Question

The hydrogen ion concentration of the oceans is about
2 x 10-9 M. What is the pH ?
(1) 8.85
(2) 9.3
(3) 7.85
1478.7​

Answer 1

Answer:

How to Calculate pH and [H+]

The equilibrium equation yields the following formula for pH:

pH = -log [H ]

[H ] = 10

In other words, pH is the negative log of the molar hydrogen ion concentration. Or, the molar hydrogen ion concentration equals 10 to the power of the negative pH value. It's easy to do this calculation on any scientific calculator because it will have a "log" button. This is not the same as the "ln" button, which refers to the natural logarithm.

Example:

Calculate the pH for a specific [H ]. Calculate pH given [H ] = 1.4 x 10 M

pH = -log [H ]

pH = -log (1.4 x 10 )

pH = 4.85

Example:

Calculate [H ] from a known pH. Find [H ] if pH = 8.5

[H ] = 10

[H ] = 10

[H ] = 3.2 x 10 M

Example:

Find the pH if the H concentration is 0.0001 moles per liter.

pH = -log[H ]

Here it helps to rewrite the concentration as 1.0 x 10 M because if you understand how logarithms work, this makes the formula:

pH = -(-4) = 4

Or, you could simply use a calculator and take:

pH = - log (0.0001) = 4

Usually, you aren't given the hydrogen ion concentration in a problem but have to find it from a chemical reaction or acid concentration. Whether this is easy or not depends on whether you're dealing with a strong acid or a weak acid. Most problems asking for pH are for strong acids because they completely dissociate into their ions in water. Weak acids, on the other hand, only partially dissociate, so at equilibrium, a solution contains both the weak acid and the ions into which it dissociates.

Example:

Find the pH of a 0.03 M solution of hydrochloric acid, HCl.

Hydrochloric acid is a strong acid that dissociates according to a 1:1 molar ratio into hydrogen cations and chloride anions. So, the concentration of hydrogen ions is exactly the same as the concentration of the acid solution.

[H = 0.03 M

pH = - log (0.03)

pH = 1.5

Check Your Work

When you're performing a pH calculation, it's a good idea to make sure your answer makes sense. An acid should have a pH much less than 7 (usually 1 to 3), while a base has a high pH value (usually around 11 to 13). While it's theoretically possible to calculate a negative pH , in practice pH values should be between 0 and 14. This, a pH higher than 14 indicates an error either in setting up the calculation or else using the calculator.