Mix Example - ATOMS AND MOLECULES Questions in English

(N/A) The Helium atom has only one shell ($K$-shell),which has a maximum capacity of accommodating $2$ electrons. Since this shell is completely filled,the atom is stable and does not have a tendency to gain,lose,or share electrons with other atoms. Therefore,the valency of Helium is $0$.

(C) The electronic configuration of the elements is as follows:
Element $A$ (atomic number $9$): $2, 7$. It needs $1$ electron to complete its octet,so it forms an anion.
Element $B$ (atomic number $10$): $2, 8$. It has a complete octet and is a noble gas,so it is inert.
Element $C$ (atomic number $13$): $2, 8, 3$. It has $3$ valence electrons. Metals tend to lose their valence electrons to achieve a stable octet configuration,thereby forming a cation ($+$ve ion).
Therefore,element $C$ will form a cation by losing $3$ electrons: $C (2, 8, 3) \rightarrow C^{3+} (2, 8) + 3e^-$.

(N/A) $(i)$ Molar mass: The mass of one mole of a substance in grams is called its molar mass.
$(ii)$ Valency: The combining capacity of an atom of an element is known as its valency. It represents the number of electrons an atom needs to lose,gain,or share to achieve a stable electronic configuration.

(N/A) The law of constant proportions states that in a chemical substance,the elements are always present in definite proportions by mass. This means that a pure chemical compound is always made up of the same elements combined together in the same fixed ratio of mass,regardless of the source from which it is obtained or the method of its preparation.

(N/A) mole is a fundamental unit in chemistry used to quantify the amount of a substance.
One mole of any substance contains exactly $6.022 \times 10^{23}$ elementary entities,such as atoms,molecules,ions,or electrons.
This constant value is known as Avogadro's number $(N_A)$.

(B) $1$. In the oxide $X_2O_5$,the valency of oxygen is $2$.
$2$. Let the valency of element $X$ be $v$.
$3$. According to the criss-cross method for $X_2O_5$,the valency of $X$ is $5$ (since $2 \times v = 5 \times 2$,so $v = 5$).
$4$. Chlorine $(Cl)$ has a valency of $1$.
$5$. To write the formula of the chloride of $X$,we place the valency of $X$ $(5)$ under $Cl$ and the valency of $Cl$ $(1)$ under $X$.
$6$. Thus,the chemical formula is $X_1Cl_5$,which is written as $XCl_5$.

(N/A) Atomicity is defined as the total number of atoms present in one molecule of a substance.
$(i)$ In $H_{2}SO_{4}$ (Sulfuric acid):
Number of Hydrogen atoms = $2$
Number of Sulfur atoms = $1$
Number of Oxygen atoms = $4$
Total atoms = $2 + 1 + 4 = 7$
Therefore,the atomicity is $7$.
$(ii)$ In $CCl_{4}$ (Carbon tetrachloride):
Number of Carbon atoms = $1$
Number of Chlorine atoms = $4$
Total atoms = $1 + 4 = 5$
Therefore,the atomicity is $5$.

(N/A) $2H$ represents $2$ individual atoms of hydrogen,which exist independently. In contrast,$H_2$ represents a single molecule of hydrogen,which consists of $2$ hydrogen atoms chemically bonded together.

(C) polyatomic ion is an ion composed of two or more atoms covalently bonded or of a metal complex that can be considered to be acting as a single unit.
$1$. In $NaOH$,the hydroxide ion $(OH^-)$ is a polyatomic ion.
$2$. In $NaCl$,both $Na^+$ and $Cl^-$ are monatomic ions.
$3$. In $Na_{2}O$,both $Na^+$ and $O^{2-}$ are monatomic ions.
$4$. In $NaNO_{3}$,the nitrate ion $(NO_{3}^-)$ is a polyatomic ion.
Therefore,$NaOH$ and $NaNO_{3}$ contain polyatomic ions.

(B) The laws of chemical combination were established by Antoine $L.$ Lavoisier and Joseph $L.$ Proust.
Antoine $L.$ Lavoisier is known as the 'Father of Modern Chemistry' and proposed the Law of Conservation of Mass.
Joseph $L.$ Proust proposed the Law of Definite Proportions.

