Chemistry

241. Permanent hardness of water cannot be removed by which one of the fo

Permanent hardness of water cannot be removed by which one of the following methods?

[amp_mcq option1=”Treatment with washing soda” option2=”Calgon’s method” option3=”Boiling” option4=”Ion exchange method” correct=”option3″]

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Permanent hardness of water is caused by the presence of dissolved sulfates and chlorides of calcium and magnesium. Boiling only removes temporary hardness, which is caused by dissolved bicarbonates of calcium and magnesium (these decompose upon heating to form insoluble carbonates). Methods like treatment with washing soda (sodium carbonate), Calgon’s method (using sodium hexametaphosphate), and the ion exchange method (using resins or zeolites) are effective in removing permanent hardness by precipitating or exchanging the dissolved calcium and magnesium ions. Therefore, boiling cannot remove permanent hardness.
This question distinguishes between temporary and permanent hardness of water and tests the knowledge of common methods used to remove water hardness.
Temporary hardness is removed by boiling because soluble bicarbonates (Ca(HCO₃)₂, Mg(HCO₃)₂) decompose into insoluble carbonates (CaCO₃, MgCO₃) which precipitate out. Permanent hardness requires chemical treatment or ion exchange to remove the Ca²⁺ and Mg²⁺ ions from the solution.

242. Which one of the following is called ‘syngas’?

Which one of the following is called ‘syngas’?

[amp_mcq option1=”C(s) + H₂O(g)” option2=”CO(g) + H₂O(g)” option3=”CO(g) + H₂(g)” option4=”NO₂(g) + H₂(g)” correct=”option3″]

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‘Syngas’ is a mixture of carbon monoxide (CO) and hydrogen (H₂).
– Syngas, or synthesis gas, is a fuel gas consisting primarily of carbon monoxide (CO) and hydrogen (H₂).
– It is produced through processes like gasification of coal, biomass, or waste, or steam reforming of natural gas.
– Syngas is a crucial intermediate in the production of synthetic fuels, ammonia, methanol, and other chemicals.
Option A, C(s) + H₂O(g), represents a reaction (steam-carbon reaction or gasification) used to produce syngas. Option B, CO(g) + H₂O(g), represents reactants in the water-gas shift reaction, which can be used to adjust the CO:H₂ ratio in syngas. Option D is a mix of nitrogen dioxide and hydrogen, which is not syngas.

243. What is the formula mass of anhydrous sodium carbonate? (Given that th

What is the formula mass of anhydrous sodium carbonate? (Given that the atomic masses of sodium, carbon and oxygen are 23 u, 12 u and 16 u respectively)

[amp_mcq option1=”286 u” option2=”106 u” option3=”83 u” option4=”53 u” correct=”option2″]

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The formula mass of anhydrous sodium carbonate (Na₂CO₃) is 106 u.
– The chemical formula for anhydrous sodium carbonate is Na₂CO₃.
– The formula mass is the sum of the atomic masses of all atoms in the formula unit.
– Atomic masses given: Sodium (Na) = 23 u, Carbon (C) = 12 u, Oxygen (O) = 16 u.
– Formula mass of Na₂CO₃ = (2 × Atomic mass of Na) + (1 × Atomic mass of C) + (3 × Atomic mass of O)
– Formula mass = (2 × 23 u) + (1 × 12 u) + (3 × 16 u)
– Formula mass = 46 u + 12 u + 48 u = 106 u.
Anhydrous means without water. Sodium carbonate also exists as hydrated forms, like sodium carbonate decahydrate (washing soda, Na₂CO₃·10H₂O), which would have a significantly higher formula mass due to the inclusion of water molecules.

244. Which one of the following is a heterogeneous mixture?

Which one of the following is a heterogeneous mixture?

