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CONCEPT:

Salt Hydrolysis and pH of Salt Solutions

• Salts are formed by the neutralization of acids and bases. The nature of the parent acid and base determines the pH of the salt solution.
• Based on the strength of the acid and base:
• Strong acid + strong base → Neutral salt → pH ≈ 7
• Strong acid + weak base → Acidic salt → pH < 7
• Weak acid + strong base → Basic salt → pH > 7
• If the salt contains a highly charged metal ion (like Al³⁺), it can hydrolyze water and release H⁺ ions, making the solution acidic.

EXPLANATION:

• Sodium acetate (CH₃COONa):
• Formed from a strong base (NaOH) and weak acid (acetic acid)
• Gives a basic solution → pH > 7
• Potassium nitrate (KNO₃):
• Formed from a strong acid (HNO₃) and strong base (KOH)
• Neutral salt → pH ≈ 7
• Aluminium chloride (AlCl₃):
• Formed from a strong acid (HCl) and a weak base (Al(OH)₃)
• Al³⁺ undergoes hydrolysis to release H⁺ ions → acidic solution
• pH < 7
• Sodium hydrogen carbonate (NaHCO₃):
• Amphiprotic salt (can act as acid or base), but overall mildly basic
• pH slightly > 7

Therefore, the correct answer is Aluminium chloride (Option 3) as it produces a solution with pH < 7.

CONCEPT:

Amphoteric Oxides

• Amphoteric oxides are metal oxides that react with both acids and bases to form salt and water.
• They show dual behavior—acidic as well as basic—depending on the reacting species.
• Typical amphoteric oxides include oxides of elements like aluminium, zinc, tin, and lead in certain oxidation states.

EXPLANATION:

• CuO (Copper(II) oxide):
• Basic in nature → reacts with acids
• Does not react with bases → Not amphoteric
• Na₂O (Sodium oxide):
• Strongly basic oxide → reacts only with acids
• Not amphoteric
• MgO (Magnesium oxide):
• Basic oxide → reacts with acids
• Does not react with bases → Not amphoteric
• Al₂O₃ (Aluminium oxide):
• Reacts with acids to form salts
• Also reacts with strong bases like NaOH to form aluminates
• Amphoteric in nature

Therefore, the correct answer is Option 4) Al₂O₃, which is an amphoteric oxide.

Concept:

A concave mirror has a reflective surface that is curved inward and away from the light source.

Solution:

r = 2f

f = \({r \over 2}\) = \({0.4 \over 2}\) = 0.2 m

\({1 \over v}\) +\({1 \over u}\) = \({1 \over f}\)

\({1 \over v}\) + \({1 \over 0.4}\) = \({1 \over 0.2}\)

\({1\over v}\) = \({10 \over 2}\) - \({10 \over 4}\) 

= \({5 \over 2}\)

v = \({2 \over 5}\) 

= 0.4 m

The correct answer is an option (2).

Concept:

The lens formula is given by,

1/f = 1/u + 1/v,

where f is the focal length of the lens, u is the distance of the object from the lens, and v is the distance of the image from the lens, we can solve for the focal length of the lens.

Calculation:

• Magnification: m = 1/3
• Object distance:  u = -24cm(negative as the object is on the same side as incoming light for a lens system)

The magnification m is related to the image and object distances by:

⇒m = -v/u=height of image/height of object

⇒1/3 = v/-24

⇒v = -8 cm

(The negative sign indicates that the image is virtual and on the same side as the object, which is typical for a diverging lens.)

The lens formula is:1/f = \(\frac{1}{v} - \frac{1}{u} \)

v= -8cm and u = -24cm

⇒\(\frac{1}{f} = \frac{1}{-8} - \frac{1}{-24} \)

⇒\( \frac{1}{f} = -\frac{1}{8} + \frac{1}{24} = \frac{-3}{24} + \frac{1}{24} = \frac{-2}{24} = \frac{-1}{12} \)

The focal length of the diverging lens is -12 cm. 

∴ The correct answer is option (4).

CONCEPT:

Reactivity of Metals with Water

• Metals react with water to form metal hydroxide and hydrogen gas.
• The alkali metals (group 1 elements like Na, K) are highly reactive with water, and their reactivity increases down the group.
• Group 2 metals (like Mg, Ca) also react with water, but less vigorously than alkali metals.

EXPLANATION:

• Sodium (Na):
• Reacts vigorously with cold water, forming sodium hydroxide and hydrogen gas.
• Potassium (K):
• Even more reactive than sodium.
• Reacts explosively with cold water, producing enough heat to ignite the hydrogen gas.
• Most vigorous reaction among the options
• Magnesium (Mg):
• Reacts slowly with cold water; faster with hot water or steam.
• Calcium (Ca):
• Reacts steadily with cold water, but less vigorously than Na or K.

Therefore, the correct answer is Option 2) Potassium (K) as it reacts most vigorously with water.