(N/A) To derive the molecular formula for Copper $(II)$ Bromide,follow these steps:
$1$. Identify the symbols of the elements involved: Copper is represented by $Cu$ and Bromide is represented by $Br$.
$2$. Determine the valency or charge of each ion: Copper $(II)$ indicates a charge of $+2$ $(Cu^{2+})$,and Bromide has a charge of $-1$ $(Br^-)$.
$3$. Use the criss-cross method to balance the charges: The charge of Copper $(2)$ becomes the subscript for Bromide,and the charge of Bromide $(1)$ becomes the subscript for Copper.
$4$. Write the formula: By crossing the charges,we get $Cu_1Br_2$,which is written as $CuBr_2$.

(N/A) $O_3$ (Ozone) is a triatomic element because it consists of three oxygen atoms.
$(b)$ $Ar$ (Argon) is a monoatomic molecule as it is a noble gas and exists as a single atom.
$(c)$ $S_8$ (Sulphur) is a polyatomic element because its molecule consists of eight sulphur atoms.
$(d)$ $SO_4^{2-}$ (Sulphate) or $NH_4^+$ (Ammonium) are examples of polyatomic ions,which are ions consisting of more than one atom.

(N/A)

$(i)$ Molar mass	Avogadro's constant
It is the mass of one mole of a substance expressed in grams $(g/mol)$.	It is the number of particles (atoms,molecules,or ions) present in one mole of a substance,equal to $6.022 \times 10^{23} \text{ mol}^{-1}$.
$(ii)$ Molecular mass	Formula unit mass
It is the sum of the atomic masses of all atoms present in a molecule of a covalent compound.	It is the sum of the atomic masses of all atoms in a formula unit of an ionic compound.

(N/A) To derive the molecular formula for Ammonium carbonate,we follow these steps:
$1$. Identify the ions and their respective charges:
- Ammonium ion: $NH_4^+$ (charge = $+1$)
- Carbonate ion: $CO_3^{2-}$ (charge = $-2$)
$2$. Write the symbols and their charges side-by-side:
- Symbol: $NH_4$ and $CO_3$
- Charge: $+1$ and $-2$
$3$. Use the criss-cross method to balance the charges:
- The charge of the Ammonium ion $(+1)$ becomes the subscript for the Carbonate ion.
- The charge of the Carbonate ion $(-2)$ becomes the subscript for the Ammonium ion.
$4$. Combine them to form the neutral compound:
- Since we need two Ammonium ions to balance one Carbonate ion,the formula is $(NH_4)_2CO_3$.

(N/A) molecular formula represents the actual number of atoms of each element present in one molecule of a compound.
For example,the molecular formula of aluminium sulphate is $Al_{2}(SO_{4})_{3}$. It provides the following information:
$(i)$ The compound is composed of three different types of atoms: aluminium $(Al)$,sulphur $(S)$,and oxygen $(O)$.
$(ii)$ There are a total of $17$ atoms in one molecule ($2$ atoms of $Al$,$3$ atoms of $S$,and $12$ atoms of $O$).
$(iii)$ The valency of aluminium $(Al^{3+})$ is $3$,while the valency of the sulphate ion $(SO_{4}^{2-})$ is $2$.

(N/A) Avogadro's constant $(N_A)$ is defined as the number of constituent particles (atoms,molecules,or ions) present in one mole of any substance. Its value is approximately $6.022 \times 10^{23} \text{ particles/mol}$.
Relationship to a mole: $A$ mole is a unit that represents a specific quantity of substance. By definition,one mole of any substance contains exactly $6.022 \times 10^{23}$ elementary entities. Therefore,the number of particles in one mole is equal to Avogadro's constant.

(N/A)

Cation	Anion
$1.$ It is positively charged,e.g.,$Na^+$	$1.$ It is negatively charged,e.g.,$Cl^-$
$2.$ It is formed from metal atoms.	$2.$ It is formed from non-metal atoms.
$3.$ On passing electric current,it moves towards the cathode.	$3.$ On passing electric current,it moves towards the anode.
$4.$ It is smaller in size than its parent atom.	$4.$ It is larger in size than its parent atom.

