Monday, March 31Welcome Back!!!!!1. Return Quizzes

2. Finishing pHIndicatorsTitrations

Homework

Review Questions, page 586 – 3, 4

Page 591 – 43, 44

The pH scale.

Three ways to measure pH

1. pH paper

2. pH meter

3. Indicators• pH meter measures the voltage difference

between a reference solution in the probe and the solution it’s immersed in and translates it into a pH reading.

Indicators

• Indicators – weak acids/bases that dissociates in a certain pH range, changing colors as it does so.

HIn + H2O H3O+ + In-

yellow red

Indicators

HIn + H2O H3O+ + In-

yellow red• What color will it be when you add acid? • (yellow)• What color will it be when you add a

base? • (red)

Pg. 580

Indicators

• The change in the two forms occurs in a range of about 2 pH units for most indicators. – In this range it is a mix of the two colors. – Below this range it has the acidic color. – Above this range it has a basic color.

• Which indicator would you use to show that a reaction solution has changed from pH 10-11?

• Alizarin Yellow R

Limitations of Indicators

• Solution should be colorless• Need ability to detect slight color changes• Can only give approximate information on pH• Is dependent on temperature

To test the pH over a wide range a combination of indicators are needed. (Universal Indicator)

Titrations

Titration – a method in which a standard solution is used to determine the concentration of another solution.

Standard solution – one for which the concentration is known.

Titration

Titration and Neutralization

• Break HCl into ions and NaOH into ions.

• React the two together and we get HOH and NaCl

• If we can get rid of all the H+ and OH- ions, this neutralizes the acid and base

• How will we know when we get there?

• Meter reads pH 7 (neutral)

Titration

• Titration uses a buret to slowly add one solution to the other

• One of the solutions is known

• Compare the number of moles added to reach the endpoint

• The endpoint is known as the equivalence point (when it is neutral)

Titration and Neutralization

• What has to be true about the amounts when the neutralization is complete?

• The moles of OH- added must be exactly the same as the moles of H+.

• How would we know with an indicator?

• Have an indicator that changes colors when it changes from acid to base!

Titration

Endpoint or Equivalence point –when enough titrant has been added to react exactly with the substance in solution that is being titrated. The portion of the curve with the steepest slope.

Occurs when solution stays colored for 15 sec with swirling

Titration

• What happens to the graph as the titration nears its equivalence point?

• Steep slope

• What happens to the pH if you continue to add base?

• increases

Titration

How many mL of 0.200 M KOH will neutralize 15.0 mL of 0.400M HCl?

1. Write a balanced chemical equation.

HCl + KOH KCl + HOH

2. Develop a conversion strategyL HCl mol HCl mol KOH L KOH

mL 30.0 L 0.0300 KOH mol 0.200

L 1 x

HCl mol 1

KOH mol 1 x

L 1

HCl mol 0.400 x HCl L 015.0

Titration

• What would happen if we left everything the same, but used H2SO4 instead?

• It would take twice as much KOH because we have two H+ in every acid that need to be neutralized.

• The amount of H+ and OH- must be equal if complete neutralization is to occur.

Titration

1. Write a balanced chemical equation.

H2SO4 + 2KOH K2SO4 + 2HOH

2. What is the mole ratio when we are looking for amount of base?

3. mol H2SO4 x mol ratio = mol KOH

4. Ma. Va

. b/a = Mb . Vb

5. b/a is the mole ratio of base to acid (put on the acid side)