XI Chemistry - Viva Notes Volumetric Analysis

VIVA NOTES

(A) Volumetric Analysis (Titrations)




Qs.1 What is Titration?

Ans. The process of adding one solution from the burette into another in the conical flask in order to determine its volume after the completion of the chemical reaction is known as Titration.

Qs.2 What is Neutralization?

Ans. Neutralization is simply a reaction between Hydrogen ion (H+) of an acid and Hydroxide ion (OH-) of the base to form water. In this reaction another class of compound known as “Salt is also produced which remains in the solution as ions, when water is boiled off these ions re-unite to form salt.

Acid + Base → Salt + Water

Qs.3 What is an acid?

Ans. An acid is a substance which gives off proton (H+) in solution or in other words it is a donor of proton e.g. HCl, H2SO4 or HNO3. When dissolve in water ionizes to give Hydrogen ion (H+)

HCL ↔ H+ + Cl-

H2SO4 ↔ 2H+ + SO4--

HNO3 ↔ H+ + NO3-

Qs.4 What do you mean by basicity of an acid?

Ans. It is the number of ionizable Hydrogen presents in the molecule of an acid.

e.g. In the above examples basicity of HCl and HNO3 is one, while that of H2­SO4 is 2.

Qs.5 What is a base?

Ans. A base is now regarded as a molecule or an ion which furnishes OH- ion or accepts a proton given up by an acid. Thus it is proton acceptor.

e.g. NaOH, KOH, Ca (OH)2

NaOH ↔ Na+ + OH-

KOH ↔ K+ + OH-

Ca(OH)2 ↔ CA++ + 2(OH)-

Qs.6 What is meant by acidity of a base?

Ans. It is the number of hydroxyl (OH-) groups present in the molecule of a base.

e.g. In the above examples the acidity of NaOH and KOH is one while that of

Ca (OH)2 is 2.

Qs.7 Why Phenolphthalein is added into the solution of titration flask?

Ans. Phenolphthalein serves as an indicator for determining the end point. It gives pink colour in presence of an alkali and becomes colourless with slight excess of an acid.

Qs.8 While using a burette what precautions are necessary to be observed?

Ans. Burette must be washed first with ordinary water and then rinsed with the solution which is to be taken in it. It must be held vertically and air bubbles must be removed.

While taking a reading the eyes should be in level with the surface of the liquid.

Qs.9 What is a pipette?

Ans. It is an instrument which delivers a definite volume of a liquid. It consists of a glass tube with a cylindrical bulb in the middle and the lower end is drawn into a jet. There is a circular mark on the upper tube.

Qs.10 What is a standard solution?

Ans. A standard solution is a solution of known strength.

Qs.11 What do you mean by strength of a solution?

Ans. Strength of a solution is the quantity of a substance present in any known volume of the solution.

Qs.12 Define a “Normal solution”?

Ans. A standard solution which contains 1 gram equivalent of a substance per dm3 is known as a Normal solution and is denoted by 1 N.

Qs.13 What is a decinormal solution?

Ans. A decinormal solution contains 1/10 fraction of gram equivalent weight of a substance dissolved per dm3 and is denoted by 0.1 N or N/10.

Qs.14 What is the relationship between Normality and Strength per dm 3 of solution?

Ans. Normality =

Qs.15 What is “Acidimetry”?

Ans. It is an operation by which the strength of an alkali is determined by neutralizing it with an acid of known strength in presence of an indicator.

Qs.16 What is “Alkalimetry”?

Ans. It is an operation by which the strength of an alkali is determined by neutralizing it with an alkali of known strength in presence of an indicator.

Qs.17 Define “Equivalent Weight”?

Ans. It is the number of parts by weight that will combine with or displace from 1 part by weight of H2, or 8 parts by weight of O2 or 35.5 by weight of Cl2.

Qs.18 What is “Gram Equivalent Weight”?

Ans. It is the equivalent weight of a substance expressed in gram.

Qs.19 How equivalent weight of an acid is determined?

Ans. Equivalent weight of an acid =

Where basicity is the total number of replaceable Hydrogen.

Qs.20 How equivalent weight of a base is determined?

Ans. Equivalent weight of a base =

Where acidity is the total number of hydroxyl (OH-) groups.

Qs.21 What do you mean by “Standardization of a solution”?

Ans. To standardize means to determine its strength by titration against some standard solution.

Qs.22 What is the equivalent weight of NaOH?

Ans. Eq. Wt. Of a base = = = 40

Therefore, equivalent weight of NaOH is 40.

Qs.23 How a decinormal solution of NaOH is prepared?

Ans. A decinormal solution (0.1 N) of NaOH can be prepared by dissolving 1/10 fraction of its equivalent wt. i.e. 40/10 = 4 gms, in one dm3 of distilled water.

Qs.24 What do you mean by “End-point”?

Ans. It is the exact stage at which the chemical reaction of the titrating solutions is just completed.