(N/A) To write the chemical formula,we use the criss-cross method where the valency of one ion becomes the subscript of the other.
$(i)$ For $Fe^{3+}$ and $SO_{4}^{2-}$:
- The valency of $Fe$ is $3$ and $SO_{4}$ is $2$.
- By crossing the valencies,we get $Fe_{2}(SO_{4})_{3}$.
- The name of the compound is Iron$(III)$ sulphate.
$(ii)$ For $NH_{4}^{+}$ and $CO_{3}^{2-}$:
- The valency of $NH_{4}$ is $1$ and $CO_{3}$ is $2$.
- By crossing the valencies,we get $(NH_{4})_{2}CO_{3}$.
- The name of the compound is Ammonium carbonate.

(N/A) $(i)$ The body that approves the nomenclature of elements and compounds is the International Union of Pure and Applied Chemistry $(IUPAC)$.
$(ii)$ The symbol of sodium is $Na$ because it is derived from its Latin name,$Natrium$. The symbol $S$ is reserved for sulfur.
$(iii)$ $(a)$ An element that forms a diatomic molecule is Hydrogen $(H_2)$.
$(b)$ An element that forms a tetraatomic molecule is Phosphorus $(P_4)$.

(N/A) Atomicity is defined as the total number of atoms present in one molecule of an element or a compound.
$(b)$ Monoatomic molecule: $He$ (Helium) or $Ne$ (Neon) are examples of monoatomic molecules because they consist of a single atom.
Polyatomic molecule: $S_8$ (Sulphur) is an example of a polyatomic molecule because it consists of eight atoms.

(N/A) The law of constant proportions states that in a chemical substance, the elements are always present in definite proportions by mass.
Example: In water $(H_2O)$, the ratio of the mass of hydrogen to the mass of oxygen is always $1:8$, regardless of the source of water.
$(b)$ The postulate of Dalton's atomic theory that is in agreement with this law is: "The relative number and kinds of atoms are constant in a given compound."

(B) Given mass $(m) = 5.75 \, g$.
Atomic mass of sodium $= 23 \, u$.
Molar mass $(M) = 23 \, g/mol$.
The formula for the number of moles $(n)$ is given by: $n = \frac{m}{M}$.
Substituting the values: $n = \frac{5.75}{23} = 0.25 \, \text{moles}$.
Therefore, the number of moles in $5.75 \, g$ of sodium is $0.25 \, \text{moles}$.

(A) $1$. Calculate the number of atoms in $11.5 \, g$ of $Na$:
- Molar mass of $Na = 23 \, g/mol$.
- Number of moles of $Na = \frac{11.5 \, g}{23 \, g/mol} = 0.5 \, mol$.
- Number of atoms in $Na = 0.5 \times 6.022 \times 10^{23} = 3.011 \times 10^{23}$ atoms.
$2$. Calculate the number of atoms in $15 \, g$ of $Ca$:
- Molar mass of $Ca = 40 \, g/mol$.
- Number of moles of $Ca = \frac{15 \, g}{40 \, g/mol} = 0.375 \, mol$.
- Number of atoms in $Ca = 0.375 \times 6.022 \times 10^{23} = 2.258 \times 10^{23}$ atoms.
$3$. Conclusion:
- Since $3.011 \times 10^{23} > 2.258 \times 10^{23}$,$11.5 \, g$ of sodium contains a greater number of atoms.

(B) The molar mass of an oxygen atom $(O)$ is $16 \text{ g/mol}$.
The formula to calculate mass is: $\text{Mass} = \text{number of moles} \times \text{molar mass}$.
Given that the number of moles is $1 \text{ mol}$,the calculation is:
$\text{Mass} = 1 \text{ mol} \times 16 \text{ g/mol} = 16 \text{ g}$.
Therefore,the mass of one mole of oxygen atoms is $16 \text{ g}$.

(N/A) The chemical formula of sodium sulphate is $Na_{2}SO_{4}$.
$(b)$ $A$ diatomic molecule consists of two atoms of the same element. Examples include Oxygen $(O_{2})$,Hydrogen $(H_{2})$,or Nitrogen $(N_{2})$. Any one of these can be cited.
$A$ tetraatomic molecule consists of four atoms of the same element. Phosphorus $(P_{4})$ is a common example of a tetraatomic molecule.

