Lab - Measurement of pH

in aqueous solutions the pH scale extends from about

0-14

the measurement of pH required 2 electrodes:

1) a sensing electrode (sensitive to the [H+] of a solution 2) reference electrode

measurement of pH lab purpose

1) to use a pH meter to measure the pH of several aqueous solutions and to 2) measure the pH change produced by diluting the solutions and adding acid to these solutions. 3) and to then classify substances as strong acids, weak acids, strong bases, weak bases based on the pH values

strong electrolytes ____________ in dilute soltuions

100% dissociate

strong bases are ______________ in solution

100% ionized

acidic solutions have a pH

<7

basic solutions have a pH

>7

Beer's Law

A = ebc

diluting a weak acid solution wil

INCREASE the PERCENT IONIZATION of the weak acid

neutral salt

KNO3 (potassium nitrate)

weak bases examples

NH3 (ammonia)

strong bases examples

NaOH (sodium hydroxide)

blood is an example of

a highly buffered solution

pH meter is

a very sensitive voltmeter that measures the electrical potential of a solution

A

absorbance of light

pH 10.0 =

blue solution

combination electrode

both electrodes are incorporated into a single probe

changing [H+] by a factor of 2,

changes pH by 0.3 since log(2) = 0.3

changing [H+] by a factor of 10,

changes pH by 1 since log(10) = 1

c

concentration

strong base pH will _____________ when diluted by a factor of 2

decrease by 0.3 units (bases decrease)

weak base pH will ____________ when diluted by a factor of 2

decrease by LESS than 0.3 units (bases decrease)

experimental measurement of pH is done with an

electronic instrument known as a pH meter

in aqueous solutions the hydrogen ion, H+, does not

exist as a separate entity; it is always bound to 1 or more water molecules; this specie is referred to as the hydronium ion, H3O+

salts formed by the reaction of a strong acid with a strong base will

give solutions with a pH of 7

buffer

hep maintain a constant pH and they do have a capacity; after that capacity they won't work anymore

strong acids pH will __________ when diluted by a factor of 2

increase by 0.3 units (acids increase)

weak acid pH will ____________ when diluted by a factor of 2

increase by LESS than 0.3 units (acids increase)

strong acids are ____________________ solutions

ionized in dilute

buffer solutions are specials solutions which

maintain a fairly constant pH in spite of chemical stresses

e

molar absorptivity

diluting any solutions will

move it closer to pH = 7 • Diluting an acidic solution decreases the concentration of H+(aq) ions. This causes the pH to increase towards 7. • Diluting an alkaline solution decreases the concentration of OH-(aq) ions.This causes the pH to decrease towards 7.

weak acids are ______________________ solutions

only partially ionized

weak bases are ________________ in solution

partially ionized

pH 4.0 =

pink solution

diluting a strong acid by a factor of 2 should

raise the pH by 0.3 since dilution lowers the concentration of H+ by a factor of 2 (∆pH = log(2) = 0.3)

salts formed by the reaction of a strong acid with a weak base will give

solutions with a pH <7

salts formed by the reaction of weak acid with a strong base will give

solutions with a pH >7

chloride ion is a

spectator ion (it does not affect the pH of the solution) Na is also a spectator ion

a DECREASE in the pH of a solution by 1 unit represents a

ten-fold INCREASE in [H+]

the electrical potential of a solution depends upon

the [H+] in the solution (measurement of this potential allows us to measure the pH of s solution)

the pH of a solution is measured by

the hydrogen ion concentration in the solution pH = -log 10 [H+]

pH = -log 10 [H+]; [H+] represents

the molar concentration of hydrogen ions in the solution

b

thickness of cells

most buffers contain a

weak acid and the conj. base of that weak acid in a solution *quite resistant to to changes in pH, even if small amounts of acid or base are added to the solution examples: • acetic acid + sodium acetate • ammonia + ammonium chloride

weak acid examples

• HC2H3O2 (acetic acid) • HF (hydrogen fluoride)

strong acid examples

• HCl (hydrochloric acid) • HNO3 (nitric acid)

from your pH data, list the name and formula of substances used in this experiment in the following categories: • strong acid; weak acid • strong base; weak base • acidic salt; neutral salt; basic salt

• strong acid: HCl (hydrochloric acid) • weak acid: CH3COOH (acetic acid) • strong base: NaOH (sodium hydroxide) • weak base: NH3 (ammonia) • acidic salt: NH4Cl (ammonium chloride) • Neural salt: KNO3 (potassium nitrate) • basic salt: NaCH3CO2 (sodium acetate)

good buffer examples:

• weak acid + conj. base • weak base + conj. acid