# NCERT Exemplar Problems and their Solutions

Reviewed By:
Krishna Kant Majee
M.Sc., B.Ed.

### Chapter 2-Acid Bases and Salts-Short Answer

Q.26. Equal volumes of hydrochloric acid and sodium hydroxide solutions of same concentration are mixed and the pH of the resulting solution is checked with a pH paper. What would be the colour obtained? (You may use colour guide in the figure given below).
(a) Red
(b) Yellow
(c) Yellowish green
(d) Blue

Explanation: Hydrochloric acid and sodium hydroxide are strong acid and base respectively. So mixing of strong acid and strong base will form neutral salt and water giving the pH value as 7.

Q.27. Which of the following is/are true when HCl(g)is passed through water?
(i) It does not ionise in the solution as it is a covalent compound.
(ii) It ionises in the solution.
(iii) It gives both hydrogen and hydroxyl ion in the solution.
(iv) It forms hydronium ion in the solution due to the combination o hydrogen ion with water molecule.
(a) Only (i)
(b) Only (iii)
(c) (ii) and (iv)
(d) (iii) and (iv)

Explanation: HCI being a polar covalent compound which can easily ionize in water to form hydronium ions $(H_{3}O^{+})$ and chloride ions (Cl-).

Q.28. Which of the following statement is true for acids?
(a) Bitter and change red litmus to blue
(b) Sour and change red litmus to blue
(c) Sour and change blue litmus to red
(d) Bitter and change blue litmus to red

Explanation: Acids have sour taste and can turn blue litmus solution to red.

Q.29. Which of the following are present in a dilute aqueous solution of hydrochloric acid?
(a) $H_{3}O^{+} + Cl^{-}$
(b) $H_{3}O^{+} + OH^{-}$
(c) $Cl^{-} + OH^{-}$
(d) Unionised HCl

Explanation: A strong acid like HCl dissociates completely in its aqueous solution and form hydronium ions with its respective anion.
$HCl + H_{2}O \rightarrow H^{+} + Cl^{-} + H_{2}O$
$H_{2}O + H^{+} → H_{3}O^{+} (Hydronium ion)$

Q.30. Identify the correct representation of reaction occurring during chloralkali process.
(a) $\ce{2NaCl (l) + 2H2O (l) ->2NaOH (l) + Cl2 (g) + H2 (g)}$
(b) $ce{2NaCl (l) + 2H2O (aq) → 2NaOH (aq) + Cl2 (g) + H2 (aq)}$
(c) $\c2NaCl (aq) + 2H2O (l) → 2NaOH (aq) + Cl2 (aq) + H2 (eaq)$
(d) 2NaCl (aq) + 2H2O (l) → 2NaOH (aq) + Cl2 (g) + H2 (g)

Explanation: Chlor-alkali process is an industrial process that is used to form sodium hydroxide by electrolysis of aqueous solution of sodium chloride. It forms sodium hydroxide with hydrogen and chlorine gas as by-products.

Q. 31. Match the acids given in Column I with their correct source given in Column II

Q.32.  Match the important chemicals given in Column I with the chemical formulae given in Column II.

Q.33. What will be the action of the following substances on litmus paper?
Dry HCl gas, Moistened $NH_{3}$ gas, Lemon juice, Carbonated soft drink, Curd, Soap solution

• Dry HCI gas: It will show no effect on litmus paper as no ionization will occur in gaseous state.
• Moistened $NH_{3}$ gas: It is alkaline in nature so red litmus will turn blue.
• Lemon Juice- it contains citric acid therefore it turns blue litmus to red.
• Carbonated soft drinks: They have carbonic acid which is a weak acid and can turns blue litmus to red.
• Curd: It contains lactic acid so blue litmus will turn red.
• Soap solution: They are alkaline in nature and can turn red litmus to blue.

Q.34. Name the acid present in ant sting and give its chemical formula. Also give the common method to get relief from the discomfort caused by the ant sting.

Ans. Acid present in ant sting is formic acid which is also named as Methanoic acid.
Chemical formula for formic acid is HCOOH.
It is acidic in nature, so an alkaline compound like wet baking soda can be used to neutralize its effect to get relief from discomfort.

Q.35. What happens when nitric acid is added to egg shell?

Calcium carbonate is main component of egg shell so when we add nitric acid to egg shell, it reacts with calcium carbonate to form soluble calcium nitrate and water with brisk effervescence of carbon dioxide gas.
$\ce{CaCO3(s) + 2HNO3 (aq)->Ca(NO3)2(aq) + CO2 (g) + H2O (l)}$

Q.36. A student prepared solutions of (i) an acid and (ii) a base in two separate beakers. She forgot to label the solutions and litmus paper is not available in the laboratory. Since, both the solutions are colourless, how will she distinguish between the two?