[amp_mcq option1=”Hydrochloric acid” option2=”Vinegar” option3=”Milk” option4=”Soda water” correct=”option3″]

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Milk is a heterogeneous mixture (specifically, a colloid/emulsion).
– A heterogeneous mixture has a non-uniform composition where different components can often be seen or separated.
– A homogeneous mixture (solution) has a uniform composition throughout.
– Hydrochloric acid (HCl in water), vinegar (acetic acid in water), and soda water (CO₂ in water) are all solutions, meaning the solute is dissolved at a molecular level, forming homogeneous mixtures.
– Milk is an emulsion of fat globules and proteins dispersed in water. While it may appear homogeneous, these particles are large enough to scatter light (Tyndall effect) and are not dissolved at a molecular level, making it heterogeneous.
Colloids like milk are considered heterogeneous mixtures because the dispersed phase (e.g., fat particles) is not uniformly distributed throughout the dispersion medium at the molecular level and can be observed or separated under certain conditions (e.g., centrifuging milk to separate cream).

245. A sample of oxygen contains two isotopes of oxygen with masses 16 u an

A sample of oxygen contains two isotopes of oxygen with masses 16 u and 18 u respectively. The proportion of these isotopes in the sample is 3 : 1. What will be the average atomic mass of oxygen in this sample?

[amp_mcq option1=”17.5 u” option2=”17 u” option3=”16 u” option4=”16.5 u” correct=”option4″]

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The average atomic mass of oxygen in this sample is 16.5 u.
– Average atomic mass is calculated as the weighted average of the masses of its isotopes based on their relative abundance.
– The sample contains isotopes with masses 16 u and 18 u in a proportion of 3:1.
– This means out of 4 parts, 3 parts have mass 16 u and 1 part has mass 18 u.
– Relative abundance of 16u isotope = 3 / (3+1) = 3/4 = 0.75
– Relative abundance of 18u isotope = 1 / (3+1) = 1/4 = 0.25
– Average atomic mass = (Mass₁ × Abundance₁) + (Mass₂ × Abundance₂)
– Average atomic mass = (16 u × 0.75) + (18 u × 0.25)
– Average atomic mass = 12 u + 4.5 u = 16.5 u.
Naturally occurring oxygen is predominantly isotope ¹⁶O (around 99.76%), with smaller amounts of ¹⁷O (around 0.038%) and ¹⁸O (around 0.205%). The standard average atomic mass of oxygen listed on the periodic table is approximately 15.999 u, reflecting the natural isotopic abundance. This question presents a hypothetical sample with a specific, non-natural isotopic ratio.

246. The atomic number of an element is 8. How many electrons will it gain

The atomic number of an element is 8. How many electrons will it gain to form a compound with sodium?

[amp_mcq option1=”One” option2=”Two” option3=”Three” option4=”Four” correct=”option2″]

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The element with atomic number 8 (Oxygen) will gain two electrons to form a compound with sodium.
– The element with atomic number 8 is Oxygen (O). Its electronic configuration is 2, 6.
– To achieve a stable octet configuration in its outermost shell, Oxygen needs to gain 8 – 6 = 2 electrons, forming the O²⁻ ion.
– Sodium (Na, atomic number 11) has electronic configuration 2, 8, 1 and tends to lose 1 electron to form the Na⁺ ion.
– In the formation of sodium oxide (Na₂O), two sodium atoms each lose one electron, and one oxygen atom gains two electrons.
Elements form chemical bonds (ionic or covalent) to achieve a stable electron configuration, usually a full valence shell (like noble gases). Oxygen is a non-metal in Group 16 and typically forms -2 ions by gaining two electrons. Sodium is an alkali metal in Group 1 and typically forms +1 ions by losing one electron.

247. Consider the following reaction: CH₄ + 2O₂ → CO₂ + 2H₂O Which of the f

Consider the following reaction: CH₄ + 2O₂ → CO₂ + 2H₂O
Which of the following about the reaction given above is/are correct ?

  • 1. Carbon is oxidized.
  • 2. Hydrogen is oxidized.
  • 3. Hydrogen is reduced.
  • 4. Carbon is reduced.

