Volumetric AnalysisTitration

Dr.Riham Hazzaa

Volumetric AnalysisTitration

Dr.Riham Hazzaa

• Titration is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of a known reactant.

• Because volume measurements play a key role in titration, it is also known as volumetric analysis.

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• Standard solution is a chemical term which describes a solution of known concentration.

• Primary Standard: Substances whose exact solution concenteration can be determined by transferring a known weight of the reagent to a volumetric flask and diluting to the mark with solvent

• Secondary standard: A titrant is prepared in an approximate concentration and then titrated against a primary standard to determine its exact concenteration

• Standardization process of calibrating the secondary standard against our primary standard

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• Molarity: is defined as the number of moles of solute divided by the number of liters of solution containing the solute.

M = Amount of moles of solute

liters of solution

Amount of moles of solute = Weight

MW

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• Normality: is defined as the number of equivalents (eq) of solute divded by the number of liters of solution containing the solute.

N = Amount of equivalent(eq) of solute

Liters of solution

Amount of equivalents (eq) = weight (grams)

EW

Equivalent weight= Molecular weight

Z5Dr.Riham Hazzaa

Normality is related to molarity:

• M = Weight OR MW = Weight

MW×volume M×volume• N = Weight OR EW = Weight

EW×volume N×volume• Equivalent weight = Molecular weight

Z

Weight × Z = Weight

N×volume M ×volume • Thus, Normality (N) = Molarity (M) (Z)

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• Normality is related to molarity:

• Normality (N) = Molarity (M) (Z)

• 1M sulfuric acid H2SO4 is 2N for acid-base reactions because each mole of sulfuric acid provides 2 moles of H+ ions (Z).

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Before Dilution After Dilution

Solution1 Solution2

M1 =moles1

litre1M2 =

moles2

litre2

moles1 = moles2

M1 × litre1 = M2 × litre2

Dr.Riham Hazzaa

Acids and Bases Titration

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Properties of Acids

• An acid is any substance that releases hydrogen ions, H+, into water.

• Blue litmus paper turns red in the presence of hydrogen ions. Blue litmus is used to test for acids.

• Acids have a sour taste; lemons, limes, and vinegar are acidic.

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Properties of Bases

• A base is a substance that releases hydroxide ions, OH –, into water.

• Red litmus paper turns blue in the presence of hydroxide ions. Red litmus is used to test for bases.

• Bases also have a bitter taste; soapy feel.

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Neutralization Reactions

• If we have an acid with two hydrogens (sulfuric acid, H2SO4), we need two hydroxide ions to neutralize it.

H2SO4(aq) + 2 NaOH(aq) → Na2SO4(aq) + 2 H2O(l)

H2SO4(aq) + Ca(OH)2(aq) → CaSO4(aq) + 2 H2O(l)

Dr.Riham Hazzaa

Acid-Base Reactions with Gas Formation

1. Carbonates react with acids to form CO2, carbon dioxide gas.

NaCO3+ 2HCl → 2NaCl + H2O +CO2

2. Sulfites react with acids to form SO2, sulfur dioxide gas.

Na2SO3 2HCl → 2NaCl + H2O + SO2

3. Sulfides react with acids to form H2S, hydrogen sulfide gas.

Na2S + 2HCl → 2NaCl + H2SDr.Riham Hazzaa 13

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Acid/Base Neutralization• An acid and a base react with each other in a

neutralization reaction.

• When an acid and a base react, water and a salt are produced.

• For example, nitric acid reacts with sodium hydroxide to produce sodium nitrate and water:

HNO3(aq) + NaOH(aq) → NaNO3(aq) + H2O(l)

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The pH Scale• A pH value expresses the acidity or basicity of a

solution.

• Most solutions have a pH between 0 and 14.

• Acidic solutions have a pH less than 7.

– As a solution becomes more acidic, the pH decreases.

• Basic solutions have a pH greater than 7.

– As a solution becomes more basic, the pH increases.pH = –log[H+]

[H+] = 10–pH

[H+] is the molar hydrogen ion concentration

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Acid/Base Classifications of Solutions• Strongly acidic solutions

have a pH less than 2.

• Weakly acidic solutions have a pH between 2 and 7.

• Weakly basic solutions have a pH between 7 and 12.

• Strongly basic solutions have a pH greater than 12.

• Neutral solutions have a pH of 7.

Dr.Riham Hazzaa

Acid – base theories

1. Arrhenius theory

2. The Brønsted –Lowry theory

3. Lewis theory

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Arrhenius Theory

• Svante Arrhenius proposed the following definitions for acids and bases in 1884:

– An Arrhenius acid is a substance that ionizes in water to produce hydrogen ions.

– An Arrhenius base is a substance that ionizes in water to release hydroxide ions.

• For example: HCl is an Arrhenius acid

NaOH is an Arrhenius base.

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Strengths of Acids

• Acids have varying strengths.

• The strength of an Arrhenius acid is measured by the degree of ionization in solution.

• Ionization is the process where polar compounds separate into cations and anions in solution.

• The acid HCl ionizes into H+ and Cl– ions in solution.

HCl + H2O → H3O+ + Cl-

hydronium ions Dr.Riham Hazzaa

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Arrhenius Acids in Solution

• All Arrhenius acids have a hydrogen atom bonded to the rest of the molecule by a polar bond. This bond is broken when the acid ionizes.

