## Calculate the number of aluminium ions present

[question] Question. Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.(Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u) [/question] [solution] Solution: 1 mole of aluminium oxide $\left(\mathrm{Al}_{2} \mathrm{O}_{3}\right)=2 \times 27+3 \times 16=102 \mathrm{~g}$ i.e., $102 \mathrm{~g}$ of $\mathrm{Al}_{2} \mathrm{O}_{3}=6.022 \times 10^{23}$ molecules of $\mathrm{Al}_{2} \mathrm{O}_{3}$ Then, $0.051 \mathrm{~g}$ ...

## Calculate the number of molecules of sulphur

[question] Question. Calculate the number of molecules of sulphur $\left(\mathrm{S}_{8}\right)$ present in $16 \mathrm{~g}$ of solid sulphur. [question] [solution] Solution: 1 mole of solid sulphur $\left(\mathrm{S}_{\mathrm{o}}\right)=8 \times 32 \mathrm{~g}=256 \mathrm{~g}$ i.e., $256 \mathrm{~g}$ of solid sulphur contains $=6.022 \times 10^{23}$ molecules Then, $1 \mathrm{~g}$ of solid sulphur contains $=\frac{6.02 \times 10^{23}}{256}$ $16 \mathrm{~g}$ of solid sulphur contains $=\frac{6.02 ...

## What is the mass

[question] Question. What is the mass of : (a) 0.2 mole of oxygen atoms ? (b) 0.5 mole of water molecules ? [/question] [solution] Solution: (a) Mass of one mole of oxygen atoms $=16 \mathrm{~g}$ mass of $0.2$ mole of oxygen atoms $=0.2 \times 16 \mathrm{~g}=3.2 \mathrm{~g}$ (b) Mass of one mole of water molecule $=18 \mathrm{~g}$ mass of $0.5$ mole of water molecules $=0.5 \times 18 \mathrm{~g}=9 \mathrm{~g}$ [/solution]...

## Convert into mole

[question] Question. Convert into mole. (a) 12 g of oxygen gas (b) 20 g of water (c) 22 g of carbon dioxide [/question] [solution] Solution: (a) $32 \mathrm{~g}$ of oxygen gas $=1$ mole Then, $12 \mathrm{~g}$ of oxygen gas $=\frac{12}{32}$ $=0.375 \mathrm{~mole}$ (b) $18 \mathrm{~g}$ of water $=1 \mathrm{~mole}$ Then, $20 \mathrm{~g}$ of water $=\frac{18}{20}$ $=1.11$ moles (approx) (c) $44 \mathrm{~g}$ of carbon dioxide $=1$ mole Then, $22 \mathrm{~g}$ of carbon dioxide $=\frac{22}{44}$ $=0.5$ ...

## What is the mass

[question] Question. What is the mass of (a) 1 mole of nitrogen atoms? (b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)? (c) 10 moles of sodium sulphite $\left(\mathrm{Na}_{2} \mathrm{SO}_{3}\right)$ ? [/question] [solution] Solution: (a) The mass of 1 mole of nitrogen atoms is $14 \mathrm{~g}$. (b) The mass of 4 moles of aluminium atoms is $(4 \times 27) g=108 g$ (c) The mass of 10 moles of sodium sulphite $\left(\mathrm{Na}_{2} \mathrm{SO}_{3}\right)$ is $10 \times[2 \times 23+32...

## Calculate the molar mass

[question] Question. Calculate the molar mass of the following substances (a) Ethyne, $\mathrm{C}_{2} \mathrm{H}_{2}$ (b) Sulphur molecule, $S_{8}$ (c) Phosphorus molecule, $\mathrm{P}_{4}$ (atomic mass of phosphorus $=31$ ) (d) Hydrochloric acid, $\mathrm{HCl}$ (e) Nitric acid, $\mathrm{HNO}_{3}$ [/question] [solution] Solution: (a) Molar mass of ethyne, $\mathrm{C}_{2} \mathrm{H}_{2}=2 \times 12+2 \times 1=26 \mathrm{~g}$ (b) Molar mass of sulphur molecule, $\mathrm{S}_{\mathrm{g}}=8 \times 32...

