# pH

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```					                 Concentration
   How much of a given substance is mixed within
(or dissolved in) another substance
   For example, some people buy frozen orange
juice concentrate from the store, then mix it with
water to make drinkable juice
Acids and Bases
   Consider acids and bases to be extremes, the
way hot and cold are extremes for temperature
   pH is a way to measure those extremes
Concentration and pH
   Water molecules collide with one another and
ionize each other in the following reaction:
   2 H2O → H3O+ + OH-
   The concentration of ions is measured in units
called “Molarity” or moles per liter (M).
   1 mole = 6.02 x 1023 water molecules, or about
18 mL of water.
Logarithms
   A number written in scientific notation (as ion
concentrations often are) has a coefficient
multiplied by a power of ten
   Example: 1.0 x 10-7
   A logarithm is the exponent that you have to
raise a number to, to make it equal another
number:
   bx = y        b = base, x = exponent, y =
   In our example, b = 10, x = -7, y = 0.0000001
   The logarithm = -7
Definition of pH
   pH means “power of concentration of hydrogen
ion”
   pH is equal to the negative log of hydrogen ion
concentration
Neutral Water
   The ionization of water is reversible, so at any
given time, the H3O+ concentration is 1.0 x 10-7
moles per liter (M).
   The pH comes from the exponent—in neutral
water, the pH is 7 (the negative log of 1 x 10-7 = 7)
Acidic Solutions
   Have a pH below 7, so a concentration of
1.0 x 10-6, 1.0 x 10-5, etc.
   The smaller the digit in the exponent, the more
acidic
Basic Solutions
   Have a pH above 7, so a concentration of
1.0 x 10-8, 1.0 x 10-9, etc.
   The larger the digit in the exponent, the more
basic
   The highest pH is 14
Basic Solutions
   If the concentration is 1.0 x 10-x, pH is easy!
   The answer is simply x
   Ex: 1.0 x 10-13            pH = 13
       1.0 x 10-2         pH = 2
   What if the concentration is 2.5 x 10-7?
   pH cannot = 7, because the coefficient is not
1.0
Solving pH on a calculator
   There is a button on the calculator for entering
exponents: either EE or EXP, normally
   For some calculators, it may be a 2nd function
key
   Enter the concentration as 2.5 EE -7
Solving pH on a calculator
   After entering the concentration, hit the log
button
Solving pH on a calculator
   You should have a negative number, with
decimals
   Press the (-) button (sometimes +/-) on the
calculator to get the final answer, or just take
the absolute value
Example
   What is the pH of a substance with 3.2 x 10-7 M?
   Enter 3.2 EE -7 on your calculator
   Press “log” button
   Answer = 7.5 (slightly basic)
pOH
   This is the OH- ion concentration
   The pH and pOH should add up to 14

   An acid that has a pH of 3 has a pOH of 11

   A base with a pH of 12 has a pOH of 2

   Neutral water has a pH of 7 and a pOH of 7

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 views: 25 posted: 11/23/2011 language: English pages: 14