(B) The postulate of Dalton's atomic theory that led to the law of conservation of mass is: $Atoms$ can neither be created nor destroyed in a chemical reaction.
To calculate the molecular mass of water $(H_2O)$:
Atomic mass of $H = 1 \text{ u}$
Atomic mass of $O = 16 \text{ u}$
Molecular mass of $H_2O = (2 \times \text{atomic mass of } H) + (1 \times \text{atomic mass of } O)$
$= (2 \times 1) + (1 \times 16) = 2 + 16 = 18 \text{ u}$.

(N/A) The formula unit mass of a substance is the sum of the atomic masses of all atoms in a formula unit of a compound. It is calculated in the same manner as molecular mass,with the distinction that the term 'formula unit mass' is used for substances whose constituent particles are ions,such as $NaCl$.
$(a)$ $Na_{2}CO_{3} \cdot 10H_{2}O = (2 \times \text{Mass of } Na) + (1 \times \text{Mass of } C) + (3 \times \text{Mass of } O) + 10 \times (2 \times \text{Mass of } H + 1 \times \text{Mass of } O)$
$= (2 \times 23) + (1 \times 12) + (3 \times 16) + 10 \times (2 \times 1 + 16)$
$= 46 + 12 + 48 + 10 \times (18)$
$= 106 + 180 = 286 \text{ u}$
$(b)$ $CuSO_{4} = (1 \times \text{Mass of } Cu) + (1 \times \text{Mass of } S) + (4 \times \text{Mass of } O)$
$= 63.5 + 32 + (4 \times 16)$
$= 63.5 + 32 + 64 = 159.5 \text{ u}$

(A) Methane: Formula is $CH_{4}$. The ratio by mass of $C:H$ is $12:(4 \times 1) = 12:4 = 3:1$.
$(b)$ Carbon dioxide: Formula is $CO_{2}$. The ratio by mass of $C:O$ is $12:(2 \times 16) = 12:32 = 3:8$.
$(c)$ Hydrogen sulphide: Formula is $H_{2}S$. The ratio by mass of $H:S$ is $(2 \times 1):32 = 2:32 = 1:16$.

(N/A) To write the chemical formula,we use the criss-cross method where the valency of one ion becomes the subscript of the other.
$(a)$ For $Mg^{2+}$ and $S^{2-}$:
- The charges are $+2$ and $-2$. Since the magnitudes are equal,they cancel out to form a $1:1$ ratio.
- Chemical Formula: $MgS$
- Name: Magnesium sulphide
$(b)$ For $Cu^{2+}$ and $OH^-$:
- The valency of $Cu$ is $2$ and $OH$ is $1$. By criss-crossing,we get $Cu(OH)_2$.
- Chemical Formula: $Cu(OH)_2$
- Name: Copper$(II)$ hydroxide
$(c)$ For $Al^{3+}$ and $Br^-$:
- The valency of $Al$ is $3$ and $Br$ is $1$. By criss-crossing,we get $AlBr_3$.
- Chemical Formula: $AlBr_3$
- Name: Aluminium bromide

(B) To find the formula of ammonia,we calculate the molar ratio of the constituent elements:

Element	Mass Ratio / Atomic Mass = Molar Ratio
$N$	$14 / 14 = 1$
$H$	$3 / 1 = 3$

The ratio of the number of atoms of $N$ to $H$ is $1:3$.
Therefore,the formula of ammonia is $NH_3$.

(N/A) An atom is considered electrically neutral because it contains an equal number of positively charged protons and negatively charged electrons,which cancel out each other's charge.
$(b)$ The valency of Helium is $0$ because its outermost shell is completely filled (it has a stable duplet configuration).
$(c)$ The formula for atomic mass is: $\text{Atomic mass} = \text{Number of protons} + \text{Number of neutrons}$.
Given: $\text{Atomic mass} = 4$,$\text{Number of protons} = 2$.
Substituting the values: $4 = 2 + \text{Number of neutrons}$.
Therefore,$\text{Number of neutrons} = 4 - 2 = 2$.