CONCEPT:

Redox Reactions and Electrochemical Series

• A metal can reduce the ion of another element only if it is more reactive (i.e., lies above it in the reactivity series).
• Zinc (Zn) lies above Cu²⁺, H⁺, and Ag⁺ in the electrochemical series, so it can reduce them.
• But Al³⁺ lies above Zn, meaning aluminium is more reactive than zinc, and hence, Zn cannot reduce Al³⁺ to Al.

EXPLANATION:

Reactivity Series (from most to least reactive):

K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au > Pt

• Zn + Cu²⁺ → Zn²⁺ + Cu
• Possible (Zn reduces Cu²⁺)
• Zn + 2H⁺ → Zn²⁺ + H₂
• Possible (Zn reduces H⁺ to H₂ gas)
• Zn + 2Ag⁺ → Zn²⁺ + 2Ag
• Possible (Zn reduces Ag⁺)
• Zn + Al³⁺ → ?
• Not possible (Zn is less reactive than Al)

Therefore, the correct answer is Option 4) Al³⁺.

CONCEPT:

Electrolytic Refining of Copper

• Electrolytic refining is used to purify metals like copper by using an electrolytic cell.
• In this process:
• The impure copper acts as the anode.
• The pure copper sheet acts as the cathode.
• The electrolyte is usually an aqueous solution of copper(II) sulfate (CuSO₄).
• During electrolysis:
• Cu atoms from the anode (impure copper) lose electrons and go into solution as Cu²⁺ ions.
• Cu²⁺ ions from the solution gain electrons and deposit on the cathode as pure copper.

EXPLANATION:

• Anode: Impure copper (loses electrons, gets oxidized)
• Cathode: Pure copper (gains electrons, gets reduced)
• Reference electrode / Indicator electrode: Used in electrochemical cells for measurement purposes, not involved in refining

Therefore, the correct answer is Option 1) Anode, as impure copper serves as the anode in electrolytic refining.

CONCEPT:

Reaction of Metals with Dilute Acids

• When a reactive metal like zinc reacts with a dilute acid (e.g., H₂SO₄), it undergoes a single displacement reaction.
• The metal displaces hydrogen from the acid, producing a salt and hydrogen gas (H₂).
• This is a characteristic reaction of metals with acids and is used to identify hydrogen gas in laboratory experiments.

EXPLANATION:

• Reaction:

Zn (s) + H₂SO₄ (aq) → ZnSO₄ (aq) + H₂ (g)

• Hydrogen gas (H₂) is evolved as a product of the reaction.
• Other options:
• Sulphur dioxide (SO₂): Not formed in this reaction; usually released during combustion or some redox reactions.
• Oxygen (O₂): Not involved in this acid-metal reaction.
• Hydrogen sulphide (H₂S): A toxic gas formed when sulfides react with acids, not involved here.

Therefore, the correct answer is Option 2) Hydrogen, which is evolved when zinc reacts with dilute sulphuric acid.

The correct answer is ketone

Concept:-

Alcohols (option 1): These are organic compounds where the functional group (-OH, a hydroxyl) is attached to a carbon atom. An example of an alcohol is ethanol (CH3CH2OH).

Aldehydes (option 2): An aldehyde has a carbonyl (C=O) group as well, but the difference is that this group is located at the end of the carbon chain in aldehydes. An example of an aldehyde is acetaldehyde (CH3CHO).

Ketones (option 3): In ketones, the carbonyl (C=O) group is present within the carbon chain, i.e., it is bonded to two carbon atoms. Therefore, CH3CH2COCH3 is classified as a ketone.

Ethers (option 4): Ethers have an oxygen atom bonded to two carbon groups; the general structure is R-O-R'. An example of an ether is dimethyl ether (CH3OCH3).c

Explanation:-

: In the CH3CH2COCH3 molecule, the CO refers to a carbonyl (C=O) group, and this carbonyl group is bonded to two other carbon atoms or groups (CH3CH2- and CH3). This arrangement is indicative of a ketone.

CH₃CH₂-CO-CH₃ --> This is 2-butanone, a ketone.

CONCEPT:

Complete Combustion of Hydrocarbons

• Combustion is a chemical reaction where a substance reacts with oxygen and releases energy.
• In the complete combustion of hydrocarbons (like alkanes), the carbon atoms are fully oxidized to carbon dioxide (CO₂), and hydrogen atoms are converted to water (H₂O).
• This type of reaction occurs in the presence of a sufficient supply of oxygen.

EXPLANATION:

• Butane (C₄H₁₀) is an alkane and undergoes complete combustion as follows:

2 C₄H₁₀ + 13 O₂ → 8 CO₂ + 10 H₂O

• Thus, the products are:
• Carbon dioxide (CO₂)
• Water (H₂O)
• Other options:
• Carbon: Formed in incomplete combustion, not in complete combustion.
• Butanone: A ketone, not a combustion product.
• Hydrogen gas: Not formed during hydrocarbon combustion.

Therefore, the correct answer is Carbon dioxide and Water.