Chemistry I Honors

Acids and BasespH and Associated Calculations

Molarity

• This is actually “old business” as we covered Molarity in our lessons involving solutions.

• The “Molarity Pyramid” still works for these systems.

• We can calculate the molarity of an acid solution the same way we would calculate the molarity of any other solution.

Molarity

Moles of Solute

Liters of Solution

The Concentration of Hydronium

• This is an easy one.• If you look back at the first presentation, you

see that acid molecules react with water to form hydronium ( H3O+ ).

• In every single example presented, one acid molecule reacted with one water molecule to produce one hydronium ion.

example: HCl + H2O H3O+ + Cl-

Meaning ????• If you look back at the

reaction on the previous slide, you will see that the ratio of H3O+ to HCl is 1 : 1

• Meaning that the concentration of hydronium will be exactly what the concentration of the acid solution was.

• So, if the HCl solution is prepared to be 0.50 molar, the concentration of H3O+ will be 0.50 - molar

No Big Deal There…What’s Next ?

• What’s next is something called pH .

• pH is a measurement that communicates the relative acidity of an acid (or base – but that discussion occurs later) solution.

• It is simply a number value – and it does not have units

• The best place to begin a discussion of pH is to understand the pH scale.

• As you will see on the next slide, the common scale runs from 0 to 14 with 7 in the middle.

• Acid solutions have pH values less than 7 and base solutions have pH values greater than 7

• An actual value of 7 is considered “neutral” – neither acidic nor basic.

The pH Scale

Note that the word “alkaline” is a synonym for “basic”. While this visual does not show it, the scale does

extend to 0 on the left and 14 on the right.

More on the pH Scale

• Need to realize that the “ends” of the scale are “more”.– As you move to the left,

the solutions become more acidic.

– As you move to the right, the solutions become more basic.

• That means that a pH = 2 solution is more acidic than a pH = 3 solution. (and a pH 1 solution would be more acidic than a pH 2 solution)

• Or, a pH 10 solution is more basic than a pH 9 solution.

What about the Calculations?

• The pH scale is actually a logarithmic scale, so we will be taking the logs of numbers.

• The equation to calculate pH is:

pH = - log [H3O+]pH is the opposite of the log of the concentration

of the hydronium in a solution.The square brackets around the hydronium mean

“concentration of” (in terms of molarity)

Keystrokes

• So, once you have the molarity of the acid solution…

• On a TI-30 series calculator– Start with the numerical

value of the molarity– Push the “log” button– Change the sign

• You have the pH

• On the TI – 80 series calculator– Push the sign change

button (-)– Push the “log” button– Enter the numerical

value of the molarity– Hit “enter”

• Now you have the pH

What if You Have a Base?

• Remember from the slide on the pH scale that bases have pH values that are greater than 7.

• We also have to keep in mind that bases do not really have concentrations of H3O+ because that ion is produced by acids.

• What we learned in Acids Lesson #1 is that bases are metal hydroxides that dissociate in water to yield OH- ions.

• For monobasic compounds (have 1 OH- in the formula), the [OH-] will be the same as the molarity of the base solution (just like the acids).

Showing the Base Dissociation

• Again, consider the dissolving of sodium hydroxide (monobasic) in water.

Na(OH) Na+1 (aq) + (OH)-1 (aq)

• There are no coefficients written – they are all 1’s. So… the ratio of (OH)-1 to Na(OH) is 1 : 1.

• Therefore, the [OH-1] will be equal to the molarity of the Na(OH) solution.

• If the Na(OH) solution has a concentration of 0.30 molar, the [(OH)-1] will be 0.30 molar.

pOH The Calculation for Bases

• Chemistry has a “parallel” calculation (to the determination of pH) just for Bases.

• It is called pOH and it is calculated virtually the same way as we calculate pH.

pOH = - log [OH-1]• The keystrokes on the calculator will be exactly the

same as described for pH. But the results will be “curious” – look at the example on the next slide.

Sample Problem

“A solution of Na(OH) is prepared to have a concentration of 0.0025 molar. What is the pOH of this solution?”First - Since the Na(OH) is 0.0025 Molar, the [OH-1] is also 0.0025 Molar.Then: pOH = - log [OH-1]

= - log (0.0025)= 2.60 (no units)

The “Curious” Part

• You have already seen that the pH scale is structured so that bases have pH values greater than 7.

• The answer you calculated in the previous slide was 2.60 (obviously less than 7).

• How is this possible?

Explanation

• The calculation that we did was for pOH , not pH.

• The pOH scale actually runs backwards compared to the pH scale– The zero value will be on the right– The 14 value will be on the left– This is because bases are the “chemical opposites”

of acids.

Comparing the Scales

• Notice how the scales run in opposite directions to each other.• Notice too how low values on each scale correspond to “more”• Low pH value is more acidic• Low pOH value is more basic

Here is the Cool Part

• If we draw a vertical line anywhere on the diagram above and add the values of pH and pOH that the line would intersect on the two scales, the result of that adding will be “14”. (Try it!)

• That allows us to create an equation that connects pH and pOH

pH + pOH = 14

Back to the Original Problem

“A solution of Na(OH) is prepared to have a concentration of 0.0025 molar. What is the pOH of this solution?”• Looking back at the calculation (5 slides

earlier), the pOH = 2.60• But, what if the problem had actually asked

you to calculate the pH ? (instead of the pOH).

Now You Can Do This

• If the pOH is 2.60 , we can convert this measurement to a pH using the relationship presented two slides earlier.

pH + pOH = 14So: pH + 2.60 = 14

Subtracting: pH = 14 - 2.60pH = 11.4

Which is in the Base range on the pH scale. (as it should be – NaOH is a base.)

SummarizingAcids

• Have the formula “HX”• Ionize to form H3O+1

• Will have [H3O+1] equal to the acid solution molarity

• You will calculate a pH using the [H3O+1]

• You can convert this to a pOH using pOH = 14 - pH

Bases• Are metal hydroxides• Dissociate in water to form

the OH-1 ion• Will have [OH-1] equal to the

base solution molarity• You will calculate a pOH

using the [OH-1]• You can convert this to a pH

using pH = 14 - pOH

Your Task• There are two different worksheets that will

provide some quick practice in calculations involving molarity and pH and pOH.