The PH Scale

The pH Scale

Reporter: Krisha Laine Rizala Sindalan

Instructor:Mrs. Karen Sale

•P- power•H- hydrogen• Thus we define pH as a negative logarithm of

the molar concentration of hydrogen ions. • pH=-log[H+]

The pH scale, first proposed in 1909 by the Danish biochemist S.P.L. Sorensen, to describe the degree of acidity or basicity.

• The neutral point on the scale is 7, with values below 7, indicating increasing acidity and those above 7, increasing basicity.

We might expect that pure water would be completely in the form of H2O molecules, but, it turns out that: about 1 out of every 500 million molecules is split into H+ and OH- ions

Concentration of hydrogen ions and of hydroxide ions in pure water of 0.0000001 mol/L, or 1x10-7 M.

• Note that its value goes down when acidity goes up. Not only in the relationship an inverse one, but also logarithmic.

A decrease of 1 pH unit represents a tenfold increase in

acidity, and when pH goes down by 2 units, acidity increases factor by 100.

Relationship between pH and concentration of Hydronium Ions.

Concentration of H3O+ (mol/L) pH1x10-0 01x10-1 11x10-2 21x10-3 31x10-4 4

EXAMPLE:What is the pH of a solution that has a hydrogen ion concentration of 1x10-5 M?

Answer: 5

WHAT IS THE HYDROGEN ION CONCENTRATION OF A SOLUTION THAT

HAS A pH of 4?• Solution:

• The pH value is 4. This means the exponent of 10 is -4. the hydrogen ion concentration is

therefore 1x10-4 M.

Jwk lol.

GET ¼ SHEET OF PAPER MY GORGEOUS AND HANDSOME

CLASSMATES.

pH from hydrogen Ion concentration

1.What is the pH of a solution that has a hydrogen ion

concentration of 1x10-11 M?

pH from hydrogen Ion concentration

2.What is the pH of a solution that is 0.0010 M HCI?

hydrogen Ion concentration from pH

3.What is the hydrogen ion concentration of a solution

that has a pH of 2?

• 1. The hydrogen ion concentration, [H+], of a 0.0010 M HNO3 solution is

A.1.0 x 10-4 M B. 1.0 x 10-3 MC. 1.0 x 10-2 M D. 1.0 M

• 2. What is the pH of a solution that has a hydrogen ion concentration of 1.0 x 10-11

A. 1B. 3C. 10D. 11

• 3. Pool water with pH of 8 has a hydrogen ion concentration of

A. 8.0 MB. 8.0 x 10-8 MC. 1.o x 10-8 MD. 1.0 x 108 M

• The pH of pure water isA. 0B. 1C. 7D. 10E. 14

• Which of the following is a reasonable pH for 0.15 M HCI?

A. 0.15B. 0.82C. 8.24D. 13.18

• Which of the following is a reasonable pH for 0.15 M NaOH?

A. 0.15B. 0.82C. 8.24D. 13.18

• Physiological pH (7.4) is the average pH of blood. Which of the following is a reasonable hydrogen ion concentration of a solution at physiological pH?

A. -7.4 M B. 0.6 M C. 6 x 10-7 M

D. 1 x 10-8 M E. 4 x 10-8

1. Buffers and

Conjugate Acid-Base

PairsConjugate acid base pair

-In the Bronsted-Lowry theory, a pair of compounds or ions That differ by one proton (H+).

examples:HF and F-

NH3 and NH4+ ion.

NH4+ is an acid; it can donate extra

proton.

Example:When a base NH3 accepts a proton, it becomes an acid because it now has a proton that it can donate.

EXAMPLESWhat is the conjugate base of

(a) HBr and (b) of HNO3 and what is the conjugate acid of (c) OH- and (d) of HSO4

-? Solution:A. HBr leaves Br- the conjugate base of HBr is Br-.B. Removing a proton from NHO3 leaves NO3

-; the conjugate base of NHO3 is No3

-

C. Adding a proton to OH- gives H2O: the conjugate acid of OH- is H2OD. Adding a proton to HSO4

- gives H2SO4; the conjugate acid of HSO4

- is H2SO4

EXERCISE •WHAT IS THE CONJUGATE

BASE OF A (a) HCN AND (b) OF H3O+ AND WHAT IS THE

CONJUGATE ACID OF (c) SO42-

AND (d) OF HCO3-?

BUFFER SOLUTION

DEFINITION:A buffer solution is one which resists changes in pH when small quantities of an acid or an alkali are added to it. To be able to add a strong acid or base to a solution without causing a large change in the pH, we need to create a buffer solution. A buffer solution contains both a weak acid (HA) and its conjugate base (A-).

Consider the equation for the ionization of acetic acid:

HC2H3O2(aq) H+(aq) + C2H3o2-(aq)• Where the slight ionization is indicated by a

double arrow. If we add sodium acetate to a solution of acetic acid, we are adding the conjugate base of acetic acid- That is, acetate ion thus forming a buffer solution. If we add a little strong base to this solution, it will react with the weak acid:

OH- + HC2H3O2 H20 + C2H3O2-

Do you see that the solution no longer contains the strong base? Instead, it has a little more weak base and a little less weak acid that it did before the reaction. The pH remains very nearly constant. Likewise, if a little strong acid is added it will react with the weak base:

H+ + C2H302- HC2H3O2

The strong acid is consumed, a weak acid takes place, and the solution pH increases slightly.

SELF-ASSESMENT QUESTIONS

1. Which of the following pairs is a conjugate acid-base pair?

A.CH3COOH and OH-

B. HCN and CN-

C. HCN and OH-

D. HCI and OH-

2. Which of the following is not paired with its Bronsted-Lowry

conjugate base or conjugate acid?

A. CH3COO-/Ch3COOHB. F-/HFC. H2O/H3O+

D. NH3/H3O+

3. A buffer solution is made from formic acid (HCOOH) and sodium

formate (HCOONa). Added acid will react with

A. HCOOB. HCOOHC. Na+

D. OH

Which of the following pairs could be combines to make a buffer?

A. C6H5COOH and C6H5COONaB. HCI and NaCIC. HCI and NaOHD. NH3 and NO3

Thank you……….