Thursday, June 30, 2011

solved questions acid base and salt for class 10

 Question (1): Define the terms: Acid, alkali and salt.
 Answer:  An acid is a compound, which releases hydronium ions (H3O+) as the only positive ions in solution. An alkali is a compound, which releases hydroxyl ions (OH-) as the only negative ions in solution. A salt is one of the products of neutralization between an acid and a base; water being the only other product. OR A salt gives positive ions other than H+ ion and negative ions other than OH- ion in solution.
Question (2): Identify the number of replaceable hydrogen ions (H+) in the following acids: HCl, CH3COOH, H2SO4, H3PO4.
Answer:  HCl = 1    CH3COOH = 1       H2SO4 = 2     H3PO4 = 3.
Question (3): What is a neutralization reaction?
Answer:  Neutralization is essentially a chemical reaction between H3O+ ions of an acid with OH- ions of the base, to give undissociated molecules of water.
Question (4): What are strong and weak acids? Give one example of each?
Answer:  A strong acid is one, which is almost completely dissociated in solution. Examples: Dilute nitric acid, dilute sulphuric acid and dilute hydrochloric acid. A weak acid is one, which is only partially ionized in solution (degree of dissociation is >30%). Examples: Acetic acid, carbonic acid and sulphurous acid.
Question (5): Why is acetic acid called a weak acid though there are 4 'H' atoms in the molecule?
Answer:  Acetic acid is called a monobasic acid because only one of the 4 'H' atoms of the acid is released as H+ ion in solution.
Question (6): How does a strong acid differ from a concentrated acid?
Answer:  The strength of an acid depends upon its dissociation power whereas concentration depends on water content in the acid.
Question (7): Name a salt of a strong acid HNO3 and a weak base like NH4OH. Represent the reaction that takes place.
Answer:  The salt that results due to the above reaction is Ammonium nitrate (NH4NO3). The reaction is represented as:  HNO3         +             NH4OH         --------à     NH4NO3    
Question (8):  1) Name a strong base and a weak base. 2) Name a hydrated salt.
1) A strong base is sodium hydroxide (NaOH) and a weak base is ammonium hydroxide (NH4OH).
2) A hydrated salt is copper sulphate crystals (CuSO4.5H2O).
Question (9): Name the following:
1) Two non-hydrated crystalline salts  2) Two neutral salts  3) Two basic salts  4) Two acid salts
Answer:   1) Two non-hydrated crystalline salts are: sodium chloride (NaCl) and potassium nitrate (KnO3)
2) Two neutral salts are: sodium chloride (NaCl) and sodium sulphate (Na2SO4)
3) Two basic salts are: basic copper carbonate (CuCO3.Cu(OH)2) and basic lead carbonate (PbCO3.Pb(OH)2)
4) Two acid salts are: sodium bicarbonate (NaHCO3) and sodium phosphate (NaH2PO4)
Question (10): Name the salts of sulphuric acid.
Answer:  The salts of sulphuric acid are bisulphate and sulphate. Examples: NaHSO4, KHSO4 and Na2SO4.
 Question (11): Define the term "pH"; what does" pH" stand for?
Answer:  The term "pH" is defined as the negative logarithm of H+ ion concentration of a given solution; the concentration being expressed as moles per litre. Mathematically pH = - log [H+] 'pH' stands for: Power of hydrogen ion concentration, 'p' for power and 'H' for H+ ion concentration.
Question (12): What is 'pH' scale? Explain briefly.
Answer:  The strength of an acid or a base is expressed in terms of hydronium ion concentration. This is expressed on a scale known as 'pH' scale. It is a 14 point scale; i.e., it has values ranging from 0 to 14, indicating the value of negative logs of H+ ion concentration of the solution. Some important benchmark values in the pH scale are: pH = 7 indicates neutral solutions e.g., aqueous solutions. pH > 7 to 14 indicates alkaline solutions and pH < 7 to 0 indicate acidic solutions
