Chemistry Unit 9 Lesson 3: pH

It is a logarithmic scale used to specify the acidity or basicity of aqueous solutions.
The acidic or basic nature of a solution is based on the concentration of H⁺ ions present in a solution.

The logarithmic nature of pH tells us that each number represents 10X an increase or decrease in H⁺ ion concentration.

The pH scale ranges from 0-14. Much like a shower faucet where in the middle is a mixture of hot and cold water, pH is neutral in the middle of the scale at 7. As you move away from this number or turn the shower handle we move towards one extreme or the other.

So with pH numbers larger than 7 are basic. The larger we get the more basic a solution is being measured as.
On the opposite end of the spectrum, numbers smaller than 7 reflect a solution that is acidic.

Remember each number represents a factor of 10X more concentrated.

So for example a 5 is 10X more acidic than a 6 but a 4 is 100X more acidic than a 6.

Earlier this year we learned that acid commonly begin their formula with an H i.e. HCl or Hydrochloric acid.

This is because acids are commonly called Hydrogen donors. As they dissolve in water they release these H's in the form of H+ ions.

This means that acids have a higher concentration of H+ ions

Earlier this year we learned that bases commonly end their formula with an OH i.e. NaOH or Sodium hydroxide

This is because acids are commonly called Hydrogen acceptors. As they dissolve in water they absorb these H+ ions.

This means that bases have a low concentration of H+ ions

This explains why acid and bases "neutralize" one another.
As the acid releases H ions and the Base asorbs them with their OH ions, these two ions combine to simply form H₂O or water.

pH is a measure of these H+ ions. You can change their concentration by neutralizing them back into water by adding more acid or base.

As mentioned earlier pH is based off a logarithmic scale.
It's also based off a negative logarithmic scale.
So as H+ ion concentration goes up the pH actually goes down and vice versa.

pH = - log (M of H+ ions)

pH is equal to the negative log of the Molar concentation or the Hydrogen ions.

So in our formula we are using the Log with a base of 10. The most common log.
If we need to solve for H+ ion concentration we simply take 10 to the negative power of our pH.
Like the picture but we divide over the negative sign in from of our log before reorganizing.

pH = - log (M of H+ ions)

If a solution has a H+ ion concentration of 0.0075 M, what is it's pH?

pH = - log (M of H+ ions)

If a solution has a H+ ion concentration of 0.0075 M, what is it's pH?

pH = -log (H+)

pH = -log (0.0075)

pH = - log (M of H+ ions)

If a solution has a H+ ion concentration of 0.0075 M, what is it's pH?

pH = -log (H+)

pH = -log (0.0075)
2.12 = -log(0.0075)

pH = - log (M of H+ ions)

If a solution has a pH of 12.5, what is the H+ ion concentration?

pH = - log (M of H+ ions)

If a solution has a pH of 12.5, what is the H+ ion concentration?


pH = -log (H+)
12.5 = -log(H+)
-12.5 = log(H+)

pH = - log (M of H+ ions)

If a solution has a pH of 12.5, what is the H+ ion concentration?


pH = -log (H+)
12.5 = -log(H+)
-12.5 = log(H+)

10^-12.5 = H+
3.16 x 10^-13 M H+ ions

pH = - log (M of H+ ions)

It makes sense that our ion concentration was very low because we have a very strong base.

3.16 x 10^-13 M H+ ions

pH = - log (M of H+ ions)

The math can be a little tricky at first but understand that it is very repetitive.

In the future, we will look at how the pOH scale plays a role in this calculation as well.