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Lecture 2

CHEM 112 Lecture Notes - Lecture 2: Ph Meter, Ph Indicator, Ph


Department
Chemistry
Course Code
CHEM 112
Professor
John Carran
Lecture
2

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Acid-Base Chemistry
-pH can be determines by:
-pH indicator dye –changes colour because it is a weak acid which changes colour when
converted to its conjugated base.
-pH indicator paper – paper impregnated with indicator dyes which exhibit a range of colours
-pH meter – this device measures the H3O+ concentration electrochemically using a voltmeter
between two electrodes placed in the solution
-What is the pH of:
-Solution with .0350 M HCL
-log(.0350)=1.46
-Solution with .107 M KOH
-Kw/.107=9.354 x10-14
-log(9.354 x10-14) = 13.03
-What is the pOH of:
- =Solution with .0765 M NaOH
-long(.0765)= 1.12 <-should be less than 7 for a base
Solution with .0025 M HCL
-log(.0025)=2.6
pOH=14-2.6
Calculate the Hydronium ion concentration of:
-a solution with a pH of 4.83
=10-4.83=.000015
-a solution with a pOH of 3.78
10-(14-3.78)=6x10-11
-Calculate the hydroxide ion concentration of:
-A solution with a pOH of 4.67:
10-4.67
A solution with a pH of 10.55
10-(14-10.55)
-Most acids and bases are not fully deprotonated or protonated so they are weak acids and bases
-to calculate the pH of a solution of a weak acid/base we need to quantify the strength of weak
acid or base
-Weak Acids:
-need a Ka value
-CH3COOH(aq) + H2O(l)  H3O+ (aq) + CH3COO- (aq)
- Ka=[H3O+][CH3COO-]/[CH3COOH]
-pKa=-logKa
-in general: HA(aq) + H2O(l) H3O+ (aq) + A- (aq)
-Weak Bases:
-B(aq) + H2O(l)  HB+ (aq) + OH- (aq)
-Kb = [HB+][OH-]/[B]
-pKb=-logKb
-higher the number weaker the base
-Carboxcylic acids:
-have the –COOH functional group with a C attached to an O with a double bond and to an OH
group
-most simple one is formic acid and is a week acid
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