PH Scale In this presentation you will: explore how to measure the strength of acids and bases Next...

pH Scale

In this presentation you will: explore how to measure the strength

of acids and bases Next >

Next >

Introduction

Acids and bases can be measured with the pH scale which specifies the concentration of hydrogen ions, H+, in a solution.

To show the concentration, the ion is enclosed in square brackets, [H+].

pH Scale

Pure (distilled) water has a pH of 7. The strongest acids are around pH 0, and the stronger bases around pH 14.

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The pH scale is shown for some common substances. Household Lye

Bleach

Ammonia

Borax

Baking SodaSea WaterBloodDistilled WaterMilkCorn

Boric Acid

Orange Juice

VinegarLemon Juice

Battery Acid

pH values of some common substances

Milk of Magnesia

pH Scale

As the pH scale is logarithmic, change in the acidity or basicity of a solution of one whole pH unit actually represents a change of ten times the concentration of hydrogen ions in the solution. Next >

Household Lye

Bleach

Ammonia

Borax


Boric Acid

Orange Juice

VinegarLemon Juice

Battery Acid


H+ concentration, M in moles per liter

Milk of Magnesia

The pH value is the negative logarithm of the H+ ionconcentration:

pH Scale

pH = - log10 [H+]Next >

The pH of an acidic or basic solution can be calculated from the concentrationof H+ ions in the solution.

Household Lye

Bleach

Ammonia

Borax


Boric Acid

Orange Juice

VinegarLemon Juice

Battery Acid



Milk of Magnesia

pH Scale

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Household Lye

Bleach

Ammonia

Borax


Boric Acid

Orange Juice

VinegarLemon Juice

Battery Acid



Milk of Magnesia

The concentration of hydrogen ions in most aqueous solutions is between 1×10-14 M and 1 M.

pH Scale

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Concentration M in moles per liter is sometimes expressed as mol/dm3. These are equivalent terms.

Household Lye

Bleach

Ammonia

Borax


Boric Acid

Orange Juice

VinegarLemon Juice

Battery Acid



Milk of MagnesiaFrom pH = - log10 (10-14) = 14

to pH = - log10 (1) = 0

Therefore, the range of pH values for these solutions is between 14 and 0:

Ionized

pH Calculations – Strong Acids

A strong acid is fully dissociated into H+ ions and its associated base ions (known as its conjugate base) in aqueous solution.

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A strong acid is one that fully ionizes, or dissociates into its ions, in an aqueous solution.

Hydrogen Ion

Chlorine Ion(Conjugate base)

Ionized


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This makes pH calculations of strong acids, of known concentration, simple to work out.

Hydrogen Ion

Chlorine Ion(Conjugate base)

The molar concentration of H+ will be equal to the concentration of the acid.


In a 0.25 M solution of HCl, the concentration of H+ ions is also 0.25 M.

HCl(aq) → H+(aq) + Cl-(aq)

pH = 0.6

pH = - log10 [0.25]

pH = - log10 [H+]

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[HCl] = [H+] = [Cl-] = 0.25 mol/L

Hence:

pH Calculations – Strong Bases

The ionic product of water (Kw) is used to calculate the pH of bases.

H2O(l) ⇌ H+(aq) + OH-(aq)

However, in alkaline solutions, the concentration of H+ ions is extremely low.

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Like strong acids, strong bases are fully dissociated into ions.

Kw = [H+] × [OH-] = 1 × 10−14 mol/L


H2O(l) ⇌ H+(aq) + OH-(aq)

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To calculate the concentration of H+ ions in a basic solution, we have to rearrange this equation.

Water naturally dissociates into H+ and OH− ions with equal concentrations of 1×10−7 mol/L at 298 K.

Kw = [H+] × [OH-] = 1 × 10−14 mol/L

[H+] = Kw / [OH-]

Consider a solution of NaOH of 0.25 M concentration at 298 K:

[H+] = 1 × 10−14 / 0.25

= 4 × 10-14 mol/L

To calculate the ionic product of water:

[NaOH] = [Na+] = [OH-] = 0.25

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pH = - log10 (4 × 10-14 ) = 13.4

Kw = [H+] × [OH-] = 1 × 10−14 mol/L

pH = - log10 [H+]

Therefore, the pH of NaOH is:

It is also possible to use the concentration of the hydroxide ion (OH-) to measure the strength of an acid or base, as pOH.

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Relationship between pH and pOH

Kw = [H+] × [OH-] = 1 × 10−14 mol/L

pH = - log10 [H+]

For a given concentration of hydrogen, say [H+] = 1 × 10−7 mol/L

In this case, pH = 7 and pOH = 7 Next >

Relationship between pH and pOH

then, [OH-] = Kw / [H+] = 1 × 10−14 / 1 × 10−7 = 1 × 10−7 mol/L

pOH = - log10 [OH-]Since, pH = - log10 [H+], then

pOH = - log10 [OH-] = - log10 (1 × 10−7 ) = 7

Example

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Relationship between pH and pOHIn all cases, the sum of pOH and pH will equal 14 because of the relationship:

Given either pOH or pH, then for any acid or base the other can be calculated.

Kw = [H+] × [OH-] = 1 × 10−14 mol/L

pOH + pH = 14

Measuring pH

It is often necessary to know the pH of a solution, in order to identify it as acidic or basic.

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Additionally, pH changes during a reaction must often be continuously monitored, for example in a titration experiment.

Measuring pH

The most common experimental techniques used for finding pH are:

Indicators, or chemical substances that react to different pH levels by changing color.

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Measuring pH

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The most common experimental techniques used for finding pH are:

pH meter, an instrument which measures the pH directly.

Universal pH indicator paper, which changes color with pH. A chart is used to identify the pH range shown by each color.

The concentration of hydrogen ions in a solution with a pH of 1 is...?

Question 1

A) twice the concentration of a solution with a pH 2.B) half the concentration of a solution with a pH 2.C) ten times the concentration of a solution

with a pH 2.

D) a tenth of the concentration of a solution with a pH 2.

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Question 1

The concentration of hydrogen ions in a solution with a pH of 1 is...?

A) twice the concentration of a solution with a pH 2.B) half the concentration of a solution with a pH 2.C) ten times the concentration of a solution

with a pH 2.

D) a tenth of the concentration of a solution with a pH 2.

What is the hydroxide ion concentration for a 1 × 10-4 M HCl solution?

Question 2

A) 1 × 10-4 M

B) 1 × 10-6 M

C) 1 × 10-8 M

D) 1 × 10-10 M

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Question 2

What is the hydroxide ion concentration for a 1 × 10-4 M HCl solution?

A) 1 × 10-4 M

B) 1 × 10-6 M

C) 1 × 10-8 M

D) 1 × 10-10 M

In this presentation you have seen:

Summary

an explanation of the pH scale

End >

pH calculation of an acid and a base

the relationship between pH and pOH

PH Scale In this presentation you will: explore how to measure the strength of acids and bases Next...

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Transcript of PH Scale In this presentation you will: explore how to measure the strength of acids and bases Next...