Select the correct answer using the code given below :

[amp_mcq option1=”1 only” option2=”1 and 2 only” option3=”2 and 3 only” option4=”2 and 4 only” correct=”option1″]

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Statement 1 is correct: Carbon is oxidized. Statements 2, 3, and 4 are incorrect.
Oxidation is the process where an atom or molecule loses electrons or increases its oxidation state. Reduction is the process where an atom or molecule gains electrons or decreases its oxidation state. In the given reaction (combustion of methane), let’s look at the oxidation states:
– In CH₄: Hydrogen is +1, so Carbon is -4 (since CH₄ is neutral).
– In O₂: Oxygen is 0 (elemental form).
– In CO₂: Oxygen is -2, so Carbon is +4 (since CO₂ is neutral).
– In H₂O: Hydrogen is +1, so Oxygen is -2 (since H₂O is neutral).
Carbon’s oxidation state changes from -4 to +4 (increase), so Carbon is oxidized.
Hydrogen’s oxidation state remains +1 (no change), so Hydrogen is neither oxidized nor reduced.
Oxygen’s oxidation state changes from 0 to -2 (decrease), so Oxygen is reduced.
This reaction is a classic example of a redox (reduction-oxidation) reaction. Methane (CH₄) acts as the reducing agent (gets oxidized), and Oxygen (O₂) acts as the oxidizing agent (gets reduced). Combustion reactions are typically redox reactions where a fuel is oxidized rapidly, usually releasing heat and light.

248. The desirable range of pH for drinking water is

The desirable range of pH for drinking water is

[amp_mcq option1=”6.5 to 8.5″ option2=”5.0 to 6.5″ option3=”6.5 to 7.0″ option4=”7.0 to 8.5″ correct=”option1″]

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The desirable range of pH for drinking water is 6.5 to 8.5.
pH is a measure of the acidity or alkalinity of water. A neutral pH is 7.0. Water with a pH less than 7 is acidic, and water with a pH greater than 7 is alkaline. The standard recommended range for potable water generally falls within the slightly acidic to slightly alkaline range, as extremely low or high pH can affect palatability, corrosion potential, and the effectiveness of disinfection.
Standards like those set by the World Health Organization (WHO) and the Indian Standard (IS 10500:2012) specify the acceptable limit for pH in drinking water as 6.5 to 8.5. While some standards might allow for slightly wider ranges in specific circumstances, 6.5-8.5 is the widely accepted desirable range to ensure safety and palatability.

249. Why is potassium permanganate used for purifying drinking water ?

Why is potassium permanganate used for purifying drinking water ?

[amp_mcq option1=”It kills germs” option2=”It dissolves the impurities” option3=”It is a reducing agent” option4=”It is an oxidizing agent” correct=”option4″]

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The correct answer is It is an oxidizing agent.
Potassium permanganate (KMnO₄) is a strong oxidizing agent. Its oxidizing properties are utilized in water purification to kill bacteria and other microorganisms (germs), and to oxidize impurities like dissolved iron and manganese, causing them to precipitate and be easily removed.
While potassium permanganate does kill germs, this action is a result of its oxidizing property. It does not dissolve impurities; rather, it helps in their precipitation after oxidation. It is distinctly an oxidizing agent, not a reducing agent. Therefore, its role as an oxidizing agent is the fundamental reason for its use in purifying drinking water.

250. Which one among the following chemicals is used as washing soda ?

Which one among the following chemicals is used as washing soda ?

[amp_mcq option1=”Calcium carbonate” option2=”Calcium bicarbonate” option3=”Sodium carbonate” option4=”Sodium bicarbonate” correct=”option3″]

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The correct answer is Sodium carbonate.
Washing soda is the common name for sodium carbonate (Na₂CO₃). It is typically found as a hydrated salt, sodium carbonate decahydrate (Na₂CO₃·10H₂O).
Sodium bicarbonate (NaHCO₃) is commonly known as baking soda. Calcium carbonate (CaCO₃) is found in substances like limestone, marble, and chalk. Calcium bicarbonate is a temporary form of calcium carbonate that exists in aqueous solution and is not a stable solid chemical compound used as “washing soda”. Washing soda is used as a cleaning agent and water softener.
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