Question 1 - Case Based Questions (MCQ) - Chapter 2 Class 10 - Acids, Bases and Salts

Frothing in Yamuna: The primary reason behind the formation of the toxic foam is high phosphate content in the wastewater because of detergents used in dyeing industries, dhobi ghats and households Yamuna's pollution level is so bad that parts of it have been labelled 'dead' as there is no oxygen in it for aquatic life to survive. 

4.1 Predict the pH value of the water of river Yamuna if the reason for froth is high content of detergents dissolved in it.

a) 10 - 11

b) 5 - 7

c) 2 - 5

d) 7

Answer

Detergents are bases . Bases have a pH greater than 7

So, the correct answer is (a) 10 - 11

4.2 Which of the following statements is correct for the water with detergents dissolved in it?

• a) low concentration of hydroxide ion (OH ^- ) and high concentration of hydronium ion (H ₃ O ⁺ )

• b) high concentration of hydroxide ion (OH ^- ) and low concentration of hydronium ion (H ₃ O ⁺ )

• c) high concentration of hydroxide ion (OH ^- ) as well as hydronium ion (H ₃ O ⁺ )

• d) equal concentration of both hydroxide ion (OH ^- ) and hydronium ion (H ₃ O ⁺ )

Answer

Detergent is a Base .

• And when bases are dissolved in water, they ionise to form OH ^- ,
• Thus, there will be high concentration of OH ^-

Also, Acid ionise to form H ₃ O ⁺

Since Detergent is a base, it will have low concentration of H ₃ O ⁺

So, the correct answer is (b) high concentration of hydroxide ion (OH-) and low concentration of hydronium ion (H3O+)

4.3 The table provides the pH value of four solutions P, Q, R and S

Which of the following correctly represents the solutions in increasing order of their hydronium ion concentration?

a) P > Q > R > S

b) P > S > Q > R

c) S < Q < R < P

d) S < P < Q < R

Answer

Acid ionise to form Hydronium Ions (H ₃ O ⁺ )

• As the hydronium concentration increases, the pH decreases , causing the solution to become more acidic

Since order of pH is → 11 < 9 < 5 < 2

So, hydronium ion concentration will be of the order, S < Q < R < P

So, the correct answer is (c)

4.4 High content of phosphate ion in river Yamuna may lead to: 

a) decreased level of dissolved oxygen and increased growth of algae 

b) decreased level of dissolved oxygen and no effect of growth of algae 

c) increased level of dissolved oxygen and increased growth of algae 

d) decreased level of dissolved oxygen and decreased growth of algae 

Answer

The phosphate ions promote the growth of algae in water

More the algae present, more the surface area of the waterbody covered and oxygen cannot penetrate into the depth of the water, it is utilized by the algae

As a result, there will be an increased growth of algae and decrease in dissolved oxygen

So, the correct answer is (a)

4.5 If a sample of water containing detergents is provided to you, which of the following methods will you adopt to neutralize it? 

a) Treating the water with baking soda 

b) Treating the water with vinegar 

c) Treating the water with caustic soda 

d) Treating the water with washing soda  

Answer

Neutralization takes place when Acid and Base react to form Salt and Water

Acid + Base → Salt + Water

Since Detergent is a base , we need an Acid

Here,

• Baking Soda is a Base
• Vinegar is an Acid
• Caustic Soda is a Base
• Wahsing Soda is a Base

Thus, we use vinegar

So, the correct answer is (b)