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Acids, Base and Salts: Short Questions and Answer

Question: What are acid, basic and amphoteric substance. Give one example of each substance. 
Answer. 
Acid and base can be defined in different ways. 
Acidic substance: According to Lowery Bronsted,  a substance which acts as proton donor is called acidic substance. 
Basic substance: A substance which acts as proton acceptor is called basic substance. 
For example , HCl donates the proton to water , so HCl is an acid and water accepts proton, so water is a base. 

Amphoteric substance: 
A substance which act as either proton donor or proton acceptor depending upon the condition is called amphoteric substance. 
For example Water is an amphoteric substance as it acts as a base (proton acceptor) when it reacts with HCl while acts as an acid (proton donor) when reacts with ammonia.
Question: What is Bronsted –lowery acid base theory? Give examples. 
Answer.
J. Bronsted and T. Lowery independently proposed acid base theory. 
According to this theory, 
Bronsted acid is a proton donor and Bronsted base is a proton acceptor. 

For example, HCl donates proton, So, HCl is Bronsted acid while H2O accepts the protons , so H2O is Bronsted base. 
There is another example. Here, H2O donates proton, So, H2O is Bronsted acid while NH3 accepts the protons , so NH3 is Bronsted base. 
Question: What are conjugate acid-base pairs? Explain with example. 
Answer.

Conjugate acid and Conjugate base 
Conjugate acid is formed by gain of proton while conjugate base is formed by loss of proton. 
Conjugate acid –base pair:  
A combination of two substances which differ from each other by a proton and easily transfer proton between them is called conjugate acid-base pair. 
Consider reaction of HCl with water. There are two examples of conjugate acid base pair. One conjugate acid pair consist of HCl as acid and its conjugate base Cl- while other conjugate acid pair consists of H2O as base and its conjugate acid (H3O).
Question: Classify each of the following as Bronsted acid or Bronsted base a. HCO3- b. HBr c. CH3COO- 
Answer. 
a. HCO3-  :
HCO3- is a Bronsted base as it accepts a proton when dissolves in water. 
e.g.,  
b. HBr :
HBr is a Bronsted acid as it donates a proton when dissolves in water. 
e.g.,

c. CH3COO- 
 CH3COO-  is a Bronsted base as it accepts a proton when dissolves in water. 
e.g., 
Question: Explain gastric acidity and use of anti-acid drug.
Answer.
 A human stomach naturally produces gastric acid to digest the food. The main component of stomach acid is HCl. Sometimes,  stomach produces too much gastric acid  which can cause gastric acidity and several unpleasant symptoms such as nausea , vomiting, heartburn, diarrhea and unexpected weight loss. The excessive gastric acidity is neutralized by anti-acid(antacid) drugs. The most commonly used antacids are Al(OH)3 and Mg(OH)2. These antacids reduce the sever acidity in stomach by neutralizing the acid. 


Question: Write briefly about the ionization of water. 
Answer.
Water undergoes self ionization to small extent. The self ionization of water is also known as auto ionization. 
 Chemical equilibrium expression can be written as follows 
 Since, concentration of unionized water is almost constant. Thus, 

For pure water, at 25 C, [H+]= [OH-] = 10-7 M 
                                                 Kw=(10-7)×(10-7) 
                                                 Kw=10-14 

Question: Define pH . What are values of pH for acids, basic and neutralization solution.
Answer. 
pH may be defined as The negative logarithm of H+ ion concentration is called pH. 
Mathematically,   
                            pH = -log[H+] 
Acidic solution:  [H+] > 10-7 M , pH < 7 
Basic solution:    [H+] < 10-7 M , pH >7 
Neutral solution [H+]= 10-7 M , pH = 7  

Question: What are Ka and pKa? 
Answer.
Ka is ionization constant of weak acid. 
For any weak acid 

As [H2O] remains constant for weak acid,
pKa:
pKa may be defined as the negative logarithm of ionization constant of weak acid (Ka) is called pKa.
Mathematically, 
 
pKa= -logKa 

Significance
Ka values are used to compare the relative strength of weak acids. Greater the Ka, stronger the acid and vice versa.
Ka values are used to determine the percentage ionization of weak acid. 
pKa values are used to compare the relative strength of weak acids. 
Greater the pKa, weaker the acid and vice versa. 

Question: What are Kb and pKb ?
Answer. 
Kb is ionization constant of weak base. For any weak base.
As [H2O] remains constant for weak base 

pKb 
pKb may be defined as the negative logarithm of ionization constant of weak acid (Kb) is called pKb. 
Mathematically, 
pKb= -logKb 

Significance 
Kb values are used to compare the relative strength of weak bases. Greater the Kb, stronger the base and vice versa. 
Kb values are used to determine the percentage ionization of weak base. 
pKb values are used to compare the relative strength of weak base. 
Greater the pKb, weaker the base and vice versa. 

Question: What is the relationship between Ka and Kb 
Answer. The Ka is inversely proportional to Kb 
As     Ka x Kb = Kw     OR
Where Ka = Ionization constant of an acid 
Kb = Ionization of conjugate base of the acid 
Kw = ionization constant of water 
From this relation, it is clear that greater the Ka values, smaller the Kb value and vice versa. It means stronger the acid, weaker is its conjugate base and vice versa. 
Greater the Kb values, smaller the Ka value vice versa. It means stronger the base, weaker is its conjugate acid and vice versa. 

Question: Give two examples of a buffer solution. 
Answer. 
Example.1
A buffer solution may contain a weak acid and its salt with strong base. e.g., CH3COOH + CH3COONa This solution is a type of acidic buffer solution with with pH less than 7.
Example.2.
A buffer solution may contain a weak base and its salt with strong acid. e.g., NH4OH + NH4Cl This solution is a type of basic buffer solution with pH more than 7.
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