## Give the names of the elements

[question] Question. Give the names of the elements present in the following compounds: (a) Quick lime (b) Hydrogen bromide (c) Baking powder (d) Potassium sulphate [/question] [solution] Solution: [/solution]...

## Write the chemical formulae

[question] Question. (a) Magnesium chloride (b) Calcium oxide (c) Copper nitrate (d) Aluminium chloride (e) Calcium carbonate [/question] [solution] Solution: (a) Magnesium chloride: $\quad \mathrm{MgCl}_{2}$ (b) Calcium oxide : $\mathrm{CaO}$ (c) Copper nitrate: $\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}$ (d) Aluminium chloride : $\mathrm{AlCl}_{3}$ (e) Calcium carbonate: $\mathrm{CaCO}_{3}$ [/solution]...

## What are polyatomic ions

[question] Question. What are polyatomic ions? Give examples? [/question] [solution] Solution:\ A polyatomic ion is a group of atoms carrying a charge (positive or negative). For example, ammonium ion $\left(\mathrm{NH}_{4}^{+}\right)$, hydroxide ion $\left(\mathrm{OH}^{-}\right)$, carbonate ion $\left(\mathrm{CO}_{3}^{2-}\right)$, sulphate ion ( $\left.\mathrm{SO}_{4}^{2-}\right)$. [solution]...

## When 3.0 g of carbon is burnt in 8.00 g oxygen

[question] Question. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combinations will govern your answer? [/question] [solution] Solution: Carbon $+$ Oxygen $\rightarrow$ Carbon dioxide 3 g of carbon reacts with 8 g of oxygen to produce 11 g of carbon dioxide. If 3 g of carbon is burnt in 50 g of oxygen, then 3 g of carbon will react with 8...

## A 0.24 g sample of compound of oxygen

[question] Question. A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight. [/question] [solution] Solution: Mass of boron $=0.096 \mathrm{~g}$ (Given) Mass of oxygen $=0.144 \mathrm{~g}$ (Given) Mass of sample $=0.24 \mathrm{~g}$ (Given) Percentage of boron $=\frac{\text { Mass of boron }}{\text { Mass of sample }} \times 100$ $=\frac{0.096}{0.24} \times 100=40 \%$ P...

## Which has more number of atoms,

[question] Question. Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of $\mathrm{Na}=23 \mathrm{u}, \mathrm{Fe}=56 \mathrm{u})$ ? [/question] [solution] Solution: 100 grams of sodium No. of sodium atoms $=\frac{\text { (Givenmass) }}{(\text { Gramatomic mass })} \times($ Avogadro's no. $)$ $=\frac{(100 g)}{(23 g)} \times\left(6.022 \times 10^{23}\right)=2.618 \times 10^{24}$ atoms 100 grams of iron No. of iron atoms $=\frac{\text { (Givenmass) }}{\tex...

## If one mole of carbon atoms weighs

[question] Question. If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon? [question] [solution] Solution: One mole of carbon atoms weighs $12 \mathrm{~g}$ (Given) i.e., mass of 1 mole of carbon atoms = 12 g One atom of carbon has mass $=12 \mathrm{~g} \times \frac{1}{6.022 \times 10^{23}}=1.99 \times 10^{-23} \mathrm{~g}$ [/solution]...

## Calculate the molecular masses

[question] Question. Calculate the molecular masses of $\mathrm{H}_{2}, \mathrm{O}_{2}, \mathrm{Cl}_{2}, \mathrm{CO}_{2}, \mathrm{CH}_{4}, \mathrm{C}_{2} \mathrm{H}_{6}, \mathrm{C}_{2} \mathrm{H}_{4}, \mathrm{NH}_{3}, \mathrm{CH}_{3} \mathrm{OH}$. [/question] [solution] Solution: Molecular mass of $\mathrm{H}_{2}=2 \times$ Atomic mass $(\mathrm{H})$ $=2 \times 1=2 \mathrm{u}$ Molecular mass of $\mathrm{O}_{2}=2 \times$ Atomic mass $(\mathrm{O})$ $=2 \times 16=32 \mathrm{u}$ Molecular mass of $\m...