(N/A) The number of atoms $= \frac{\text{Given mass}}{\text{Molar mass}} \times \text{Avogadro constant}$
$\Rightarrow N = \frac{m}{M} \times N_A = \frac{48}{24} \times 6.022 \times 10^{23} = 2 \times 6.022 \times 10^{23} = 12.044 \times 10^{23} \text{ atoms}$.
$(b)$ The number of molecules $= \frac{\text{Given mass}}{\text{Molar mass}} \times \text{Avogadro constant}$
$\Rightarrow N = \frac{8}{32} \times 6.022 \times 10^{23} = 0.25 \times 6.022 \times 10^{23} = 1.5055 \times 10^{23} \text{ molecules}$.
$(c)$ The number of particles (atoms) $= \text{Number of moles} \times \text{Avogadro constant}$
$\Rightarrow N = 0.1 \times 6.022 \times 10^{23} = 6.022 \times 10^{22} \text{ atoms}$.

(N/A) The molar mass of $CO_2$ is $44 \, g/mol$ and the molar mass of $H_2O$ is $18 \, g/mol$.
Mass of $5$ moles of $CO_2 = 5 \times 44 = 220 \, g$.
Mass of $5$ moles of $H_2O = 5 \times 18 = 90 \, g$.
Since $220 \, g > 90 \, g$, it is verified that $5$ moles of $CO_2$ has a higher mass.
$(b)$ Molar mass of $Ca = 40 \, g/mol$ and $Mg = 24 \, g/mol$.
Moles of $120 \, g$ of $Ca = 120 / 40 = 3 \, moles$.
Moles of $120 \, g$ of $Mg = 120 / 24 = 5 \, moles$.
Therefore, the mole ratio of $Ca$ to $Mg = 3:5$.

(N/A) $(1)$ The valencies or charges on the ions must be balanced.
$(2)$ When a compound consists of a metal and a non-metal,the name or symbol of the metal is written first.
$(3)$ In compounds formed with polyatomic ions,the ion is enclosed in a bracket before writing the number to indicate the ratio. In case the number of the polyatomic ion is one,the bracket is not required.

$(i)$ Molar mass of $NH_3 = 14 + (3 \times 1) = 17 \, g/mol$.
$(ii)$ Number of moles $= \frac{\text{Given mass}}{\text{Molar mass}} = \frac{1.7 \, g}{17 \, g/mol} = 0.1 \, mol$.
$(iii)$ Number of molecules $= \text{Number of moles} \times \text{Avogadro's number} = 0.1 \times 6.022 \times 10^{23} = 6.022 \times 10^{22}$ molecules.

(A) $1$. For Calcium $(Ca)$: Atomic mass $= 40 \, g/mol$. Moles $= \frac{120 \, g}{40 \, g/mol} = 3 \, mol$. Number of atoms $= 3 \times 6.022 \times 10^{23} = 18.066 \times 10^{23}$ atoms.
$2$. For Iron $(Fe)$: Atomic mass $= 56 \, g/mol$. Moles $= \frac{120 \, g}{56 \, g/mol} \approx 2.143 \, mol$. Number of atoms $= 2.143 \times 6.022 \times 10^{23} \approx 12.905 \times 10^{23}$ atoms (using precise division $120/56 \approx 2.1428$).
$3$. Comparing the two,Calcium has more atoms.
$4$. Difference $= (18.066 - 12.905) \times 10^{23} = 5.161 \times 10^{23}$ atoms.

(N/A) Mass $=$ Molar mass $\times$ Number of moles
$m = M \times n$
$m = 16 \ g/mol \times 0.5 \ mol = 8 \ g$
$(b)$ Number of moles $(n) = \frac{\text{Given mass } (m)}{\text{Molar mass } (M)}$
$n = \frac{90 \ g}{180 \ g/mol} = 0.5 \ mol$
Since $1 \ mol$ contains $6.022 \times 10^{23}$ molecules,
Therefore,$0.5 \ mol$ will contain $0.5 \times 6.022 \times 10^{23} = 3.011 \times 10^{23}$ molecules.
$(c)$ Number of moles $(n) = \frac{\text{Given mass } (m)}{\text{Molar mass } (M)}$
Given mass of water $= 2 \ g$,Molar mass of water $(H_2O) = (2 \times 1) + 16 = 18 \ g/mol$
$n = \frac{2}{18} = \frac{1}{9} \approx 0.11 \ mol$

