Lesson 7.03: pH

Concepts: slides 1-16

Lab: Slides 17-39

Lesson Standard & Objectives

• SC.912.P.8.11—Relate acidity and basicity to hydronium and hydroxyl ion concentration and ph.

• SC.912.P.8.8—Characterize types of chemical reactions, for example: redox, acid-base, synthesis, and single and double replacement reactions.

After completing this lesson, you will be able to:– Determine the relationships between pH,

hydronium, and hydroxide ions in a given solution.

– Describe the process of titration and apply it to a laboratory investigation.

Mastery of Previous Material Required

• Stoichiometry– Chemicals react based upon moles (review lesson

4.06)– Molar mass (review lesson 2.04, 3.09, 4.06)

• Molar mass is the mass in grams of one mole of your chemical, according to the periodic table

• Molarity– Lesson 5.06/5.07

• Molarity formula: M = mole solute liter solution

• Remember: the brackets around a formula indicates concentration in Molarity: [H2O]

pH and Water

• Water is amphoteric• Water has the ability to behave as both an acid and a

base, depending on the properties of the other reactant.

• Water molecules can react with each other.

• One water molecule, acting as an acid, donates a hydrogen ion to another water molecule, acting as a base.

H2O + H2O OH- + H3O+

pH and water

H2O + H2O OH- + H3O+

• Oftentimes, we say “hydrogen” ions in solution. However, it really is the hydronium ion, H3O+

• In pure water, the concentrations of hydroxide and

hydronium ions are equal.

• Pure water behaves as a very weak acid or base.

• Pure water is a very poor electrolyte. Do you know why?

• Pure water is neutral because it contains equal amounts of hydroxide and hydronium ions.

Acidic Solutions

In an acidic solutions, the concentration of hydronium ions is greater than the concentration of the hydroxide ions present.

H2O + H2O OH- + H3O+

Acidic Solution = [H3O+] > [OH-]

Basic Solutions

In a basic solutions, the concentration of hydroxide ions is greater than the concentration of hydronium ions.

Basic Solution = [H3O+] < [OH-]

H2O + NH3 OH- + NH4+

pH and Water

Acidic Solution = [H3O+] > [OH-]

Neutral Solution = [H3O+] = [OH-]

Basic Solution = [H3O+] < [OH-]

There are more hydronium (hydrogen) ions in solution

There are more hydroxide ions in solution

pH of a Solution

The pH scale is a numeric scale used to indicate the hydronium ion concentration of a solution.

pH = - log [H3O+]Recall from module 5: Brackets are used to represent concentration in molarity

Review Molarity:

M = moles solute liter solution

Using Concentration to Solve for pH

What is the pH of a 1.2 x 10-3 molar nitric acid, HNO3 solution?

pH = - log [H3O+]

pH =

pH =

Using Concentration to Solve for pH

When you are given the [OH-] in a basic solution, you can use that value to find the [H3O+] present using the following relationship:

[H3O+] x [OH-] = 1.0 x 10-14 M

The value 1.0 x 10-14 is called the ionization constant of water.

Ionization Constant of Water – Formula Sheet

• We can rearrange the formula to solve for any of the variables

[H3O+] x [OH-] = 1.0 x 10-14 M

[OH-] = 1.0 x 10-14 M [H3O+]

[H3O+] = 1.0 x 10-14 M

[OH-]

Using Concentration to Solve for pH – Practice #1

What is the pH of a 2.3 x 10-2 molar NaOH solution?

Practice #2

What is the pH of a solution with a concentration of 6.8 x 10-4 molar H3O+?

Practice #3

What is the pH of a solution with a concentration of4.2 x 10-5 molar OH-?

Using pH to solve for Concentration

[H3O+] = 10-pH

We can determine the concentration of hydronium and hydroxide ions in a solution that has a pH of 2.3 by plugging into the formula above.

[H3O+] = 10-2.3

[H3O+] = 0.0050 M

Rearrange the pH formula: pH = - log [H3O+]

Titration Lab - Vocabulary

• Titration• Buret• Stopcock• Stoichiometrically

Equivalent• Endpoint• Acid-base indicator• Titrant

What is a Titration?

• Titration is a laboratory method that allows you to determine the concentration of an acid or a base by reacting it with a known concentration of the other.

• Imagine you have a beaker of HCl, but you don’t know the concentration. Well, if have some NaOH with a KNOWN concentration, we can react them! Once, they are neutralized, we will know how many moles of acid we have! Then, we can calculate its molarity.

• Confused?

• Don’t worry…let’s talk about this one step at a time…

Titration Apparatus

Buret – (burette) narrow piece of glassware that is used to deliver small amounts of one reactant to a flask containing the other reactant.

Stopcock – The bottom of the buret can be adjusted, using the stopcock , to release a small stream of liquid or add the reactant drop by drop. Think of turning on/off a faucet.

Titration is a laboratory method that allows you to determine the concentration of an acid or a base by reacting it with a known concentration of the other.

