Acids & Bases

ACIDS & BASES

pHHow W

e Measure Acid

s and Bases

Blue Base

What is pH?A pH scale is how we measure how acidic

or basic something is.

When we dissolve an acid in water, it ionizes and produces excess Hydrogen Ions, H+

pH is formally how we measure the activity of those Hydrogen Ions in solutions.

WHERE DO COMMON SUBSTANCES FALL ON THE SCALE?

pH is a range that usually is expressed as going from 0 to 14, though it can go beyond that range.

What Exactly does the pH Number Mean?

• When we studied solutions, we learned about Molarity, or moles-per-liter.

• pH is a special way of measuring the moles of Hydrogen Ions in a solution.

• The problem is that the number of moles can vary by many orders of magnitude.

• To solve that problem, we use a logarithmic scale, proposed by the Danish Scientist Soren Sorensen in 1909.

WHERE DO THE NUMBERS COME FROM?

In a liter of water, a very small potion of the molecules will self-ionize, and split into a hydrogen ion (H+) and a hydroxide (OH-) ion.

In this process the Hydrogen attaches to another water and creates something called a hydronium ion (H3O+).

Hydronium ion (H3O+) and Hydrogen ion (H+) can be used interchangeably in discussions or math with no problem .

Water is Amphoteric – Both an Acid and a Base

HOW DO WE GET THE NUMBERS?

When water self-ionizes, the concentrations of the hydrogen ions and the hydroxide ions are 1 × 10-7 moles/liter (they are the same, because each split water molecule produces one of each)

We show concentrations by placing them in square brackets, like this:

[H+] = 1.0 × 10-7 mols/LThe pH is the negative Log of the Hydrogen Ion

Concentration, [H+], so for water:pH = - Log [1.0 × 10-7mol/L]pH = 7

LET’S LOOK AT SOME OF THE CONCENTRATIONSHow do the concentrations of [H+] compare to pH?

1×10-14 mol/L

1×10-10

1×10-12

1×10-8

1×10-2

1×10-4

1×10-6

1×100

Let’s Practice

pH is a measure of the hydrogen ion concentration

pH is calculated using the following formula: pH = -log10[H+]

Find the pH of a 0.2mol/L-1 (0.2M) solution of HCl

Write the balanced equation for the dissociation of the acid HCl -----> H+(aq) + Cl-(aq)

Use the equation to find the [H+]: 0.2 mol/L- HCl produces 0.2 mol/L-1 H+ since HCl is a strong acid that fully dissociates

Calculate pH: pH = -log10[H+] pH = -log10[0.2] = 0.7

Let’s Practice (2)

Find the pH of a 0.2 mol/L (0.2M) solution of H2SO4

Write the balanced equation for the dissociation of the acid

H2SO4 -----> 2H+(aq) + SO42-(aq)

Use the equation to find the [H+]:

0.2 mol/L H2SO4 produces 2 x 0.2 = 0.4 mol/L H+ since H2SO4 is a strong acid that fully dissociates

Calculate pH: pH = -log10[H+]

pH = -log10[0.4] = 0.4

Let’s Practice (3)

A hydrogen ion concentration is found to be [H+] = 2.3×10-5 mol/L what is the pH between (without a calculator).

1×10-5 would be a pH of 5, but this is more than that.

1×10-4 would be a pH of 4, but is less, so it is between the two.

What is the actual pH? (us your calculator)

pH = - log[H+]

pH = - log(2.3×10-5)

pH = 4.6

pH and pOH – What is pOH?Remember that water splits into both an OH- and an H+ when it ionizes. Because of this, we

define the pOH just like pH, but it is the negative log of the hydroxide ion concentration instead of the hydrogen ion concentration:

pOH = - log[OH-]so for water:pOH = - Log [1.0 × 10-7 mol/L]pOH = 7Interestingly, the pOH and the pH are the same for water. That

is because the concentration of H and OH are the same.

[H+] = [OH-]

Can we convert between pH and pOH?

Yes – Start with what you have and use the formulas at the bottom to convert between pH and pOH

Let’s Practice pOHThe hydroxide ion concentration of a solution is found to be 3.4 × 10-4 mol/L What is the

pOH?

pOH = - log [OH-]

pOH = - log (3.4 × 10-4 mol/L )

pOH = 3.5

What is the pH of that same solution?

14 = pH + pOH

pH = 14 – pOH

pH = 14 – 3.5

pH = 10.5

Is the solution an acid or a base?

Let’s Practice pOH

A reference solution of sodium hydroxide is 0.005 M (mol/L). What should its pH be?

Write the balanced equation for the dissociation of the base

NaOH-----> Na+(aq) + OH-(aq)

there is one mole of OH- for each mole of base, so [OH-] = 0.005 mol/L

Find the pOH first:

pOH = - log [OH-]

pOH = - log (0.005 mol/L )

pOH = 2.3

What is the pH of that same solution?

14 = pH + pOH

pH = 14 – pOH

pH = 14 – 2.3

pH = 11.7

Is the solution an acid or a base?

Is it Acid Rain?

A solution of rainwater has a hydronium ion concentration of 3.98 × 10-5 mol/L . What is its pH? Is it acid rain?

pH = - log [H3O+] (Remember that H3O+ = H+)

pH = - log (of 3.98 × 10-5 )pH = 4.4

Acid rain has a pH of less than 5, so yes – it is acid rainpOH = 14 – pH, sopOH = 14 - 4.4 pOH = 9.6

EXTRA DIAGRAM