To determine the pH of various samples of NaOH solution using: (i) pH paper, and (ii) universal indicator solution

To determine the pH of various samples of NaOH solution using: (i) pH paper, and (ii) universal indicator solution

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Requirements

Test tubes, measuring cylinder, dropper, pH paper, universal indicator solution, 0.1 M NaOH,
0.01 M NaOH, 0.001 M NaOH, 0.0001 M NaOH and 0.00001 M NaOH.

Procedure

1. Using pH Paper. Put 2-3 drops of the sample solution on pH paper by means of a glass rod and observe the colour on the pH paper. Now compare the shade of colour formed with various colours given on the ‘pH indicator chart’. From this, note the approximate pH of the sample solution and record in the table.
2. Using Universal Indicator Solution. Take five test tubes, clean and dry them. Take 5 ml of each given solution in different test tubes with the help of measuring cylinder. Now put 2-3 drops of universal indicator in each test tube by means of a dropper. Note the colour of solution in each test tube and compare its colour with different colour shades as given in the ‘pH indicator chart’. After comparing the colour in each tube, note the pH of solution and record in the table.

Observations And Results

Approximate pH of Sample Solutions of NaOH

Sample No. Molarity of NaOH solution For pH paper For universal indicator Calculated pH  = – log [H3O+]
Colour produced an pH paper Approximate pH value Colour produced in solution Approximate pH value
1. 10-1 M
2. 10-2 M
3. 10-3 M
4. 10-4 M
5. 10-5M

For calculating pH,
find H3O+ ion concentration
[OH–] [H30+] = 1 x 10-14

Or

[H30+] =\(\frac { 1\times { 10 }^{ -14 } }{ [{ OH }^{ – }] }\)

Now,

pH = – log [H3O+]

Conclusion

(i) The pH of NaOH solutions is greater than 7.
(ii) The pH of NaOH solution ………… with decrease in concentration.

Determine the pH values of hydrochloric acid at different strengths using; (i) pH papers and (ii) universal indicator solution

Determine the pH values of hydrochloric acid at different strengths using; (i) pH papers and (ii) universal indicator solution

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Requirements

Test tubes, measuring cylinder, glass rod, universal indicator solution, pH paper, 0.1 M \(HCl\), 0. 01 M \(HCl\), 0.001 M \(HCl\), 0.0001 M \(HCl\) and 0.00001 M \(HCl\).

Procedure

1. Using pH Paper. Put 2-3 drops of the sample solution on pH paper by means of a glass rod and observe the colour on the pH paper. Now compare the shade of colour formed with various colours given on the ‘pH indicator chart’. From this, note the approximate pH of the sample solution and record in the table.
2. Using Universal Indicator Solution. Take five test tubes, clean and dry them. Take 5 ml of each given solution in different test tubes with the help of measuring cylinder. Now put 2-3 drops of universal indicator in each test tube by means of a dropper. Note the colour of solution in each test tube and compare its colour with different colour shades as given in the ‘pH indicator chart’. After comparing the colour in each tube, note the pH of solution and record in the table.

Observations And Result

Approximate pH of Sample Solutions of hydrochloric acid

Sample No. Molarity of acid For pH paper For universal indicator Calculated pH  = – log [H3O+]
Colour produced an pH paper Approximate pH value Colour produced in solution Approximate pH value
1. 10-1 M
2. 10-2 M
3. 10-3 M
4. 10-4 M
5. 10-5M

Conclusion

(i) The pH of acid solutions is less than 7.
(ii) The pH of an acid solution ………… with decrease in concentration.

Experiments Based On pH Change

Experiments Based On pH Change

pH SCALE

In order to express the hydronium ion (H3O+) concentration in a solution P.L. Sorensen (1909) devised a logarithmic scale. This scale is known as pH scale. The pH of a solution is defined as the negative logarithm of hydronium ion concentration in moles per litre.

\(pH=-\log { { [H }_{ 3 } } { O }^{ + }]\)

= \(log\frac { 1 }{ { H }_{ 3 }{ O }^{ + } } \)

Acidity, Alkalinity, Neutrality of Solutions

neutral solution : \({ H }^{ + }\) = \({OH}^{-}\) = \({10}^{-7}\)M ; pH = – 7
acidic solution : \({ H }^{ + }\) > \({OH}^{-}\), \({ H }^{ + }\)  > \({10}^{-7}\)M, pH < 7.
basic solution: \({OH}^{-}\) > \({ H }^{ + }\), \({ H }^{ + }\) < \({10}^{-7}\)M, pH > 7. (also called an alkaline solution)

