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