(N/A) The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
Activity:
$1$. Prepare separate solutions of $X$ and $Y$ in water. Take a little amount of $X$ solution in a conical flask and some $Y$ solution in a small ignition tube.
$2$. Hang the ignition tube in the flask carefully by a thread. Put a cork on the flask.
$3$. Weigh the flask with its contents on a balance.
$4$. Now,tilt and swirl the flask so that the solutions $X$ and $Y$ get mixed.
$5$. Weigh the flask again.
Observation:
It is observed that the mass of the flask and its contents remains the same before and after the reaction,demonstrating that mass is conserved.
Example:
Reaction between Barium Chloride $(BaCl_2)$ and Sodium Sulphate $(Na_2SO_4)$:
$BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)$

(D) To find the ratio by number of atoms,we divide the mass ratio by the atomic mass of each element:

Element	Ratio by mass / Atomic mass
$H$	$1 / 1 = 1$
$O$	$8 / 16 = 0.5$

Dividing by the smallest value $(0.5)$:
$H = 1 / 0.5 = 2$
$O = 0.5 / 0.5 = 1$
Thus,the ratio of the number of atoms of $H$ to $O$ is $2:1$.
$(b)$ $(i)$ Ammonium sulphate: The valency of $NH_{4}^{+}$ is $+1$ and $SO_{4}^{2-}$ is $-2$. By criss-cross method,the formula is $(NH_{4})_{2}SO_{4}$.
$(ii)$ Magnesium chloride: The valency of $Mg^{2+}$ is $+2$ and $Cl^{-}$ is $-1$. By criss-cross method,the formula is $MgCl_{2}$.

(A) Given that the formula of the metal chloride is $MCl$,the valency of metal $M$ is $+1$ because the valency of chlorine $(Cl)$ is $-1$.
$1$. Oxide: The valency of oxygen $(O)$ is $-2$. By criss-cross method,the formula is $M_2O$.
$2$. Bicarbonate: The valency of the bicarbonate ion $(HCO_3)$ is $-1$. Combining $M^{+1}$ and $HCO_3^{-1}$,the formula is $MHCO_3$.
$3$. Sulphide: The valency of sulphur $(S)$ is $-2$. By criss-cross method,the formula is $M_2S$.

(N/A) $(i)$ Mole: One mole of any species (atoms,molecules,ions,or particles) is the quantity in number equal to Avogadro's constant $(6.022 \times 10^{23})$ having a mass equal to its atomic or molecular mass in grams.
$(ii)$ Gram atomic mass: The mass of one mole of atoms of an element expressed in grams.
$(iii)$ Gram molecular mass: The mass of one mole of molecules of a substance expressed in grams,which is the sum of the atomic masses of all atoms in the molecule.
$(b)$ $(i)$ The molar mass of $CO_2$ is $(12 + 2 \times 16) = 44 \text{ g/mol}$. Therefore,the mass of $2$ moles of $CO_2$ is $2 \times 44 = 88 \text{ g}$.
$(ii)$ Since $6.022 \times 10^{23}$ molecules represent $1$ mole of $CO_2$,the mass is equal to its molar mass,which is $44 \text{ g}$.

(N/A) Molar mass of $H_{2}O_{2} = (2 \times 1) + (2 \times 16) = 34 \, g/mol$. Percentage of oxygen $= (32 / 34) \times 100 = 94.12 \%$.
$(b)$ Molar mass of $N_{2} = 2 \times 14 = 28 \, g/mol$. Number of moles $= 56 \, g / 28 \, g/mol = 2 \, moles$.
$(c)$ Mass of $Ca = \text{moles} \times \text{atomic mass} = 0.50 \times 40 = 20 \, g$.
$(d)(i)$ Number of molecules $= \text{moles} \times N_{A} = 5 \times 6.022 \times 10^{23} = 30.110 \times 10^{23}$ particles.
$(d)(ii)$ Number of atoms $= (\text{given mass} / \text{atomic mass}) \times N_{A} = (92 / 23) \times 6.022 \times 10^{23} = 4 \times 6.022 \times 10^{23} = 24.088 \times 10^{23}$ particles.