Question (13): What is the 'pH' of pure water and that of rain water? Explain the difference.
Answer:  The pH of pure water is seven. Rain water is slightly acidic because as rain drop fall, the carbon dioxide in the air dissolves with drops to form very weak carbonic acid. Accordingly, rain water has a pH that is slightly below 7.
Question (14): What is the pH of solution 'A' which liberates CO2 gas with a carbonate salt? Give the reason?
Answer:  The pH of solution 'A' is lesser than 7. Carbonates salts react with acids (A) to liberate CO2 gas.
Question (15): What is the pH of solution 'B' which liberates NH3 gas with an ammonium salt? Give reason?
Answer:  The pH of solution 'B' is lesser than 7 because 'B' is an alkali as it liberates NH3 gas. Question (16): How do you increase or decrease the pH of pure water? Answer:  By adding a few drops of alkali to pure water, it's pH increases; and by adding a few drops of an acid decreases the pH of pure water.
Question (17): What are indicators?
Answer:  Indicators are chemicals that show whether the given solution is acidic or basic, by the sudden change of color.
Question (18): Name the common acid-base indicators used in the laboratory with their color change.
Answer:  The three common indicators used in the laboratory are:
Final colour
Methyl orange
Deep pink
Question (19): What is a universal indicator? What is its advantage?
Answer:  A universal indicator is a mixed indicator of organic chemicals which not only shows whether the given solution is acidic or basic, but also shows the approximate pH values by giving a wide particular colour for a specific value of pH.
Question (20): What is the action of litmus on ferric chloride solution and why?
Answer:  An aqueous solution of FeCl3 salt undergoes hydrolysis and releases H+ ions from the stronger acid HCl formed. Hence, the resulting solution is acidic and turns blue litmus turns red.
          Fecl3    +   3 H2O          -----------------------------à     Fe (OH)3    + 3 H Cl
                                                          HCl -------------------------à   H+                 + Cl -
Question (21): What is the action of Na2CO3 solution on litmus and why?
 Answer:  In solution Na2CO3 salt undergoes hydrolysis and releases OH- ions from the stronger alkali NaOH. Consequently, the resulting solution is basic and turns red litmus blue.
                      Na2CO3   +    2H2O                -----------------------------------------à 2NaOH   + H2CO3
                    Na OH                                     ----------------------------------------à Na+        +                OH-
Question (22): What is the action of NaHSO4 solution on litmus and why?
Answer:  Blue litmus turns red in NaHSO4 solution due to the release of H+ ions along with Na+ ions. Thus, NaHSO4 is an acid salt.
Question (23): Though NaHSO4 solution releases H+ ions, why is it not called an acid?
Answer:  NaHSO4 solution is not called an acid because an acid should release only H+ ions as positive ions and not any other positive ions. But NaHSO4 is solution releases H+ ions as well as Na+ ions also as positive ion.
Question (24): Define the term ' normal salt ' with examples.
Answer:  A normal salt is produced when all the replaceable 'H' atoms of an acid are completely replaced by a metal or NH4 group, during neutralization with a base. Examples: NaCl, K2CO3and (NH4)3PO4.