Strong and Weak Acids and Bases

strong acid—an acid that is a strong electrolyte and has a pH < 3.
For example, \(H_{2}SO_{4}\), \(HCl\), \(HBr\), \(HI\), \(HNO_{3}\)

weak acid—an acid that is a weak electrolyte or an ionic compound that partially reacts with water to form hydrogen ions in aqueous solution. It will have a pH greater than 3 but less than 7.
For example, \(H_{2}S\), \(H_{3}PO_{4}\), \(CH_{3}COOH\), \(H_{2}CO_{3}\).

strong base—a hydroxide that is a strong electrolyte and has a pH >11.
For example, \(NaOH\), \(KOH\), \(Ba(OH)_{2}\).

weak base—a hydroxide that is a weak electrolyte or a compound that partially reacts with water to form hydroxide ions in aqueous solution. Its pH will be less than 11 but greater than 7.
For example, carbonates, bicarbonates, ammonia (ammonium hydroxide), phosphates.

salt—an ionic compound produced by reacting an acid and a base. It will have a pH close to 7.

Ioconic Product Of Water

Pure water is very weakly ionised. So there is an equilibrium between ionised and unionised molecules.

experiments-based-ph-change-1

In general, it has been observed that at room temperature all neutral solutions have pH equal to 7, all acidic solutions have pH less than 7 and all basic solutions have pH more than 7.

Common Ion Effect

Common ion effect may be defined as the supression of degree of dissociation of a weak electrolyte by the addition of a small amount of some strong electrolyte having a common ion with that of the weak electrolyte.
Consider for example, \(NH_{4}OH\) which is a weak electrolyte and there is an equilibrium between unionised molecules and its ions.
experiments-based-ph-change-2

When \(NH_{4}Cl\), a strong electrolyte, is added to it, \(NH_{4}Cl\) ionises as

experiments-based-ph-change-3

Due to the presence of common \(NH_{4}^+\) ions the equilibrium (6.1) shifts in the backward direction and degree of dissociation of \(NH_{4}OH\) is supressed. So the concentration of \({OH}^{-}\) ions decreases and hence concentration of \(H_{3}O^+\)  ions increases. Thus, pH of the solution is lowered.
Similarly consider acetic acid, a weak electrolyte

experiments-based-ph-change-4

When sodium acetate, a strong electrolyte is added to it, CHgCOONa ionises as :

experiments-based-ph-change-5

Due to the presence of common \(CH_{3}COO^{-}\) ions the equilibrium (6.2) shifts in the backward direction and so concentration of \(H_{3}O^+\) ions decreases and hence that of OH- ions in¬creases. Therefore, pH of solution increases.

Salts when dissolved in water may undergo hydrolysis producing acidic or basic solutions. Hydrolysis of salts may be defined as the interaction of ions of the salt with water producing acidic or basic solution.

Hydrolysis of salts of strong bases and weak acids produces alkaline solution on hydrolysis. For example, the aqueous solution of sodium acetate is alkaline due to the presence of excess hydroxyl ions in the solution.

experiments-based-ph-change-6

Hydrolysis of salts of strong acids and weak bases produces acidic solution due to the presence of excess hydronium ions in the solution. For example, an aqueous solution of ammonium chloride is acidic in nature.

experiments-based-ph-change-7

Hydrolysis of salts of weak acids and weak bases gives almost neutral solutions. For example,

experiments-based-ph-change-8

Salts of strong acids and strong bases donot undergo hydrolysis and hence their aqueous solutions are neutral.

Table 6.1. Colour Changes and pH range of Certain Indicators

S.No. Indicator pH range Colour in acidic medium Colour in alkaline medium
1. Thymol blue 1.2-2.8 Red Yellow
2. Methyl yellow 2.9-4.0 Red Yellow
3. Bromophenol blue 3.0-4.6 Yellow Blue
4. Congo red 3.0-5.0 Violet Red
5. Methyl orange 3.1-4.4 Red Yellow
6. Methyl red 4.2-6.3 Red Yellow
7. Phenol red 6.8-8.4 Yellow Red
8. Phenolphthalein 8.3-10.0 Colourless Pink
9. Thymolphthalein 9.4-10.5 Colourless Blue

Universal Indicator

A universal indicator is prepared by mixing a number of common indicators together so that the mixture obtained can pass through a series of colour changes over a much wider pH range. For example, one such mixture which may show various colours at different pH is as given in Table 6.2.

