Transcript pH and [H+] Chemistry Notes pH and [H+] Working with the Arrhenius acid definition, we say that acids produce hydrogen ions: HX a H+ +

pH and [H+]
Chemistry Notes
pH and [H+]
Working with the Arrhenius acid
definition, we say that acids
produce hydrogen ions:
HX a H+ + XAnd bases produce hydroxide
ions:
XOH a X+ + OH-
Water, being made up of both H+
and OH- can dissociate giving
us the formula:
H2O a H+ + OHBut it only does this once out of
every 500,000,000 molecules!
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.
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 ]
pH = ‘power of hydrogen ion’
pH scale…
0 -14
7 = neutral
0 - 7 = acidic
7 - 14 = basic/alkaline
0 = strongest acid
14 = strongest base
logarithmic scale…each step is ten times
Stronger/weaker than the one next to it!
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).
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 are chemical dyes used
to determine pH of substances.
They are cheap, but only
precise at specific temperatures
and have limited range
A pH meter is a device that uses
two electrodes to measure pH.
Copy the Following Formulas
pH = 14 – pOH
pOH = 14 – pH
pH = - log [H+]
pOH = - log [OH-]
[H+] = antilog – pH
[OH-] = antilog - pOH
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.