How do I find pH and pOH when given molarity?

Question

exmaple....pH of 0.0025 M H3O+

and molarity from pH and pOH

Answer 1

pH=-log[H+]
pOH=-log[OH-]

some useful equations to obtain a term given only the other:
[H+][OH-]=1E-14
pH+pOH=14


brackets indicate concentration.

Answer 2

To give you more detail in answering the question, convert the given molarity 0.0025 M into scientific notation for easier calculation and checking if the answer is in the right "ballpark".

This gives 2.5 x 10-3 or to the power -3 ( sorry unable to do negative exponents directly on yahoo). Then, take the negative logarithm of the molarity. Note that the logarithm of 2 multiplied factors is the sum of the logarithm of each factors, (2.5) and (10 to the power -3). Please note that pH and pOH are NEGATIVE logarithms. So, after taking the logs, multiply the answer by -1.

This will give you 0.40 and -3. Adding them gives you -2.6. Then taking the negative or multiplying by -1, you get +2.6 (close to +3) as your final answer for pH. It is an acidic solution. The corresponding pOH of this same solution is calculated by just SUBTRACTING the pH value from the number 14. Or, pOH = 14 -pH; 14-2.6 or 11.4.

The usefulness of converting to scientific notation is that you can easily ESTIMATE the pH of any solution. For example, if the exponent of the base 10 is -4, the pH is close to 4 with maximum at 4. Likewise, if the pH is -6, the pH is close to 6 with maximum at 6. Then, use the formula for getting the corresponding pOH.