## How many atoms a

[question] Question. How many atoms are present in a (i) $\mathrm{H}_{2} \mathrm{~S}$ molecule and (ii) $\mathrm{PO}_{4}^{3}$ ion? [/question] [solution] Solution: (i) In a $\mathrm{H}_{2} \mathrm{~S}$ molecule, three atoms are present; two of hydrogen and one of sulphur. (ii) In a $\mathrm{PO}_{4}^{3-}$ ion, five atoms are present; one of phosphorus and four of oxygen [/solution]...

## What is meant by the term

[question] Question. What is meant by the term chemical formula? [/question] [solution] Solution: The chemical formula of a compound means the symbolic representation of the composition of a compound. For example, from the chemical formula CO2 of carbon dioxide, we come to know that one carbon atom and two oxygen atoms are chemically bonded together to form one molecule of the compound, carbon dioxide. [/solution]...

## Write down the names of compounds

[question] Question. (i) $\mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}$ (ii) $\mathrm{CaCl}_{2}$ (iii) $\mathrm{K}_{2} \mathrm{SO}_{4}$ (iv) $\mathrm{KNO}_{3}$ (v) $\mathrm{CaCO}_{3}$ [/question] [solution] Solution: (i) $\mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}$ : Aluminium sulphate (ii) $\mathrm{CaCl}_{2}$ : Calcium chloride (iii) $\mathrm{K}_{2} \mathrm{SO}_{4}$ : Potassium sulphate (iv) $\mathrm{KNO}_{3}$ : Potassium nitrate (v) $\mathrm{CaCO}_{3}$ : Calcium carbonate [/solution]...

## Write down the formulae of

[question] Question. (i) sodium oxide (ii) aluminium chloride (iii) sodium sulphide (iv) magnesium hydroxide [question] [solution] Solution: (i) Sodium oxide : $\mathrm{Na}_{2} \mathrm{O}$ (ii) Aluminium chloride : $\mathrm{AlCl}_{3}$ (iii) Sodium suphide : $\mathrm{Na}_{2} \mathrm{~S}$ (iv) Magnesium hydroxide : $\mathrm{Mg}(\mathrm{OH})_{2}$ [/solution]...

## Why is it not possible to see an atom

[question] Question. Why is it not possible to see an atom with naked eyes? [/question] [solution] Solution: The size of an atom is too small thats why it is not possible to see it with naked eyes [/solution]...

## Define atomic mass unit?

[question] Question. Define atomic mass unit. [/question] [solution] Solution: Atomic mass unit equal to exactly one-twelth the mass of one atom of carbon-12 is called one atomic mass unit. It is written as ‘u’. [/solution]...

## Which postulate of Dalton’s atomic

[question] Question. Which postulate of Dalton’s atomic theory can explain the law of definite proportions? [question] [solution] Solution: The postulate of Dalton’s atomic theory based on the law of definite proportion is: "The relative number and kind of atoms in a given compound remains constant." [/solution]...

## Which postulate of Dalton’s atomic theory is the result of the law

[question] Question. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass? [/question] [solution] Solution: The postulate of Dalton’s atomic theory is, atoms are indivisible particles, which can neither be created nor destroyed in a chemical reaction. [/solution]...

## Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water.

[question] Question. Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas? [/question] [solution] Solution: It is given that the ratio of hydrogen and oxygen by mass to form water is $1: 8$. Then, the mass of oxygen gas required to react completely with $1 \mathrm{~g}$ of hydrogen gas is $8 \mathrm{~g}$. Therefore, the mass of oxygen gas required to react completely with $3 \mathrm{~g}$ of hy...

## In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid.

[question] Question. In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass. [/question] [solution] Solution: Sodium carbonate reacts with ethanoic acid converted into sodium ethanoate, carbon dioxide,and water $\therefore$ Total mass before the reaction $=$ Total mass after the reaction Hence, the given obse...