Transcript Chapter 20 Notes, part III pH and [H+] Hydrogen and hydroxide in H2O •Working with the Arrhenius acid definition, we say that acids are: –HX.

Chapter 20 Notes, part III
pH and [H+]
Hydrogen and hydroxide in H2O
•Working with the Arrhenius
acid definition, we say that
acids are:
–HX a H+ + X•And that a base would be:
– XOH a X+ and OH-
Hydrogen and hydroxide in H2O
•Water, being made up of both
H+ and OH- can dissociate
giving us the formula:
–H2O a H+ + OH•But it only does once out of
every 500,000,000 molecules!
Hydrogen and hydroxide in H2O
•We can measure how much H+
and OH- there is in regular water.
•[H+]=1x10-7M and [OH-]=1x10-7M
•From this information, we can
get a dissociation constant for
water.
Hydrogen and hydroxide in H2O
•Kw=[H+]x[OH-]
•Kw=(1.0x10-7M)x(1.0x10-7M)
•Kw= 1x10-14
Acids, Bases and Concentration
•Even if a solution is acidic or
basic, [H+][OH-] = 1x10-14
•Like a see-saw, raising or
lowering the amount of H+ will
change the amount of OH-.
The pH Scale
•The pH scale is a logarithmic
scale from 0 to 14. It measures
the concentration of H+ (pH
stands for “potential
hydrogen”) with the formula:
pH =
+
-log[H ]
The pH Scale
•Water has a pH of 7.0, which is
neutral.
•Less than 7.0 is acidic (the
closer to zero the stronger the
acid).
•More than 7.0 is basic (the
closer to 14 the stronger the
base).
The pH Scale
•Another way to look at it is the
concentration of hydrogen in a
solution is 1x10-xM. X is the pH.
The pOH Scale
•The pOH scale measures the
hydroxide ion concentration.
•Because Kw=[H+][OH-]=1x10-14,
14=pH + pOH
•Given this, you can solve for pH,
pOH and [OH-] given just [H+].
How to measure pH
•Indicators-chemical dyes used
to determine pH of substances
–Cheap, but only precise at
specific temperatures and
have limited range
•pH meter-device that uses two
electrodes to measure pH
Practice Problem #1
•What is the pH of a solution
where [H+] = 1.0x10-3M?
Practice Problem #2
•What is the hydrogen ion
concentration of a solution
with a pH of 9.6?
Practice Problem #3
•What is the pOH of a solution
with a pH of 6.2?
Practice Problem #4
•What is the pOH of a solution
with a [H+]=2.9x10-11M?
Practice Problem #5
•Find [OH-], pH and pOH of a
solution where [H+]=7.2x10-2M.