Introduction to Quantitative Analysis
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Questions and Answers

What is the primary objective of an analyst when presented with a completely unknown sample?

To ascertain what substances are present in it.

What is the main concern of an analyst when examining raw materials in manufacturing industries?

To ensure that there are no unusual substances present which might be deleterious to the manufacturing process or appear as a harmful impurity in the final product.

What is the role of quantitative analysis in chemical analysis?

To determine how much of each component, or of specified components, is present in a given sample.

In what ways do manufacturing industries rely on chemical analysis?

<p>To ensure that the raw materials used meet certain specifications, and to check the quality of the final product.</p> Signup and view all the answers

What is the significance of qualitative analysis in the initial stages of chemical analysis?

<p>It helps to ascertain the nature of the constituents of a given sample.</p> Signup and view all the answers

What is the ultimate goal of chemical analysis in an industrial setting?

<p>To ensure the quality of the final product.</p> Signup and view all the answers

What is the term used to describe the ratio of [salt] to [acid] in a buffer solution?

<p>Buffer ratio</p> Signup and view all the answers

Under what conditions does the pH of a buffer solution equal the pKa of the acid?

<p>When [Salt] = [Acid]</p> Signup and view all the answers

What is the range of buffer ratios within which the buffering capacity is maintained?

<p>[acid] : [salt] = 1 : 10 or 10 : 1</p> Signup and view all the answers

What is the effect of a tenfold increase or decrease in the buffer ratio on the pH of the solution?

<p>The pH of the solution will increase or decrease by one pH unit</p> Signup and view all the answers

What is the purpose of titrating an alkaline solution with a standard solution of an acid?

<p>To determine the amount of acid that is exactly equivalent chemically to the amount of base present</p> Signup and view all the answers

What is the term used to describe the point at which the amount of acid added is exactly equivalent chemically to the amount of base present?

<p>Equivalence point or stoichiometric point</p> Signup and view all the answers

What is the property of strong electrolytes, such as HCl and NaOH, in aqueous solutions?

<p>They dissociate readily in aqueous solutions with an α value of unity.</p> Signup and view all the answers

What is the relationship between the velocity of a chemical reaction and the product of the active masses of reacting substances, according to the Law of Mass Action?

<p>The velocity of a chemical reaction is proportional to the product of the active masses of reacting substances.</p> Signup and view all the answers

What is the condition for dynamic equilibrium in a reversible reaction, A + B ⇌ C + D?

<p>The forward and backward reaction velocities are equal (Vf = Vb).</p> Signup and view all the answers

What is the definition of an acid according to the Bronsted-Lowry theory?

<p>An acid is any substance that donates a proton (H+).</p> Signup and view all the answers

What is the conjugate base of an acid, according to the Bronsted-Lowry theory?

<p>The conjugate base of an acid is the species formed when the acid donates a proton.</p> Signup and view all the answers

What is the equilibrium constant (K) in a reversible reaction, A + B ⇌ C + D?

<p>K = Kf / Kb = [C] [D] / [A] [B]</p> Signup and view all the answers

What is the pH range at which phenolphthalein turns red upon the addition of alkali?

<p>pH = 8-10</p> Signup and view all the answers

According to the chromophore theory, what is the reason for the color change in indicators?

<p>An isomeric change, resulting in the formation or disappearance of chromophores or auxochromes, which influence the color.</p> Signup and view all the answers

What are the types of groups that can influence the color of organic compounds, according to the chromophore theory?

<p>Chromophoric groups and auxochromes.</p> Signup and view all the answers

What is the role of auxochromes in the chromophore theory?

<p>Auxochromes cannot confer color by themselves, but they augment the action of chromophores and deepen the color produced.</p> Signup and view all the answers

What is the result of the isomeric change in the indicator molecule, according to the chromophore theory?

<p>A change in the structure of the indicator, resulting in the formation or disappearance of chromophores or auxochromes.</p> Signup and view all the answers

Why does the color change of some indicators appear slow, despite ionic reactions being instantaneous?

<p>The color change is due to an intermolecular rearrangement, which changes the structure of the indicator, taking some time.</p> Signup and view all the answers

What is the approximate pH at the first equivalence point of phosphoric acid?

<p>4.6 (MO)</p> Signup and view all the answers

Why is direct titration of orthophosphoric acid as a tribasic acid impossible with any indicator?

<p>Because K3 is very small (5 X 10^(-13))</p> Signup and view all the answers

What is the equation used to approximate the pH at the third equivalence point of phosphoric acid?

<p>pH = ½ pKw + ½ pKa - ½ pC = 12.35</p> Signup and view all the answers

Why is phenolphthalein the only suitable simple indicator for the secondary stage of phosphoric acid?

