Saturday, June 1, 2013

10 class acids salt and base Exam oriented Board Questions

Q. 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.
Q. 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.
Q. How would you distinguish between baking powder and washing soda by heating?
Answer: Sodium hydrogencarbonate on heating gives CO2 gas which will turn lime water milky whereas no such gas is obtained from sodium carbonate.
2NaHCO3 ---Heat
Na2CO3 + H2O + CO2     
Na2CO3.10H2O -Heat
Na2CO3 + 10H2O
Q. Why aqueous solution of sodium carbonate is basic in nature?
Sodium bicarbonate is an amphoteric compound. Aqueous solutions are mildly alkaline due to the formation of carbonic acid and hydroxide ion:
NaHCO3 + H2O
H2CO3 + OH− 
Q. Why is an aqueous solution of ammonium chloride acidic in nature?
Answer: A basic component is the one that produces hydroxide (OH-) ions when dissolved in water. In aqueous solution of ammonium chloride, ammonium ions (NH4+) first associate with H2O and form ammonia and hydroxide ions.
NH4Cl + H2O = NH4+ + HCl
NH4+ + H2O = NH3 + OH-
Since ammonium ions produce hydroxide ions, NH4+ are considered to be the basic components.
Q. Write the composition of baking powder. What will happen if tartaric acid is not added to it?
Ans. Baking powder is a mixture of sodium bicarbonate and tartaric acid. This acid neutralizes sodium carbonate formed on decomposition of baking soda. If tartaric acid is not present in baking powder‘, the food materials such as cake will taste bitter due to the presence of sodium carbonate.
Q. Why is Plaster of Paris written as CaSO4. ½ H2O ? How is it possible to have half a water molecule attached to CuSO4?
Ans. The actual formula of Plaster of Paris is 2CaSO4.H2O which means that one molecule of H2O is associated with two molecules of CaSO4. The formula for simplicity is written as CaSO4. ½H2O.
Q. 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.
Q. 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 -
Q. 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   +    2H2 ----->   2NaOH   + H2CO3
Na OH                      ----->   Na+        +                OH-
Q. 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.
Q. 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.
Q. Define a base and acid in terms of proton.
Answer:  A base is a proton acceptor and an acid is a proton donor.
Q. What is deliquescent and hygroscopic substance?
Answer: Deliquescent substances and hygroscopic substances both are the water attracting or absorbing substances. Such substance becomes moist or wet when placed in damp or moist place by absorbing water.
Hygroscopic substances become wet while deliquescent substance dissolve after absorbing moisture. This is the major difference between these two closely related terms.
For e.g. CaCl2 is deliquescent while NaCl is hygroscopic.
Q What are hydrated salts? Give an example.
Ans: Salts containing a fixed number of water molecules in their crystal structure are called hydrated salts.
A molecule of sodium carbonate (Na2CO­3.10H2O) contains ten molecules of water. This is known as hydrated salt of sodium carbonate.
Q. What are Anhydrous and deliquescent salt?
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