Table 6.2. Colours of Universal Indicator at Different pH Values

pH Colour
3.0 Red
5.0 Orange red
5.5 Orange
6.0 Orange-yellow
7.0-7.5 Greenish-yellow
8.0 Green
9.5 Blue
10.0 Violet

Such mixtures are commonly known as universal indicators. Universal indicators are available commercially as solutions and as test papers. A pH paper is a strip of paper which is prepared by dipping the strip in the solutions of different indicators and then drying them.
pH paper can be used to find the approximate pH of any solution. The pH paper is dipped in a given sample of the solution, the colour developed in the paper is compared with the colour chart and approximate pH of the solution can be predicted. A pH paper is shown in Fig. 6.1.

experiments-based-ph-change-9

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Viva Questions with Answers on Purification of Chemical Substances by Crystallisation

Viva Questions with Answers on Purification of Chemical Substances by Crystallisation

1. Define the term ‘crystallisation’.
Ans. The substances when present in well-defined geometrical shapes are called crystals. These are formed when a hot saturated solution of the salt is allowed to cool slowly and undisturbed. This process is termed as crystallisation.
2. What is solubility ?
Ans. It is the amount of the solute which when dissolved in 100 gms of the solvent provides a saturated solution.
3. Why is crystallisation done ? 
Ans. Crystallisation enables to prepare a substance in state of highest purity.
4. What is filtration ?
Ans. It is a process of separating insoluble substances by passing the solution through a filter paper.
5. What is Kipp’s waste ?
Ans. It is a’ mixture left behind after production of H2S gas by reaction between FeS and dilute H2S04. It mainly contains FeS04 and unreacted dilute H2S04.
6. What is meant by the term, ‘water of crystallisation’ ? 
Ans. Water of crystallisation is the definite number of water molecules that is present in lose combination with one formula unit of the compound.
7. Explain the term—saturated solution.
Ans. A solution in which no more of solute can be dissolved at a particular temperature is known as saturated solution.
8. Why is solution not heated to dryness to get crystals ?
Ans. Heating the solution to dryness will not remove soluble impurities and crystals of very poor quality are obtained.
9. What is characteristic of crystals ?
Ans. Crystals have well defined geometry and shape.
10. Why is the hot saturated solution not cooled suddenly ?
Ans. By allowing saturated solution to cool slowly, crystals grow in size. It helps in their better separation as units rather than giving a massy substance of no proper geometry.
11. What is the term ‘seeding’ ?
Ans. Sometimes on cooling the saturated solution, crystallisation does not occur. A crystal of same substance is placed in the saturated solution which causes seeding. It helps in quick separation of crystals from saturated solution.
12. What is green vitriol ?
Ans. It is hydrated ferrous sulphate \(FeSO_{4}.7H_{2}O\).
13. What is mother liquor ?
Ans. The liquid left behind after the separation of crystals from a saturated solution is known as mother liquor.
14. Name the different steps involved in the process of crystallisation ?
Ans. The various steps are :
(i) Preparation of the solution. (ii) Filtration of the solution.
(iii) Concentration of the solution. (iv) Cooling of the solution slowly.
(v) Separation and drying of the crystals.
15. What are the formulae of blue vitriol, potash alum and green vitriol crystals ?
Ans. The formulae are (\(CuSO_{4}.5H_{2}O\)),(\({K_2}S{O_4}.A{l_2}{(S{O_4})_3}.24{H_2}O\)),\(FeSO_{4}.7H_{2}O\).
16. What is the formula of benzoic acid ?
Ans.

purify-impure-sample-benzoic-acid-process-crystallisation

17. What happens when the following crystals are heated separately ?
(i) Blue vitriol (ii) Potash alum (iii) Benzoic acid
Ans. (i) It changes into white powder due to loss of water of crystallisation.
(ii) It changes into fluffy white mass.
(iii) It undergoes sublimation.

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To purify impure sample of benzoic acid by the process of crystallisation.

To purify impure sample of benzoic acid by the process of crystallisation. 

Chemistry Lab ManualNCERT Solutions Class 11 Chemistry Sample Papers

Theory

Benzoic acid is a crystalline solid that has moderate solubility in hot water and low solubility in cold water. Its structure is :

purify-impure-sample-benzoic-acid-process-crystallisation

Benzoic acid is recrystallised by dissolving it in hot water.

Requirements

Crude sample of benzoic acid, 250 ml beakers (two), funnel, a policeman and a trough.

Procedure

1. Preparation of Solution. Take about 150 ml of water in a 250 ml beaker and keep it for boiling using tripod stand and wire gauze. In another 250 ml beaker take 2-3 gm of the crude sample of benzoic acid and add gradually with stirring minimum quantity of boiling water just sufficient to dissolve benzoic acid. Heating can be done if required.
2. Filtration of the Solution. Filter the hot solution immediately using fluted filter paper placed in a funnel. Insoluble impurities are left on the filter paper.
3. Cooling the Hot Saturated Solution. Let the filtered solution come to room temperature by itself. Now cool it by placing in cold water trough.
4. Separation of Crystals and Drying. Separate the crystals by Alteration using fun-nel and filter paper. Wash the crystals with cold water. Transfer the crystals on another filter paper and dry them by pressing gently between the folds of a filter paper. Transfer the crystals to a dry test tube and cork it.
The crystals of benzoic acid are opaque white.