Question (25): Define the term 'acid salt' with examples.
Answer:  An acid salt is formed when the available 'H' atoms of an acid are only partially replaced by a metal, during neutralization with a base, and hence there are still 'H' atoms present in the salt that are available for replacement.  Examples: NaHSO4, NaHCO3, Na2HPO4and NaH2PO4
Question (26): Define the term 'basic salt' with an example.
Answer:  A basic salt is formed by the incomplete neutralization of a base with an acid or partial replacement of hydroxyl radicals of a diacidic base or a triacidic base with an acid radical. Example: Zn(OH)Cl, Cu(OH)NO3.
Question (27): Define a double salt. Give some examples.
Answer:  A double salt is formed between two simple salts by crystallization from a saturated solution of a mixture of the two. Examples   K2SO4.Al2(SO4)3.24H2O ( potash Alum),
(NH4)2 Al2(SO4)3.24H2O (Ammonium Alum)                      FeSO4(NH4)2 SO4.6H2O (Mohr’s Salt)
Question (28): Define mixed salt giving some examples.
Answer:  Mixed salts contain more than one acid radical or basic radical other than H+ and OH- ions.
Examples:  K+Ca2+PO3- (Potassium calcium phosphate) , Na+K+SO42- (Sodium potassium sulphate).
Ca2+(OCl)-Cl- [Bleaching powder (Calcium chloro hypochlorite)]
Question (29): Define complex salts with examples.
Answer:  Complex salts dissociate to give one simple ion and one complex ion in solution. Examples:
Na [Ag(CN)2 ] (aq) ---------------------------    à Na+  +  [Ag(CN)2 ]-                 (complex Ions)
[Cu (NH3)4] SO4                ----------------------à [ Cu (NH3)4 ]2+           + SO4 2- (complex Ions)
Question (30): What happens if NaOH is added to ferrous ammonium sulphate solution and warmed? What is the inference?
Answer:  When NaOH is added to ferrous ammonium sulphate solution and warmed, both ferrous ion and ammonium ion separate and exhibit their individual properties. A green precipitate of Fe(OH)2 is formed and the smell of NH3 gas from (NH4)+ ion is observed. Hence it is an example of the formation of a double salt.
Question (31): What happens if sodium hydroxide is added to cupper  ammonium sulphate solution and what is the inference?
Answer:  When sodium hydroxide is added to cupper  ammonium sulphate solution, no blue precipitate of Cu(OH)2 is seen, which is normally observed for Cu2+ ion. Hence it is an example of a complex salt.
Question (32): When lead dioxide reacts with concentrated HCl and produces a salt and water, the reaction is not called a neutral reaction why?
Answer:  For a reaction to be called a neutral reaction the products should be only salt and water. But in the above reaction, Cl2 gas is also liberated.
Question (33): Define a base and acid in terms of proton.
Answer:  A base is a proton acceptor and an acid is a proton donor.
Question (34):  1) Name the anions that can be detected in the laboratory using dilute HCl or dilute H2SO4 and give the ionic equation common to both reactions.  2) Which property of the acid is indicated?
1) The three anions that can be identified using dilute acids with their common ionic equations are:
Carbonate and Bicarbonate Ions      2H+  + CO32-                                     -----------à   H2O + CO2 ­
Sulphate and Bi sulphate Ions          2H+  + SO32-                        --------
à  H2O + SO2  ­
Sulphide Ion                                   2H+ (dil. Acid)  + S 2-  (salt)   -------------
à     H2S­  (Gas Identified)                                                         
2) The property is that of releasing H+ ions by dilute HCl or dilute H2SO4 in solution i.e., the acidic property.
Question (37): 1) What do you observe when concentrated H2SO4 is added slowly to blue CuSO4 crystals in a test tube and why? 2) Is these any other way of observing the above result?
Answer:  1) The blue crystals turn to white amorphous anhydrous powder. Hence concentrated H2SO4 acts as a dehydrating agent, removing the water of crystallization molecules from CuSO4.5H2O crystals.
2) Another way of observing the above result is by gentle heating of blue crystals of CuSO4.5H2O directly in a test tube.
Question  (38) : Name two crystalline salts that do not have water of crystallization.
Answer:  The chlorides of K, Na and Pb and the nitrates of K, Na, Pb, Ag, [NH4+] do not have water of crystallization.
Question (39): Why do acids not show acidic behavior in the absence of water?
Ans: Acids do not show acidic behaviour in absence of water because the dissociation of hydrogen ions from an acid occurs in presence of water only. It is hydrogen ions that are responsible for the acidic behaviour.  
Question (40): How is NH4OH used as a laboratory reagent?
Answer:  NH4OH is used to detect cations in their salt solutions by precipitating their hydroxides whose colour and solubility in excess NH4OH, identifies them.
Examples of cations detected are Pb2+, Zn2+, Cu2+, Fe2+ and Fe3+.
Question (41):  Name four gases that can be prepared in the laboratory using dilute H2SO4. Show how they can be prepared?
Answer:  The four gases are: 1) H2 gas            2) CO2 gas         3) SO2 gas       4) H2S gas
Their respective methods of preparations are:
1) An active metal (above Pb) and dilute H2SO4 gives H2 gas.
2) Any carbonate salt and dilute H2SO4 gives CO2 gas.
3) Any sulphite salt and dilute H2SO4 gives SO2 gas. 4) Any sulphide salt and dilute H2SO4 gives H2S gas.
Question (42): While diluting concentrated H2SO4, what care must be taken and why?
Answer:  While diluting concentrated H2SO4 never add water to concentrated acid; instead, add concentrated H2SO4 slowly with stirring to the volume of water. As this reaction is highly exothermic the heat produced will splash the acid and may cause burns.
Question (43): What is the cause of tooth decay?
Answer:  The bacteria present in the mouth act on sugar and food particles remaining in the mouth after eating to produce acids. The acid produced lowers the pH in the mouth to 5.5 and begins to attack the hard enamel and corrode it resulting in tooth decay.
 Question (44): Plaster of Paris should be stored in a moisture-proof container. Explain why?
Answer:  Plaster of Paris (POP) or calcium sulphate with half a molecule of water per molecule of the salt (hemi-hydrate) is a hygroscopic substance. Should it absorb a little water from the surroundings, it evolves heat and quickly sets to a hard porous mass hence it should be stored in a moisture-proof container to keep it dry
Q.45  A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?  
Ans:  (a) By adding small amount of baking soda, the milkman shifts the pH of the fresh milk from 6 to slightly alkaline so that he can keep it for longer time as the milk in alkaline condition, does not set curd easily.
(b) Since, this milk is slightly basic than the usual milk, acids produced due to bacterial actions in it are neutralized by the base. Therefore, it takes longer time to set as curd.    

