3 Marks Question

Question 513 Marks

Why is water molecule polar?

Answer

Water is a polar molecule and also acts as a polar solvent. When a chemical species is said to be "polar," this means that the positive and negative electrical charges are unevenly distributed. The positive charge comes from the atomic nucleus, while the electrons supply the negative charge. It's the movement of electrons that determines polarity. Here's how it works for water.

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Question 523 Marks

An aqueous solution of an inorganic compound 'X' shows the following reactions:

It decolourises an acidified solution of $\text{KMnO}_4$
It liberates $\text{I}_2$ from acidified KI solution.
If removes black stains from red paintings.

Identify 'X' and write all the reactions involved in ionic form,

Answer

'X' is $\text{H}_2\text{O}_2$

$2\text{MnO}_4^-+5\text{H}_2\text{O}_2+6\text{H}^+\xrightarrow{\ \ \ \ \ \ \ \ \ }2\text{Mn}^{2+}+8\text{H}_2\text{O}+5\text{O}_2$
$2\text{I}^-+\text{H}_2\text{O}_2+2\text{H}^+\xrightarrow{\ \ \ \ \ \ \ \ \ }\text{I}_2+2\text{H}_2\text{O}$
$\text{PbS}+4\text{H}_2\text{O}_2\xrightarrow{\ \ \ \ \ \ \ \ \ }\text{PbSO}_4+4\text{H}_2\text{O}$

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Question 533 Marks

i. Why is $\mathrm{H}_2 \mathrm{O}_2$ syrupy liquid?
ii. Is $\mathrm{H}_2 \mathrm{O}_2$ paramagnetic or diamagnetic, why?
iii. Which has higher dipole moment $\mathrm{H}_2 \mathrm{O}_2$ or $\mathrm{H}_2 \mathrm{O}$, why?

Answer

i. $\mathrm{H}_2 \mathrm{O}_2$ is associated with intermolecular H -bonding.
ii. $\mathrm{H}_2 \mathrm{O}_2$ is diamagnetic because $\mathrm{O}^{2-}{ }_2$ does not have unpaired electrons.
iii. $\mathrm{H}_2 \mathrm{O}_2$ has higher dipole moment because it has open book structure and both $\mathrm{O}-\mathrm{H}$ group are.
not in same plane. It is more polar than water.

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Question 543 Marks

Phosphoric acid is preferred over sulphuric acid in preparing hydrogen peroxide from peroxides. Why?

Answer

In the preparation of $\mathrm{H}_2 \mathrm{O}_2$ from barium peroxide and sulphuric acid, the use of sulphuric acid has a disadvantage as it catalyses the decomposition of $\mathrm{H}_2 \mathrm{O}_2$ formed. In place of $\mathrm{H}_2 \mathrm{O}_2$, weak acids like ortho phosphoric acid, carbonic acid are preferred. All heavy metal impurities present in $\mathrm{BaO}_2$ are removed as insoluble phosphates and decomposition of $\mathrm{H}_2 \mathrm{O}_2$ is prevented.

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Question 553 Marks

A metal ' $X$' for crystalline ionic solid with dihydrogen. The solid produces hydrogen with $\mathrm{H}_2 \mathrm{O}$ and produces $\mathrm{H}_2$ gas and compound ' $Y$ ' is formed. ' $Y$ ' reacts with Aluminium metal and liberates $\mathrm{H}_2$ gas. ' $Y$ ' is also manufactured by electrolysis of brine solution.
Identify ' $X$ ' and ' $Y$ ' and write reaction involved.

