Water Chemistry pH

Chemistry Unit 9 Lesson 3: pH

Multiple Choice

If a solution has watered added until it's volume doubles, what will happen to the concentration?

• It will also double

• It will be cut in half

• It will be cut by a fourth

• It will quadruple

Multiple Choice

What will be the final concentration of a solution if it originally has a concentration of 0.25 M and volume of 500 mL but is allowed to sit out so as water evaporates it's volume becomes 300 mL?

• 0.15 M

• 0.42 M

• 1.25 M

• 0.33 M

Multiple Choice

How much more water should be added to a 4 M solution with a volume of 40 mL if I need to reduce it to 0.5 M for it to be safe? Be careful you are not simply answering the final volume.

• 320 mL

• 280 mL

• 360 mL

• 80 mL

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.

pH is a scale of measure

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

pH is a scale of measure

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.

pH is a scale of measure

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.

pH is a scale of measure

Multiple Choice

How many more times basic is a pH of 12 than a pH of 8?

• 10,000X

• 1000X

• 4X

• 40X

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.

pH is a scale of measure

Categorize

Options (10)

pH less than 7

pH greater than 7

Sour taste

Neutralizes bases

Commonly found in citrus fruits

Used in car batteries

Slippery to the touch

Tastes bitter

Neutralizes acids

Commonly found in household cleaning products

Using the knowledge gained from watching the previous video clip, drag and drop each category based on whether it describes an acid or a base.

ACID

BASE

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.

Where does this H+ come from

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.

Where does this H+ come from

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.

Where does this H+ come from

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.

Multiple Choice

Which of the following can be defined as a hydrogen donor?

• Acid

• Base

• Neutral

Multiple Choice

Which of the following can be defined as a hydrogen acceptor?

• Acid

• Base

• Neutral

Fill in the Blank

When acids and bases are mixed they will _______ one another forming water.

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.

How do we get the pH numbers

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.

How do we get the pH numbers

pH = - log (M of H+ ions)

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

Lets give it a try

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)

Lets give it a try

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)

Lets give it a try

pH = - log (M of H+ ions)

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

Lets give it a try

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+)

Lets give it a try

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

Lets give it a try

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

Lets give it a try

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.

Lets give it a try

Math Response

What is the pH of a solution if it has a concentration 3.7 x 10^-8 M of H+ ions?

ROUND YOUR ANSWER TO THE NEAREST TENTH

Type answer here

Correct Answer: 7.4

Deg^°

Rad

Math Response

If a solution has a pH of 3.5, what must be the molarity of it's H⁺ ion concentration?

ROUND YOUR ANSWER TO THE NEAREST TENTH

Type answer here

Correct Answer: 3.2\times10^{-4}

Deg^°

Rad