Qs.25 Why alkali is taken in burette when phenolphthalein is used as an indicator?

Ans. The appearance of a pink colour at the end point is easily detectable. So it is a better criterion than the disappearance of colour when acid is used in the burette.

Qs.26 How Equivalent weight of Na2CO3 is calculated?

Ans. Na2CO3 is a basic salt. Its equivalent weight can be calculated from the equation of its reaction with an acid e.g. HCl.

Hence the equivalent weight of Sodium Carbonate is determined as follows:

Na2CO3 + 2HCl 2 NaCl + H2O +CO2

2x23+12 2(1+35.5)

+3x16 = 73

= 106

2 HCl = Na2CO3

1 HCl = ½ Na2CO3

Eq. Wt. of Na2CO3= = 53.

Qs.27 What is Methyl Orange?

Ans. It is Sodium salt of an azo dye. It is very good indicator for titrating strong acid against strong base or strong acid against weak base.

Qs.28 What is meant by Anhydrous salt?

Ans. A Salt without water molecule is called Anhydrous.

Qs.29 What indicator is suitable for Sodium Carbonate titration against strong acids and why?

Ans. The pH range of Methyl Orange is pH 3.0 to 4.4.

Hence, it is very suitable indicator when a weak alkali like Sodium Varbonatye is neutralized with a strong acid. In such vases the end point would be at a pH some what below 7.0.

Qs.30 Give the structural formula of Phenolphthalein?

Ans. It is as follows:

Qs.31 Which is the suitable indicator for the titration of?

(i) Weak acid against strong alkali.

(ii) Strong acid against weak alkali.

(iii) Strong acid against strong alkali.

Ans. (i) Phenolphthalein for the titration of weak acid with strong alkali.

(ii) Methyl Orange for the titration of strong acid with a weak alkali.

(iii) Phenolphthalein or Methyl orange for the titration of strong acid with strong alkali (preferably phenolphthalein)

Qs.32 Define Oxidation?

Ans. The loss of electron from an atom, ion or molecule is called Oxidation.

Fe++ ® Fe+++ + e-

Qs.33 Define Reduction?

Ans. Gain of electron or the loss of positive valence is called reduction.

Fe+++ + e- ® Fe++

Qs.34 What are “Oxidation-Reduction Titration”?

Ans. Titrations based upon the reaction between an oxidation agent and reducing agent are known as “Oxidation-Reduction Titrations”.

Qs.35 What are Oxidizing and Reducing agents?

Ans. An oxidizing agent is that substance which oxidizes the other substance e.g., KMnO4 in this titration.

A reducing agent is that which reduces the other substance e.g., Oxalic acid in this titration.

Qs.36 Why do we add equal volume of dilute Sulphuric acid in KMnO4 titration?

Ans. In presence of acid it acts as a strong oxidizing agent and liberates atomic Oxygen from KMnO4.

2 KMnO4 + 3H2SO4 ® K2SO4 + 2 MnSO4 + 3 H2O + 5 [O]

Qs.37 Why do we heat Oxalic acid solution to 60-70º C?

Ans. Oxalic acid reacts with Potassium Permanganate very slowly at room temperature. In order to facilitate the reaction, it is heated to 60º to 70ºC.

Qs.38 What indicator is used in KMnO4 titrations?

Ans. KMnO4 itself acts as an indicator. So no external indicator is required. Near end point it produces a permanent pinkish tinge.

Qs.39 Explain how the change of colour takes place near end point while titrating Oxalic acid with KMnO4 in presence of sulphuric acid?

Ans. In the presence of Sulphuric acid, Potassium permanganate reacts with reducing agent as follows.

2 KMnO4 + 3H2SO4 ® K2SO4 + 2 MnSO4 + 3 H2O + 5 [O]

As the titration proceeds, Potassium Sulphate and Manganese Sulphate are formed, both give colourless solution. As soon as KMnO4 is in excess, the solution becomes pink and so it acts as its own indicator.

Qs.40 Why upper meniscus is noted while using KMnO4 solution in the burette?

Ans. Potassium Permanganate solution is highly coloured and lower meniscus is not distinctly visible. That is why reading of upper meniscus of the liquid is noted.

Qs.41 What is the nature of FeSO4 · 7H2O in Redox titration?

Ans. Ferrous Sulphate acts as reducing agent.

Qs.42 How equivalent weight of FeSO4 · 7H2O is calculated?

Ans. According to the equation:

10 FeSO4 · 7H2O + 5H2SO4 + 5[O] → 5Fe2 (SO4)3 + 12H2O

5[O] combines with 10 FeSO4 · 7H2O

5 x 16 parts by wt. of [O] combines with 10 x 278 parts by

wt. of FeSO4 · 7H2O

8 parts by wt. of [O] combines with = 278

Hence equivalent weight of FeSO4 · 7H2O = 278

Qs.43 Why no external Indicator is required for this titration?