Q.46: Give two important uses of washing soda and baking soda.
Ans: Uses of Washing Soda:
1. Sodium carbonate (washing soda) is used in glass, soap, and paper industries.
2. It is used in the manufacture of sodium compounds such as Borax.
Uses of Baking Soda:1. Sodium hydrogen carbonate is also ingredient in antacids. Being alkaline, it neutralizes excess acid in the stomach and provides relief.2. It is also used in soda-acid fire extinguishers.
3. It is used as baking powder in confectionery items like cake, pastry etc.  

Q. 47. What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
Ans. When a solution of sodium hydrocarbonate (sodium hydrogencarbonate) is heated, sodium carbonate and water are formed with the evolution of carbon dioxide gas.

2NaHCO3    +                heat                      Na2CO3      +   H2 O  + CO2

Q. 48. Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. In which test tube will the fizzing occur more vigorously and why?

Ans: The fizzing will occur strongly in test tube A, in which hydrochloric acid (HCl) is added. This is because HCl is a stronger acid than CH3COOH and therefore produces hydrogen gas at a faster speed due to which fizzing occurs.

Q 49. Plaster of Paris should be stored in a moisture-proof container. Explain why?

Ans. Plaster of Paris (POP) should be stored in a moisture-proof container because Plaster of Paris, a powdery mass, absorbs water (moisture) to form a hard solid known as gypsum.
Ca SO4 . ½ H2 O     +   11/2 H2 O        Ca SO4 . 2 H2O
Question 50 :Why should curd and sour substances not be kept in brass and copper vessels?
Ans:  Curd and other sour substances contain acids. Therefore, when they are kept in brass and copper vessels, the metal reacts with the acid to liberate hydrogen gas and harmful products, thereby spoiling the food.
Question 51: Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?
Ans: Hydrogen gas is usually liberated when an acid reacts with a metal.fig 2.1 page 19  
Take few pieces of zinc granules and add 5 ml of dilute H2SO4. Shake it and pass the gas produced into a soap solution. The bubbles of the soap solution are formed. These soap bubbles contain hydrogen gas.                 2H2SO4            +     Zn        Zn (SO4)2   +  2H2  
We can test the evolved hydrogen gas by its burning with a pop sound when a candle is brought near the soap bubbles.
Question 52: Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.
                        CaCO3  (s)   +    HCI(aq)       CaCI2(aq) + CO2   + H2 O (I)
Question 53 : Why do HCl, HNO3, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
Ans: The dissociation of HCl or HNO3 to form hydrogen ions always occurs in the presence of water. Hydrogen ions (H+) combine with H2O to form hydronium ions (H3O+).     HCI --------à   H+  +  CI       H+     +     H2 O →   H3 O+
Although aqueous solutions of glucose and alcohol contain hydrogen, these cannot dissociate in water to form hydrogen ions. Hence, they do not show acidic character.
Question 54  Why does an aqueous solution of an acid conduct electricity?
Ans.:   Acids dissociate in aqueous solutions to form ions. These ions are responsible for conduction of electricity.
Question 55.   Why does dry HCl gas not change the colour of the dry litmus paper?
Ans. : Colour of the litmus paper is changed by the hydrogen ions. Dry HCl gas does not contain H+ ions. It is only in the aqueous solution that an acid dissociates to give ions. Since in this case, neither HCl is in the aqueous form nor the litmus paper is wet, therefore, the colour of the litmus paper does not change.
Question 56 While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Ans.:  Since the process of dissolving an acid in water is exothermic, it is always recommended that acid should be added to water. If it is done the other way, then it is possible that because of the large amount of heat generated, the mixture splashes out and causes burns
Question 57: How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
Ans. : When an acid is diluted, the concentration of hydronium ions (H3O+) per unit volume decreases. This means that the strength of the acid decreases.

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