(N/A) $(i)$ $NaBr$: Sodium bromide
$(ii)$ $Al_{2}O_{3}$: Aluminium oxide
$(iii)$ $Zn(NO_{3})_{2}$: Zinc nitrate
$(iv)$ $HCl$: Hydrogen chloride
$(v)$ $NaCl$: Sodium chloride
$(vi)$ $CaCO_{3}$: Calcium carbonate

(N/A) $(i)$ $2O$ represents two separate atoms of oxygen.
$(ii)$ $O_{2}$ represents one molecule of diatomic oxygen.
$(iii)$ $O_{3}$ represents one molecule of triatomic oxygen (ozone).
$(iv)$ $H_{2}O$ represents one molecule of water, consisting of two atoms of hydrogen and one atom of oxygen.
$(b)$ $(i)$ Potassium carbonate: $K_{2}CO_{3}$
$(ii)$ Calcium chloride: $CaCl_{2}$
$(c)$ Formula unit mass of $Al_{2}(SO_{4})_{3}$:
$= (2 \times \text{atomic mass of } Al) + (3 \times \text{atomic mass of } S) + (12 \times \text{atomic mass of } O)$
$= (2 \times 27) + (3 \times 32) + (12 \times 16)$
$= 54 + 96 + 192 = 342 \, u$.

(N/A) $(a) (i)$ Examples include Carbon $(C)$ and Nitrogen $(N)$.
$(ii)$ Examples include Sodium ($Na$,from Natrium) and Iron ($Fe$,from Ferrum).
$(b)$ Avogadro's constant: The value $6.022 \times 10^{23}$ is defined as Avogadro's constant $(N_A)$. It represents the number of particles (atoms,molecules,or ions) present in one mole of any substance.
Molar mass: The mass of $1$ mole of a substance in grams is called its molar mass.
Relationship: One mole of any species (atoms,molecules,or ions) contains $6.022 \times 10^{23}$ number of particles,and its mass is equal to its relative atomic or molecular mass expressed in grams.

$1$ mole of particles

Number of particles = $6.022 \times 10^{23}$

Mass = Molar mass in grams

(N/A) group of atoms carrying a charge is known as a polyatomic ion.
Example: $NH_{4}^{+}, PO_{4}^{3-}, SO_{4}^{2-}$.
$(b)$ Formula unit mass of $CaCO_{3} = (1 \times \text{Atomic mass of } Ca) + (1 \times \text{Atomic mass of } C) + (3 \times \text{Atomic mass of } O)$
$= 40 \ u + 12 \ u + (3 \times 16 \ u) = 40 + 12 + 48 = 100 \ u$.
$(c)$ $(i)$ Molecular mass of $HNO_{3} = (1 \times \text{Atomic mass of } H) + (1 \times \text{Atomic mass of } N) + (3 \times \text{Atomic mass of } O)$
$= 1 \ u + 14 \ u + (3 \times 16 \ u) = 1 + 14 + 48 = 63 \ u$.
$(ii)$ Molecular mass of $CH_{3}COOH = (2 \times \text{Atomic mass of } C) + (4 \times \text{Atomic mass of } H) + (2 \times \text{Atomic mass of } O)$
$= (2 \times 12 \ u) + (4 \times 1 \ u) + (2 \times 16 \ u) = 24 + 4 + 32 = 60 \ u$.

(A) The number $6.022 \times 10^{23}$ is known as Avogadro's number or Avogadro's constant.
It represents the number of particles (atoms,molecules,or ions) present in one mole of a substance.

(N/A) $J$öns Jacob Berzelius proposed a system for representing elements using symbols. He suggested that the symbols of elements should be derived from one or two letters of their names,typically taken from their English or Latin names. For example,$H$ for Hydrogen and $Ca$ for Calcium.

(N/A) $(i)$ The international organisation that approves the names of elements is the International Union of Pure and Applied Chemistry $(IUPAC)$.
$(ii)$ The symbol of tungsten is $W$ (derived from its German name,Wolfram).

(B) The chemical formula of the sulfate ion is $SO_{4}^{2-}$.
This ion consists of one sulfur $(S)$ atom and four oxygen $(O)$ atoms.
Total number of atoms = $1$ (sulfur) + $4$ (oxygen) = $5$ atoms.
Therefore,the total number of atoms present in one $SO_{4}^{2-}$ ion is $5$.