Ans. During titration the dark purple colour of permanganate solution disappears entirely. As soon as the reaction is completed, a single drop of permanganate produces a permanent pinkish tinge to the solution. Thus KMnO4 acts as internal indicator and no external indicator is required.

Qs.44 How equivalent weight of hydrated and Anhydrous Oxalic acid are calculated?

Ans. Mol. Wt. of hydrated Oxalic acid H2C2O4 • 2H2O

2+24+64+2x18=126

Basicity of acid =2

Eq. wt. of hydrated Oxalic acid = = = 63

Mol. wt. of Anhydrous Oxalic acid H2C2O4 = 2+24+64 = 90

Eq. wt. of Anhydrous Oxalic acid = = = 45.

Qs.45 What do you mean by Mohr’s Salt?

Ans. Ferrous Ammonium Sulphate FeSO4 (NH4)2 SO4.6H2O is commonly known as “Mohr’s salt”.

Qs.46 What is the nature of Ferrous Ammonium Sulphate in redox titration?

Ans. Ferrous Ammonium Sulphate acts as a reducing agent.

Qs.47 How can you calculate the equivalent weight of KMnO4?

Ans. KMnO4 ­is an Oxidizing agent. The equivalent weight of an Oxidant is the number of parts by weight which gives 8 parts by weight of Oxygen for oxidation.

2 KMnO4 ­+ 3H2SO4 → K2SO4 + 2MnSO4 + 3H2O + 5 [O]

5x16 parts by wt. of O2 comes from 2x158 parts of KMnO4

׃٠ 8 ״ ״ ״ ״ ״ ״ = 31.6

Therefore, the equivalent weight of KMnO4 is 31.6

Qs.48 Why Mohr’s salt solution is prepared in water containing dilute Sulphuric acid?

Ans. Solution of Ferrous Ammonium Sulphate (Mohr’s salt) is always prepared by dissolving it in water containing some dilute Sulphuric acid which prevents hydrolysis.

Qs.49 What happens when KMnO4 solution is added in acidified Ferrous Ammonium Sulphate solution?

Ans. When KMnO4 solution is added to Ferrous Ammonium Sulphate is presence of dulute H2SO4, the Ferrous salt is Oxidized to ferric state.

Qs.50 What is the equivalent weight of Ferrous Ammonium Sulphate ?

Ans. Equivalent weight of Ferrous Ammonium Sulphate.

FeSO4. (NH4)2SO4.6H2O = 152+36+32+64+108 = 392







(B) Melting Point & Boiling Point




Qs.1 What is meant by melting or fusion of a substance?

Ans. It is the change of a substance from solid to the liquid state.

Qs.2 Define melting point?

Ans. The temperature at which the solid substance fuses (i.e. changes into liquid) and continue to take place until the whole of the solid is converted into liquid is known as the “Melting point” of the substance.

Qs.3 Why a thin walled capillary tube and not a thick walled tube is selected to determine the Melting point?

Ans. The wall of the capillary tube should be thin otherwise the temperature of the bath may not be equal to the temperature of the substance inside the capillary tube.

Qs.4 Why it is necessary to heat the bath slowly with constant stirring?

Ans. It is necessary to heat the bath slowly with constant stirring to ensure uniformity of temperature; otherwise the rate of rise of temperature is so rapid that it will be difficult to observe the temperature at which the substance just melts.

Qs.5 Why water as a bath cannot be used for the substance having Melting point above 100˚C?

Ans. Boiling point of water is 100˚C. So it cannot be used as a bath for determination of Melting points of such substances. Instead of water, Sulphuric acid or Glycerine can be used.

Qs.6 What is “Boiling”?

Ans. It is a rapid change from the liquid to the gaseous state.

Qs.7 What is meant by the “Boiling point” of a liquid?

Ans. The Boiling point of a liquid is the temperature at which the vapour pressure of the liquid is equal to the atmospheric pressure (i.e. 760 mm.)

Qs.8 Why temperature remains constant at boiling point of a liquid?

Ans. Because the heat is utilized for converting liquid into vapours, so temperature remains constant.

Qs.9 What is the Boiling point of pure water? Is the same at all places?

Ans. The Boiling point of pure water is 100˚C at a pressure of 760 torr. The pressure of air at all places is not the same. So B.P. differs at various places.

Qs.10 What is the difference between Boiling and Evaporation?

Ans. Boiling is a rapid change and takes place through out the mass of the liquid at a definite temperature (i.e., boiling point). While Evaporation is a slow change and takes place at the surface of the liquid at all temperature.

Qs.11 What is the effect of pressure on Boiling point?

Ans. It is raised by the increase of pressure and is lowered by the decrease of pressure. (An increase or decrease of 26.7 m.m. of pressure increase or decrease the Boiling point by 1˚ C).

Qs.12 What is the effect of height on the Boiling point of a liquid?

Ans. With heights, the boiling points are reduced as the pressure decreases.

Qs.13 Why should we stir the liquid in the beaker?

Ans. We should stir it constantly because, this helps the liquid to maintain uniform temperature.