the most common mistake is forgetting to count how many hydroxide ions come from each formula unit. Sodium hydroxide, NaOH, releases only one OH-. Calcium hydroxide, Ca(OH)2, barium hydroxide, Ba(OH)2, and strontium hydroxide, Sr(OH)2, release two. That difference doubles the hydroxide concentration and changes pOH and pH significantly.

For example, a 1.5 x 10-3 M NaOH solution would have [OH-] = 1.5 x 10-3 M, while a 1.5 x 10-3 M Sr(OH)2 solution has [OH-] = 3.0 x 10-3 M. Since pH is logarithmic, doubling [OH-] does not double the pH, but it does increase basicity in a measurable way. This is why stoichiometry matters before you use any logarithms.

Step by step method

1. Write the dissociation equation: Sr(OH)2 -> Sr2+ + 2OH-.
2. Identify the base concentration: 1.5 x 10-3 M.
3. Multiply by 2 because each formula unit releases two hydroxides.
4. Calculate hydroxide concentration: 3.0 x 10-3 M.
5. Find pOH using pOH = -log[OH-].
6. At 25 degrees C, calculate pH from pH = 14.00 – pOH.

Common student errors in this exact problem

This problem looks easy, but it produces a surprising number of mistakes. The most common errors are predictable, and if you know them, you can avoid losing points on chemistry homework, quizzes, and exams.

• Forgetting the coefficient 2 for OH-: This is the biggest mistake. If you skip it, your [OH-] becomes too small and your pH is too low.
• Using pH directly from the base concentration: You must first compute [OH-], then pOH, then pH.
• Entering the scientific notation incorrectly: 1.5 x 10-3 means 0.0015, not 0.015.
• Confusing strong and weak base behavior: Sr(OH)2 is generally treated as a strong base in introductory chemistry calculations.
• Ignoring temperature: In more advanced contexts, pKw changes with temperature, so pH + pOH may not equal 14.00.

Quick exam shortcut: For any M(OH)2 strong base in dilute solution, hydroxide concentration is approximately 2 x the listed molarity. After that, use pOH and then pH.

Worked example using the exact values

1. Write the chemical relationship

Strontium hydroxide dissociates according to:

Sr(OH)2 -> Sr2+ + 2OH-

This means every mole of dissolved Sr(OH)2 generates 2 moles of OH-. If the solution concentration is 1.5 x 10-3 M, the hydroxide concentration must be twice that amount.

2. Calculate hydroxide concentration

[OH-] = 2 x 1.5 x 10-3 = 3.0 x 10-3 M

This is the value you need for the logarithmic step.

3. Convert [OH-] to pOH

pOH = -log(3.0 x 10-3)

pOH = 2.52

If you use more decimal places on your calculator, you may get 2.5229. Rounded to two decimal places, pOH is 2.52.

4. Convert pOH to pH

At 25 degrees C:

pH + pOH = 14.00

pH = 14.00 – 2.52 = 11.48

The solution is clearly basic, as expected for a strong metal hydroxide.

Comparison table: how the stoichiometry affects [OH-] and pH

Base	Base concentration	OH- ions released per formula unit	Calculated [OH-]	pOH at 25 degrees C	pH at 25 degrees C
NaOH	1.5 x 10-3 M	1	1.5 x 10-3 M	2.82	11.18
Sr(OH)2	1.5 x 10-3 M	2	3.0 x 10-3 M	2.52	11.48
Ba(OH)2	1.5 x 10-3 M	2	3.0 x 10-3 M	2.52	11.48

This table highlights why it is dangerous to assume all strong bases behave like NaOH. The formula unit matters. The hydroxide stoichiometry controls the concentration of OH- in solution, and because pH is logarithmic, even a modest stoichiometric difference changes the answer.

What happens if temperature changes

In most introductory chemistry problems, the assumption is 25 degrees Celsius, and the relation pH + pOH = 14.00 is used automatically. However, the ionic product of water, Kw, changes with temperature. As temperature rises, Kw increases, which means pKw decreases. That changes the sum of pH and pOH. The solution is still basic when pH is greater than the neutral pH at that temperature, but neutrality itself shifts with temperature.

For practical classroom work, you usually stick with 25 degrees C unless your teacher or textbook tells you otherwise. Still, advanced students benefit from seeing the real numerical pattern.

Temperature	Approximate Kw	Approximate pKw	Meaning for calculations
0 degrees C	1.15 x 10-15	14.94	pH + pOH is about 14.94
25 degrees C	1.00 x 10-14	14.00	Standard textbook condition
50 degrees C	5.50 x 10-14	13.26	Neutral pH shifts lower
100 degrees C	5.13 x 10-13	12.29	pH + pOH is far below 14

These values explain why calculators that let you choose a temperature are more chemically realistic. The formula for [OH-] from Sr(OH)2 does not change, but the conversion from pOH to pH can change if pKw is different from 14.00.

Why the answer is basic but not extremely basic

A pH of 11.48 is definitely basic, but it is not close to the maximum pH values that highly concentrated strong bases can reach. The concentration here is only 0.0015 M in Sr(OH)2, which produces 0.0030 M in hydroxide. That is strong enough to create a clearly basic solution, but still dilute compared with concentrated laboratory base solutions. This is a helpful intuition check. If you obtained a pH near 13 or 14 from this concentration, that would usually signal a setup error.

How to check your answer quickly

There are several fast checks you can perform after finishing the math:

• If the solution contains a strong base, pH should be above 7 at 25 degrees C.
• If the concentration is around 10-3 M and the base gives two OH-, [OH-] should be on the order of 10-3 M.
• The pOH should therefore be a little above 2, not above 4 and not below 1.
• The pH should be around 11 to 12, which fits the final result of 11.48.

Authority references for pH, hydroxide, and water equilibrium

If you want to verify the chemistry background from authoritative educational or government sources, these references are useful: USGS on pH and water, NCBI Bookshelf overview of pH concepts, Purdue Chemistry guide to pH calculations from Kw.

Final answer for the problem

For 1.5 x 10-3 M Sr(OH)2 at 25 degrees C:

• [OH-] = 3.0 x 10-3 M
• pOH = 2.52
• pH = 11.48

That answer comes directly from complete dissociation of Sr(OH)2 and the fact that each formula unit contributes two hydroxide ions. If you remember to do the stoichiometry before using logarithms, you can solve this kind of problem quickly and accurately every time.