High School
SAT
ACT
University
MCAT
LSAT
DAT
Wize University Chemistry Textbook > Acids and Bases

pH

Conjugate Acid-Base PairsPrevious Section
Acid Equations (Ka, pKa)Next Section
Popular Courses
Find My Course
0:00 / 0:00

pH Scale

The pH scale is a logarithmic scale used to measure the concentration of [H+] in a solution, since this value can vary over many orders of magnitude. It ranges from 0 to 14, with 7 representing neutral pH

Note: There are some exceptions to this. For example, an acid could have a negative pH! These aren't commonly seen.

Photo by OpenStax College / CC BY

A lower pH indicates that a substance is more (acidic/basic)
acidic
and has a (higher/lower)
higher
[H+]
PAGE BREAK



We also have Ka and pKa equations:

pKa=−log⁡⁡Ka and Ka=10−pKapK_a=-\log⁡K_a\ and\ K_a=10^{-pK_a}
As well as Kb and pKb equations:

pKb=−log⁡⁡Kb and Kb=10−pKbpK_b=-\log⁡K_b\ and\ K_b=10^{-pK_b}

PAGE BREAK

What does it mean to be a logarithmic scale?

How does the [H+] change as the pH goes from 1 to 2 or 1 to 3??



Wize Concept
Since pH is a logarithmic scale, each 1 unit change in pH corresponds to a 10-fold change in [H+]!


0:00 / 0:00

Example: pH Problem

If 0.2 mols of HCl(s)\text{HCl}_{(s)} are dissolved in 1.8 L of water, what is the pH?
HCl → H+ + Cl-

Since HCl\text{HCl} is a strong acid, we will be adding 0.2 mols of H+\text{H}^+ to 1.8 L of water.

[H+]=0.2mol1.8L=0.11mol/L[\text{H}^+]=\dfrac{0.2mol}{1.8L}=0.11mol/L

pH=−log⁡[H+]=0.95pH=-\log[\text{H}^+]=0.95

0:00 / 0:00

Example: pH Problem

C6H5COOH ⇌ H+ + C6H5COO-

Ka of C6H5COOH = 6.5 x 10-7 and the concentration of C6H5COOH is 0.01 M, what is the pH of the solution?

Write Out An ICE Table

I 0.01M 0 0
C -x +x +x
E 0.01-x x x

Write a K expression and Solve for x

Ka = x²/(0.01 - x) Ka = x²/0.01
We can ignore the "-x" part since y/K > 400

x² = Ka·0.01
x² = (6.5 x 10-7)(0.01) x² = 6.5 x 10-9 x = 8.06 x 10-5
x=[H+]

Find pH

pH = -log[H+] pH = -log(8.06 x 10-5) pH = 4.09



Practice: Solve for [H+]

