10th acid base and salt solved model sample paper

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 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         -------->     NH₄NO₃    

Question (8): What is the action of Na₂CO₃ solution on litmus and why?
Examples:  K⁺Ca²⁺PO^3- (Potassium calcium phosphate) , Na⁺K⁺SO₄^2- (Sodium potassium sulphate).

Question (9): Name the following:

Answer:   1) Two non-hydrated crystalline salts are: sodium chloride (NaCl) and potassium nitrate (KnO₃)

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:

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          ------------->    Fe (OH)₃    + 3 H Cl

HCl -------------------------> H⁺      +   Cl ^-
Question (21): 1) Name a strong base and a weak base. 2) Name a hydrated salt.

Answer:  1) A strong base is sodium hydroxide (NaOH) and a weak base is ammonium hydroxide (NH₄OH). 

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   
K₂SO_4.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.

Ca²⁺(OCl)^-Cl^- [Bleaching powder (Calcium chloro hypochlorite)]

Question 29: 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.

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.

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