Answer

$2\text{Na}+\text{H}_2\xrightarrow{\ \ \ \ \ \ \ \ \ \ \ \ }2\text{NaH}\\\ \ \ \text{'X'}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{sodium hydride}\\^\text{sodium}$
$2\text{NaH}+2\text{H}_2\text{O}\xrightarrow{\ \ \ \ \ \ \ \ \ }2\text{NaOH}+3\text{H}_2\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{'Y'}$
$2\text{Al}+2\text{NaOH}+2\text{H}_2\text{O}\xrightarrow{\ \ \ \ \ \ \ \ \ \ }2\text{NaAlO}_2+3\text{H}_2\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{'Y'}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{Sodium meta}\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{aluminate}$
$2\text{H}_2\text{O}+2\text{NaCl}(\text{aq)}\xrightarrow[]{\text{electrolysis}}2\text{NaOH}+\text{H}_2+\text{Cl}_2\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{'Y'}\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{Sodium hydroxide}$

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Question 563 Marks

The hardness of water sample is 0.002 mole of magnesium sulphate dissolved in a litre of water. Express in ppm.
Which of these is peroxide?

$KO_2, BaO_2, MnO_2, NO_2$

Answer

a. Amount of $\mathrm{Mg} \mathrm{SO}_4=0.002 \times 120 \times 1000$
$=240 \mathrm{mg}[1 \mathrm{~g}=100 \mathrm{mg}]$
120 mg of $\mathrm{MgSO}_4=100 \mathrm{mg}$ of $\mathrm{CaCO}_3$
240 mg of $\mathrm{MgSO}_4=200 \mathrm{mg}$ of $\mathrm{CaCO}_3$
IL of $\mathrm{H}_2 \mathrm{O}$ contains 200 mg of $\mathrm{CaCO}_3$
$\left[I L=10^6 \mathrm{~mL}=10^6 \mathrm{mg} \because \mathrm{d}=1 \mathrm{~g} \mathrm{~mL}^{-1}\right.$ )
$10 \mathrm{mg} \mathrm{~H}_2 \mathrm{O}$ contain $=200 \mathrm{mg}$ of $\mathrm{CaCO}_3$
Degree of hardness $=200 \mathrm{ppm}$
$\left[\because 200 \mathrm{mg}\right.$ in $10^6 \mathrm{mg}$ of $\left.\mathrm{H}_2 \mathrm{O}\right]$
b. $\mathrm{BaO}_2$, is barium peroxide. $\mathrm{KO}_2$, is potassium superoxide, $\mathrm{MnO}_2$ is Manganese dioxide, $\mathrm{NO}_2$ is nitrogen dioxide.

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Question 573 Marks

Why does hard water not form lather with soap?

Answer

Soap is sodium or potassium salt of fatty acid. it is made up when alkali reacts with fatty acids. hardness of water is due to calcium and magnesium ions. sodium salts dissolve in water and hence lather is formed when soap is added to hard water, calcium and magnesium ions replaces sodium (or potassium) ions in the salt (soap) and gets precipitated hence lather formation is hindered.

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Question 583 Marks

Dihydrogen reacts with dioxygen ($\mathrm{O}_2$) to form water. Write the name and formula of the product when the isotope of hydrogen which has one proton and one neutron in its nucleus is treated with oxygen. Will the reactivity of both the isotopes be the same towards oxygen? Justify your answer.

Answer

The isotope of hydrogen which contains one proton and one neutron is deuterium (D). Thus, when dideuterium reacts with dioxygen, deuterium oxide, i.e., heavy water is produced.
$2\text{D}_2(\text{g})\ \ \ \ \ \ +\ \ \ \ \ \ \text{O}_2(\text{g})\xrightarrow{\ \ \ \ \ \ \ \ \text{Heat}\ \ \ \ \ \ \ \ \ }2\text{D}_2\text{O}\\\text{Dideuterium}\ \ \ \ \ \ \ \text{Dioxygen}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Deuterium oxide}$
The reactivity of $\mathrm{H}_2$ and $\mathrm{D}_2$ towards oxygen will be different. Since the $\mathrm{D}-\mathrm{D}$ bond is stronger than $\mathrm{H}-\mathrm{H}$ bond, therefore, $\mathrm{H}_2$ is more reactive than $\mathrm{D}_2$ towards reaction with oxygen.

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Chemistry 3 Marks Question