Acids- Base Titration and pH

Aqueous Solutions and the Concept of pH

When two molecules of water produce a hydronium and hydroxide ion by the transfer of a proton.

H2O + H2O H3O+ + OH-

At 25C the [H3O+] = [OH-]= 10-7 M So, the products of [H3O+] and [OH-] can

be represented by: [H3O+] x [OH-] 10-7 M x 10-7 M = 10-14 M2

Where 10-14 M2 Is the Kw ( ionization constant of water)

So, [H3O+] x [OH-]= Kw at 25C

Neutral solutions: [H3O+] = [OH-]= 10-7 M

Acidic solutions:[H3O+] > [OH-]

Basic solutions: [H3O+] < [OH-]

We can find the [H3O+] and[OH-] by using the formula:

[H3O+] x [OH-]= Kw= 10-14 M2

Practice p: 484

It is the negative logarithm of the hydronium ion concentrationpH= - log [H3O+]

Likewise, pOH It is the negative logarithm of the hydroxide ion concentration

pOH= - log [OH-]So, pH + p OH= 14 at 25C

[H3O+]=10-pH

Practice p: 487

1- [H3O+] x [OH-]= Kw

2- pH= - log [H3O+]

3- pOH= - log [OH-]

4- pH + p OH= 14 at 25C

5- [H3O+]=10-pH

Determining pH and

Titrations

1- Acid- Base indicators: are compounds whose colors are sensitive to pH. Indicators come in many different colors. The exact pH range over which an indicator changes color also varies.

2-Universal indicators: the pH of solution can be determined by comparing the color it turns with the scale of paper.

3- pH meter: determines the pH of a solution by measuring the voltage between the electrodes that are placed in the solution. (It’s the most accurate way to determine the pH)

Is the controlled addition and measurement of the amount of a solution of known concentration required to react completely with a measured amount of a solution of unknown concentration.

The equation of titration is:

H3O+ + OH- 2H2O

1- strong acid- strong base

2- strong acid- weak base

3- weak acid- strong base

Indicators are specific, each type of titration needs a specific indicator

For example: Strong acid- strong base titration:We use bromothymol blue (6.2- 7.6) For strong acid- weak base titration:We use bromophenol blue ( 3-4.6) For weak acid- strong base titrationWe use phenolphtalein(8-10)

The point at which the two solutions used in a titration are present in chemically equivalent amounts is called the equivalence point.

The endpoint in titration marks the point at which the color of indicator changes.

The solution that contains precisely known concentration of solute is known as the standard solution.

To find the molarity or the volume of an unknown solution we use the equation:

No of moles of acid= No of moles of base

CxV (acid)= CxV (base) Practice p: 503 (1 and 2)