Other then litmus paper, pH indicators can also be used to distinguish between an acidic and an alkaline solution. Some common pH indicators are methyl orange, phenolphthalein. They show characteristic colour change in acidic and alkaline medium. Some natural indicators such as turmeric, red cabbage juice can also used to identify acidic and alkaline behavior of solutions.

Q.37. How would you distinguish between baking powder and washing soda by heating?

Baking soda is sodium hydrogen carbonate (NaHCO3) which forms sodium carbonate $(Na_{2}CO_{3})$, water and carbon dioxide gas while heating.
$2NaHCO_{3}\rightarrow Na_{2}CO_{3} + H_{2}O + CO_{2}\uparrow$
Presence of $CO_{2}$ can be detected with the help of lime water which turns milky after reacting with carbon dioxide gas
Washing soda is sodium carbonate deca-hydrate $(Na_{2}CO_{3}.10H_{2}O)$ which means there are 10 water molecules present with crystal hence named as water of crystallization.
On heating washing soda, it becomes anhydrous and presence of water of crystallization can be detected with the help of anhydrous $CuSO_{4}$ which shows colour change from white to blue with moisture.
$Na_{2}CO_{3}.10H_{2}O\rightarrowNa_{2}CO_{3} + 10H_{2}O$

Q.38. Salt A commonly used in bakery products on heating gets converted into another salt B which itself is used for removal of hardness of water and a gas C is evolved. The gas C when passed through lime water turns it milky. Identify A, B and C.
Salt which is used in bakery product is baking soda or sodium hydrogen carbonate. On heating of baking soda, sodium carbonate is formed with evolution of carbon dioxide gas.
$2NaHCO_{3}\rightarrow Na_{2}CO_{3} + H_{2}O + CO_{2}\uparrow$
Sodium carbonate is used to remove hardness of water. Another product is carbon dioxide gas that gives lime test in which it turns lime water milky.
Thus, $A = NaHCO_{3}$; $B= Na_{2}CO_{3}$ and $C= CO_{2}$.

Q.39. In one of the industrial processes for manufacture of sodium hydroxide, a gas X is formed as by-product. The gas X reacts with lime water to give a compound Y which is used as a bleaching agent in chemical industry. Identify X and Y giving the chemical equation of the reactions involved.

Sodium hydroxide is manufactured by electrolysis of aqueous solution of sodium chloride which forms hydrogen and chlorine gas as byproducts. Reaction of chlorine gas with lime water forms bleaching powder $(CaOCl_{2})$. Thus, X = Cl2 gas and Y = Calcium oxychloride $(CaOCl_{2})$. The equation for the preparation of sodium hydroxide (NaOH) and bleaching powder $(CaOCl_{2})$, is given below:

$(i) 2NaCl (aq) + 2H_{2}O (l)\rightarrow 2NaOH (aq) + Cl_{2} (g) + H_{2} (g)$
$(ii) Cl_{2} (g) + Ca(OH)_{2}\rightarrow$CaOCl_{2}$+ H_{2}O$

Q.40. Fill in the missing data in the given table.

Q.41. What are strong and weak acids? In the following list of acids, separate strong acids from weak acids.
• Hydrochloric acid
• Citric acid
• Acetic acid
• Nitric acid
• Formic acid
• Sulphuric acid

Strong acids ionize completely in its aqueous solution therefore such acids contains high concentration of hydronium ions in their solutions. Nitric acid, sulphuric acid are good examples of strong acids.
Unlike strong acids, weak acids cannot ionize completely therefore solution of a weak acid contains some un-ionized molecules with its ions. Organic acids like citric acid, acetic acid are weak acids.
In the given acids;
• Hydrochloric acid – Strong Acid
• Citric acid- Weak Acid
• Acetic acid- Weak Acid
• Nitric acid- Strong Acid
• Formic acid- Weak Acid
• Sulphuric acid- Strong Acid

Q.42. When zinc metal is treated with a dilute solution of a strong acid, a gas is evolved, which is utilised in the hydrogenation of oil. Name the gas evolved. Write the chemical equation of the reaction involved and also write a test to detect the gas formed.

The reaction of zinc with dilute solution of strong acid like HCl forms zinc chloride $(ZnCl_{2})$ and hydrogen gas is evolved.
$Zn + 2HCl \rightarrow ZnCl_{2} + H_{2}\uparrow$