pH

Custom Search

pH=-log[H⁺]

Definition from wiki- pH is a measure of the acidity or alkalinity of a solution. Solutions with a pH less than seven are considered acidic, while those with a pH greater than seven are considered basic (alkaline). pH 7 is considered neutral because it is the pH of pure water at 25 °C. pH is formally dependent upon the activity of hydrogen ions (H⁺), but for very pure dilute solutions, the molarity may be used as a substitute with some sacrifice of accuracy. Because pH is dependent on activity, a property which cannot be measured easily or predicted theoretically, it is difficult to determine an accurate value for the pH of a solution. The pH reading of a solution is usually obtained by comparing unknown solutions to those of known pH, and there are several ways of doing this.

The concept of pH was first introduced by Danish chemist S. P. L. Sørensen. The name, pH, has been purported to come from a variety of places including: pondus hydrogenii (Latin),potentiel hydrogène (French), and potential of hydrogen (English). However pH is actually a shorthand for its mathematical approximation: in chemistry a small p is used in place of writing - log₁₀ and the H should more correctly be [H⁺], standing for concentration of hydrogen ions.

What does log mean? LINK<---

Regents pH

for every zero in a number the pH changes by 1

100 times more acidic means the pH decreases by 2.

Substance	pH
Hydrochloric Acid, 10M	-1.0
Battery acid	0.5
Gastric acid	1.5 – 2.0
Lemon juice	2.4
Cola	2.5
Vinegar	2.9
Orange or apple juice	3.5
Beer	4.5
Acid Rain	<5.0
Coffee	5.0
Tea or healthy skin	5.5
Milk	6.5
Pure Water	7.0
Healthy human saliva	6.5 – 7.4
Blood	7.34 – 7.45
Seawater	7.7 – 8.3
Hand soap	9.0 – 10.0
Household ammonia	11.5
Bleach	12.5
Household lye	13.5
Caustic Soda	13.9

Past Regents Problems involving pH

Jan 2004-25 Which of these pH numbers indicates the highest
level of acidity?
(1) 5 (3) 10
(2) 8 (4) 12

Aug 2010-9 A solution with a pH of 2.0 has a hydronium ion concentration ten times greater than a solution with a pH of
(1) 1.0 (3) 3.0
(2) 0.20 (4) 20.

Aug 2007-48 What is the pH of a solution that has a hydronium ion concentration 100 times greater than a solution with a pH of 4?
(1) 5 (3) 3
(2) 2 (4) 6

Aug 2006-50 Solution A has a pH of 3 and solution Z has a pH of 6. How many times greater is the
hydronium ion concentration in solution A than the hydronium ion concentration in solution Z?
(1) 100 (3) 3
(2) 2 (4) 1000

Aug 2003-28. When the pH of a solution changes from a pH of 5 to a pH of 3, the hydronium ion concentration is

A. 0.01 of the original content
B. 0.1 of the original content
C. 10 times the original content
D. 100 times the original content

Jan 2008-

Jan 2007-48 As the pH of a solution is changed from 3 to 6, the concentration of hydronium ions
(1) increases by a factor of 3
(2) increases by a factor of 1000
(3) decreases by a factor of 3
(4) decreases by a factor of 1000

Aug 2004-30 Which pH change represents a hundredfold increase in the concentration of H⁺?

1) pH 5 to pH 7 (3) pH 3 to pH 1

(2) pH 13 to pH 14 (4) pH 4 to pH 3

June 2009-47 Which change in pH represents a hundredfold increase in the concentration of hydronium ions in a solution?
(1) pH 1 to pH 2 (3) pH 2 to pH 1
(2) pH 1 to pH 3 (4) pH 3 to pH 1

Aug 2004-48 Which statement correctly describes a solution with a pH of 9?

(1) It has a higher concentration of H3O⁺ than OH^– and causes litmus to turn blue.

(2) It has a higher concentration of OH– than H3O⁺and causes litmus to turn blue.

(3) It has a higher concentration of H3O⁺ than OH^– and causes methyl orange to turn yellow.

(4) It has a higher concentration of OH^– than H3O⁺ and causes methyl orange to turn red.

Back to Acid Base Links

Advanced Chemistry Topics

Strong Acids- Due to the complete dissociation of strong acids in aqueous solution, the concentration of hydronium ions in the water is equal to the re-duplication of the acid introduced to solution:

[HA] = [H⁺] = [A⁻]; pH = −log[H⁺]

Calculate the pH of a 0.100 M solution of HCl.

Therefore, the [H⁺] equals 0.100 M. So, to solve it, you write:

pH = - log (0.100) = 1.000

Calculate the pH of a 1.00 M solution of HBr.

pH = - log (1.00) = 0.00 <--Answer

Strong Bases- Strong bases is pretty much the same as strong acids EXCEPT you'll be calculating a pOH first, then going to the pH.

pH + pOH = 14

Calculate the pH of a 0.100 M solution of NaOH.

Therefore, the [OH^-] equals 0.100 M. So, to solve it, you write:

pOH = - log (0.100) = 1.000

pH = 14.000 - 1.000 = 13.000

Calculate the pH of a 0.050 M solution of KOH.

pH = 14.00 - 1.30 = 12.70

AUTOIONIZATION of WATER

--->Auto Ionization of Water Tutorial<---

notes fromhttp://dbhs.wvusd.k12.ca.us/webdocs/AcidBase/Kw.html

H₂O_(l) + H₂O_(l) <==> H₃O⁺_(aq) + OH¯_(aq)

K_w = [H₃O⁺] [OH¯]

K_w = water autoionization constant=1.0 x 10^-14@ 25 °C

From the chemical equation just above, it can be seen that H₃O⁺ and OH¯ concentrations are in the molar ratio of one-to-one. This means that [H₃O⁺] = [OH¯].

Therefore the values of [H₃O⁺] and [OH¯] can be determined by taking the square root of K_w. Hence, both [H₃O⁺] and [OH¯] equal 1.00 x 10¯⁷ M in pure water.

Result #1: The pH of pure water is 7

By definition, pH = -log [H₃O⁺]

The pH of pure water then equals -log 10¯⁷, which is 7.

Result #2: If the pH or the pOH is known, the other can be found.

Take the negative logarithm of each side of the K_w equation as follows:

- log K_w = -log [H₃O⁺] + -log [OH¯]

-log 1.00 x 10¯¹⁴ = -log [H₃O⁺] + -log [OH¯]

Note the use of the add sign on the right side of the equation. The result is ususally written as:

pK_w = pH + pOH = 14

This is an extremely important equation. Learn it well.

Result #3: If the [H₃O⁺] or the [OH¯] is known, the other can be found.

Simply divide K_w by the known value to get the other.

Suppose [H₃O⁺] is known, then:

[OH¯] = K_w / [H₃O⁺]

Suppose [OH¯] is known, then:

[H₃O⁺] = K_w / [OH¯]

Result #4: If one variable ( [H₃O⁺] or [OH¯] ) changes value (either up or down), the other variable will change in the opposite direction.

The change in values will still preserve this fundamental equality:

K_w = [H₃O⁺] [OH¯]

Suppose [H₃O⁺] became larger, therefore the [OH¯] becomes smaller.
Suppose [OH¯] became larger, therefore the [H₃O⁺] becomes smaller.

Chemical Demonstration Videos