A solution of HBr has a pOH of 12.2, what is the [H+] concentration?
Extra Practice
Weak Acids and Bases
Calculate the pH of the following solution.
A solution prepared by dissolving 4.25 x 10-4 g of CH3COONa in 100.0 mL of water.
Note: Acetic Acid Ka = 1.8 x 10-5
Weak Acids and Bases
Calculate the pH and pOH of the following solutions.
a) A 6.30 x 10-2 M solution of CH3COOH in water.
b) A solution prepared by dissolving 4.25 x 10-4 g of CH3COONa in 100.0 mL of water.
Methyl diethanolamine CH3N(C2H4OH)2 is a weak base which we will abbreviate as mdea. The conjugate acid, mdeaH+, has a pKa = 8.52.
Henderson Hasselbalch: Titrations and pH
For the titration of 50.00 mL of 0.1000 M ammonia (NH3) with 0.1000 M HCl calculate the pH (For ammonia Kb​=1.8×10−5)(For\ ammonia\ Kb​=1.8\times10^{-5})
Before the addition of any HCl
After 20.00 mL of the acid has been added
Molecular formula: Concentration
A solution of HBr has a pOH of 12.2, what is the [H+] concentration?
100 mL of a 0.5 M solution of nitric acid (HNO3\rm HNO_3) was added to 75 mL of a 0.37 M solution of sodium benzoate (C6H5COONa\rm C_6H_5COONa) the conjugate base of benzoic acid (C6H5COOHKa=6.6×10−5\rm C_6H_5COOH Ka = 6.6 \times 10^{-5}). Calculate the pH\rm pH of the resulting solution.
If the pH of pure water is 6.80 at 37 °C, calculate Kw pOH and [OH- ] at this temperature.
In a 0.25 M solution, a weak acid is 3.0% dissociated. Calculate [H3O+], pH and pOH of the solution
In a 0.25 M solution, a weak acid is 3.0% dissociated. Calculate [H3O+], pH and pOH of the solution
Acids and Bases: pH of Alkaline Earth Metal Hydroxides
Calculate the pH of a 1.00 x 10-4 M Mg(OH)2 solution.
Acids and Bases: Strong Acid Mixture
Practice Question: Strong Base Mixture
Calculate the pH and pOH of a solution containing a mixture of 75.00 mL of 0.100 M HI and 25.00 mL of 0.250 M HClO4.
Weak Acids and Bases
Calculate the pH of a solution prepared by dissolving 0.500 g of sodium hydrogen phosphate (Na2HPO4, MW = 141.9579 g/mol) in 150.0 mL of water.
Weak Acids and Bases
Calculate the pH and pOH of the following solutions.
a) A 6.30 x 10-2 M solution of CH3COOH in water.
b) A solution prepared by dissolving 4.25 x 10-4 g of CH3COONa in 100.0 mL of water.
Strong Acids and Bases
Calculate the pH of a basic solution prepared by taking 10.00 mL of 0.700 M NaOH solution and diluting it to 100.00 mL with water.
Strong Acids and Bases
Calculate the pH of a solution prepared by adding 100.0 mL of 3.00 x 10-2 M KOH with 50.0 mL of 2.50 x 10-2 M Be(OH)2.
Strong Acids and Bases
Calculate the pH and pOH of the following solutions.
a) 2.75 x 10-3 M aqueous HNO3 solution.
b) 1.50 x 10-2 g of CaO dissolved in 200.0 mL of water.
Weak Acids and Bases
Calculate the pH and pOH of the following solutions, knowing that Ka = 1.8x10^-5.
a) A 5.33 x 10-2 M solution of CH3COOH in water.
b) A solution prepared by dissolving 1.15 x 10-3 g of CH3COONa in 100.0 mL of water.
Weak Acids and Bases
Calculate the pH of a solution prepared by dissolving 0.150 g of sodium hydrogen phosphate (Na2HPO4, MW = 141.9579 g/mol) in 125.0 mL of water.
Ka (HPO42−) = 4.2 x 10−13;  Kb (HPO42−) = 1.6 x 10−7. K_a\ \left(HPO_4^{2-}\right)\ =\ 4.2\ x\ 10^{-13};\ \ K_b\ \left(HPO_4^{2-}\right)\ =\ 1.6\ x\ 10^{-7}.\
Strong Acids and Bases
Calculate the pH of a basic solution prepared by taking 20.00 mL of 0.500 M Be(OH)2 solution and diluting it to 100.00 mL with water.
Strong Acids and Bases
Calculate the pH of a solution prepared by adding 25.00 mL of 0.100 M HCl with 75.00 mL of 2.50 x 10-2 M HBr.
Strong Acids and Bases
Calculate the pH and pOH of the following solutions.
a) 3.00 x 10-4 M aqueous HClO4 solution.
b) 8.00 x 10-3 g of Na2O dissolved in 150.0 mL of water.
Consider the following reaction in water:
NaOH <----> Na+ + OH-
This reaction has a Kb of 1 x 10-10 and a pOH of 3. If there is 1 M of NaOH at equilibrium, what is the concentration of Na+ in solution?
Acids and Bases: pH based on % dissociation
In a 0.25 M solution, a weak acid is 3.0% dissociated. Calculate [H3O+], pH and pOH of the solution
If an answer is an exponent such as 4.3x10-7, input the answer as: 4.3e-7
Which of the solutions below would have the HIGHEST pH value?
Acids and Bases: pH
What is the pH of a 0.10 M solution of NaF? (Ka for HF = 6.8×10-4)?
Methyl diethanolamine CH3N(C2H4OH)2 is a weak base which we will abbreviate as mdea. The conjugate acid, mdeaH+, has a pKa = 8.52.