<p>Because the secondary stage is very weak and only phenolphthalein is sensitive enough to detect the equivalence point.</p> Signup and view all the answers

How can indirect titration of phosphoric acid be achieved?

<p>By replacing phosphoric acid with an equivalent amount of HCl by adding CaCl2, and then titrating the HCl.</p> Signup and view all the answers

What is the role of CaCl2 in the indirect titration of phosphoric acid?

<p>To replace phosphoric acid with an equivalent amount of HCl, allowing the HCl to be titrated.</p> Signup and view all the answers

Study Notes

Introduction to Quantitative Analysis

  • The fundamental problem in quantitative analysis is determining the amount of each component present in a given sample.
  • This requires knowing the nature of the constituents of the sample, which is outside the scope of this volume.

Applications of Chemical Analysis

  • Analytical chemists play a crucial role in modern industrialized society.
  • Manufacturing industries rely on qualitative and quantitative chemical analysis to ensure raw materials meet specifications and to check the quality of the final product.
  • The examination of raw materials is carried out to ensure no unusual substances are present that might be deleterious to the manufacturing process or appear as a harmful impurity in the final product.

Buffer Solutions

  • pH = pKa + log [salt] / [acid] (Henderson equation)
  • The ratio [Salt] / [Acid] is known as the "Buffer ratio".
  • If [Salt] = [Acid], then pH = pKa.
  • The solution containing equal concentrations of acid and its salt has the maximum buffer capacity.
  • Buffering capacity is maintained for mixtures within the range [acid] : [salt] = 1 : 10 or 10 : 1
  • pH = pK ± 1

Buffers from Weak Bases and Their Salts

  • pH = pKb + log [salt] / [base]
  • pH = pKw – pKb – log [salt] / [base]
  • A tenfold increase or decrease of buffer ratio would raise or lower the pH of solution by one pH unit.

Examples of Buffer Solutions

  • Calculate the pH of a buffer solution containing 0.1 M acetic acid and 0.1 M sodium acetate, pKa = 4.76.
  • Calculate the pH of a buffer solution containing 0.05 M ammonia and 0.3 M ammonium chloride, pKb = 4.76.

Neutralization Indicators

  • The object of titrating an alkaline solution with a standard solution of an acid is to determine the amount of acid equivalent to the amount of base present.
  • The point at which this is reached is the equivalence point, stoichiometric point, or theoretical end point.
  • HCl, NaOH, and most salts dissociate readily in aqueous solutions (α = 1).
  • Weak electrolytes possess α values far from unity (e.g., CH3COOH, NH4OH, CuSO4).

Law of Mass Action

  • The velocity of a chemical reaction is proportional to the product of the active masses of reacting substances.
  • Active masses refer to the concentration expressed in g.mol/l.
  • Consider the simple reversible reaction: A + B ⇌ C + D
  • Vf ∝ [A] [B] or Vf = Kf [A] [B]
  • Vb ∝ [C] [D] or Vb = Kb [C] [D]
  • At dynamic equilibrium, the forward and backward reaction velocities will be equal.

Acid-Base Theories

  • Arhenius theory: An acid is any substance that ionizes into H+ while a base ionizes to give OH–.
  • Bronsted-Lowry theory: An acid is any substance that produces or donates a proton, and a base accepts a proton.
  • When an acid gives up a proton, the remaining species has a certain proton affinity and hence is a base.

Chromophore Theory

  • The color change of some indicators is slow while ionic reactions are known to be instantaneous.
  • The color change of indicators is due to the presence of unsaturated groups called chromophoric groups in the indicator molecule.
  • Examples of chromophoric groups are NO2, NO, C=C-C=C.
  • The color of organic compounds is also influenced by the presence of another type of groups known as auxochromes.
  • Auxochromes cannot by themselves confer color to a compound but when present together with chromophores, they augment the action of the latter and deepen the color produced by them.
  • The most important auxochromes are OH and NH2 groups.

Phosphoric Acid

  • Phosphoric acid is a triprotic acid with three dissociation constants.
  • The pH at the first equivalence point is given approximately by: (½pK1 + ½pK2) = 4.6 (M.O.)
  • The pH at the second equivalence point is given approximately by: (½pK2 + ½pK3) = 9.7 (ph.Ph.)
  • The third equivalence point may be calculated approximately from the equation: pH = ½pKw + ½pKa - ½pC = 7.0 + 6.15 - ½(1.6) = 12.35 for 0.1M H3PO4.

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Description

Learn about the fundamental problem of quantitative analysis, which involves identifying the substances present in a sample. This quiz covers the basics of qualitative analysis and its applications in identifying impurities and confirming their absence.

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