• Polar water molecules help ionize the acid by pulling the hydrogen atom away:

HCl(aq) + H2O(l) → H3O+(aq) + Cl–(aq) (~100%)

HC2H3O2(aq) + H2O(l) → H3O+(aq) + C2H3O2–(aq) (~1%)

• The hydronium ion, H3O+, is formed when the aqueous hydrogen ion attaches to a water molecule.

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Strong & Weak Arrhenius Acids• Strong acids ionize extensively (completly) to

release hydrogen ions into solution.– HCl is a strong acid and ionizes nearly 100%

• Weak acids only ionize slightly(partialy )in solution.– HF is a weak acid and ionizes only about 1%

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Strengths of Bases• Bases also have varying strengths.

• The strength of an Arrhenius base is measured by the degree of dissociation in solution.

• Dissociation is the process where cations and anions in an ionic compound separate in solution.

• A formula unit of NaOH dissociates into Na+ and OH– ions in solution.

NaOH Na+ + OH-

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Arrhenius Bases in Solution

• When we dissolve Arrhenius bases in solution, they dissociate giving a cation and a hydroxide anion.

• Strong bases dissociate almost fully and weak bases dissociate very little:

NaOH(aq) → Na+(aq) + OH–(aq) (~100%)

NH4OH(aq) ↔ NH4+(aq) + OH–(aq) (~1%)

Dr.Riham Hazzaa

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Strong & Weak Arrhenius Bases• Strong bases dissociate extensively to release

hydroxide ions into solution.– NaOH is a strong base and dissociates nearly 100%

• Weak bases only ionize slightly in solution.– NH4OH is a weak base and only partially dissociates

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Brønsted-Lowry Theory

• The Brønsted-Lowry definitions of acids and bases are broader than the Arrhenius definitions.

• A Brønsted-Lowry acid is a substance that donates a hydrogen ion to any other substance. It is a proton donor.

• A Brønsted-Lowry base is a substance that accepts a hydrogen ion. It is a proton acceptor.

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Brønsted-Lowry Acids & Bases• Lets look at two acid/base reactions:

– HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

– HCl(aq) + NH3(aq) → NH4Cl(aq)

• HCl donates a proton in both reactions and is a Brønsted-Lowry acid.

• In the first reaction, the NaOH accepts a proton and is the Brønsted-Lowry base.

• In the second reaction, NH3 accepts a proton and is the Brønsted-Lowry base.

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Amphiprotic Compounds

• A substance that is capable of both donating and accepting a proton is an amphiprotic compound.

• NaHCO3 is an example:

– HCl(aq) + NaHCO3(aq) → NaCl(aq) + H2CO3(aq)

– NaOH(aq) + NaHCO3(aq) → Na2CO3 (aq) + H2O(l)

• NaHCO3 accepts a proton from HCl in the first reaction and donates a proton to NaOH in the second reaction.

Dr.Riham Hazzaa

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Lewis Theory

Lewis acid-base theory relates acid-base behavior of molecules to their molecular structure.

Lewis acid- A species that is an electron pair acceptor.

Lewis base- A species that is an electron pair donor.

H+ + :NH3 H:NH3

+

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Acid-Base Titrations• A titration is used to analyze an

acid solution using a solution of a base.

• A measured volume of base is added to the acid solution. When all of the acid has been neutralized, the pH is 7. One extra drop of base solution after the endpoint increases the pH dramatically.

• When the pH increases above 7, phenolphthalein changes from colorless to pink indicating the endpoint of the titration.

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Methyl Red Bromothymol Blue Phenolphthalein

Acid-Base Indicators

Dr.Riham Hazzaa

Acid-Base Titrations: Strong Acid/Strong BaseHCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

? M KNOWN M

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Fewer moles of H2SO4 would be required to neutralize 25 mL NaOH, thus the curve would shift left

7

14

0250 50

7

14

0250 50

Because we start with pure HCl, then add NaOH the pH starts low then goes high, thus the curve flips

Titration curve

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Dr.Riham Hazzaa

The theory• Begin with a balanced equation for the

reaction: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) na = 1 nb = 1 (mole ratios of acid and base)Mole = concentration X volume

For the acid: na = MaVa

For the base: nb = MbVb

na : nb (stoichiometry mole ratio)

MaVa : MbVb 34

Dr.Riham Hazzaa

The Theory na : nb

MaVa : MbVb

i.e. na nb

MaVa MbVb

Then, MaVa na

MbVb nb

=

=

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MaVa na MbVb

nb =

36

= 0.277 M HCl

1.What is the molarity of a hydrochloric acid solution if 25.50 mL are required to neutralize 0.375 g Na2CO3?

2 HCl(aq) + Na2CO3(aq) → 2 NaCl(aq) + H2O(l) + CO2(g)

Titration Problem

the molarity of a hydrochloric acid

2. A 10.0 mL sample of 0.555 M H2SO4 is titrated with 0.233 M NaOH. What volume of NaOH is required for the titration?

H2SO4(aq) + 2 NaOH(aq) → Na2SO4(aq) + H2O(l)volume of NaOH = 49.8 mL NaOH

Dr.Riham Hazzaa