End Point

In a titration, we always react an acid with a base or vice versa. When ‘stoichiometrically equivalent’ amounts of each have reacted, the reaction will be neutral. We call this the End Point.

acid + base = neutral (pH=7)Recall that acids have a pH < 7, and bases pH > 7

Stoichiometrically equivalent means: moles of acid = moles of base

Then, we can use stoichiometry to calculate moles of the unknown. (we’ll see more about this later)

Measuring the pH

• Most of the time, acids and bases are and form colorless solution. So, how do we know when we have reacted enough?

• We must use something to measure the pH:1. *An acid-base indicator

(Chemical)2. A pH meter (instrument)

Acid Base Indicator in our lab

Acid-Base indicator– Phenolphthalein (Chemical)– Turns pink in basic solution

End Point

Part I Titration

Solution = titrant• The solution that is in a buret is often referred to

as the titrant.

• Be sure to remove all air bubbles from the buret. Why?

• Be sure the dropper end of the buret is filled with solution as well.

• It is important to record the initial volume of solution in the buret in your lab sheet and the “notebook” before the titration begins.

Unknown concentration in the flask. The purpose of lab is to determine it!

Part I Titration

http://www.youtube.com/watch?v=g8jdCWC10vQ

In part I of the lab, you will use an acid-base indicator to determine the end point of the reaction. Add a few drops of the indicator to the flask before titrating.

What would happen if we forgot?

In part II of the lab, you will use a pH meter (electronic) to determine the end point of the reaction. The pH at the end point =

Part I TitrationWhen you start the titration, click on the stopcock (turn the knob) to drop a few milliliters of titrant to the flask. Go slowly until you get the hang of it.

You will see the indicator change color in the bottom flask when the titrant makes contacts with the solution in the flask.

Be sure to constantly stir the solution inside the flask by carefully moving the flask in small circles as the titrant is being added. You will see the color change disappear as you stir. As you near the end point, the color change will begin to linger. Slow down! Add the titrant drop by drop.

If you add too much, and the flask is too pink, you will have to start over! Patience is a virtue for this lab.

Part I Titration

Your endpoint will look like the picture.

The color change should stay after the solution has been stirred for 10 seconds, but it should be so light that you may need to hold a piece of white paper behind the flask to notice the color change.

When you have reached the end point, read the final volume in the buret and record it in your data table or notebook.

Part I Titration Calculations (HINTS!)

1. Use subtraction to determine the amount of titrant added from the buret into the flask.

Final volume of titrant – initial volume of titrant = volume of titrant used

Review:

What is the titrant?How do we read a buret?

Reading the Buret (Burette)

Top: Initial Bottom: Final

The numbers go in reverse order. 0 mL is at the top (when buret is full, V = 50 mL).Report 2 decimal places! Remember those sig figs from 1st semester (lesson 1.07)

0 mL

50 mL

Part I Titration Calculations (HINTS!)

2. Starting with the volume of titrant (NaOH) added and the known molarity of the titrant (given in lab), you can use stoichiometry to solve for the molarity of the substance in the flask (in this case, HCl).

Vol. of titrant used (mL)

x1 L NaOH

1000 mL titrantx 0.25 mol NaOH

1 L NaOH

NaOH (aq) + HCl (aq) NaCl (aq) + H2O (l)

x1 mol HCl

1 mol NaOH

You will need to put in the volume of your titrant above. The rest of the math is the same!

Part I Titration Calculations

3. Once you have solved for the moles of solute in the flask, divide the moles by the measured volume of solution in the flask to determine the unknown molarity of the solution in the flask.

M (HCl) =moles of HCl (found in calc #2 you just did)

Volume (L) of HCl you put into the flask at the beginning

Part II: Titration with a pH Meter

Part II• Titration using a pH meter • Record more data, will graph

Part II Titration Lab using pH MeterSame procedure as part I, except the endpoint is detected by a rapid change in pH, rather than the color change of an indicator.

If the titration is between a strong acid and strong base, the endpoint is reached when the pH is 7.0

Note: NO COLOR CHANGE OBSERVED FOR THIS PART!!!

Part II pH Graph

Your pH graph should look similar to the graph below.

LET’S REVIEW

VOCABULARY

Match That Term!!!Vocabulary

Terms

Titration

Stopcock

Buret

End Point

Stoichiometrically Equivalent

Titrant

DefinitionThe bottom of the buret that can be adjusted to release a small stream of liquid or add the reactant drop by drop.

When stoichiometrically equivalent amounts of each have reacted and the reaction is neutral.

Moles of acid = Moles of base

The solution in a buret.

A laboratory method that allows you to determine the concentration of an acid or a base by reacting it with a known concentration of the other.

Narrow piece of glassware that is used to deliver small amounts of one reactant to a flask containing the other reactant.

Label that picture!

Terms BuretStopcockTitrantUnknown solutionHint: What’s inside?

Thank you!!!Gracias

Merci beaucoup

Dankë

Mahalo

Obrigada