Answer: Option B. -> 12 g
:
B

The chemical formula of methane is $C{H}_4$. The carbon to hydrogen mass ratio in it is $3:1$.
3 g of carbon requires one gram of hydrogen.
So, 36 g of carbon requires
=$\frac{36}{3}\times 1=12\text{g}$ of hydrogen

Answer: Option B. -> False
:
B

Law of conservation of mass states that mass can neither be created nor be destroyed in a chemical reaction. In a chemical reaction, physical and chemical properties of matter can be altered but the total mass of the matter remains the same.
Since an atom is the basic unit of mass, atoms can neither be created nor be destroyed.

Answer: Option B. -> valency
:
B
The number of atoms combining to form the molecule of a compound is determined by the valency of its constituent atoms or ions.
Valency is determined by the number of electrons an atom can accept, donate or share.

Answer: Option B. -> 11.11%
:
B

Let us consider 1 molecule of $H_{2}O$  

Mass of hydrogen in 1 molecule of $H_{2}O$ = $2g$

Mass of oxygen in 1 molecule of $H_{2}O$ = $16g$

Therefore, total mass of 1 molecule of $H_{2}O$ = $2+16$ = $18g$

Now percentage of hydrogen is given by:

 = $\frac{\text{Mass of hydrogen in 1 molecule of}{H}_{2}O}{\text{Total mass of 1 molecule of}{H}_{2}O}$

 = $\frac{2}{18}\times 100=11.11$%

Answer: Option A. -> $Ca{CO}_3$
:
A
Calcium carbonate consist of calcium ion and carbonate ion, i.e., $C{a}^{2+}$and ${{\mathrm{CO}}_3}^{2-}$, which have a charge of +2 and -2 respectively.
$$\begin{array}{ccc}\text{Symbol}& \text{Ca}& {\text{CO}}_3\\ \text{Charge}& \text{+2}& \text{-2}\end{array}$$

On cross-multiplication and balancing the charge, we get the formula as $Ca{CO}_3$.

Answer: Option A. -> Hydrogen chloride
:
A
Compounds are made up of two or more kinds of atoms, chemically combined, in a definite ratio. Here, hydrogen chloride ($\text{HCl}$) is formed by chemical combination of hydrogen and chlorine in the ratio 1:1, hence it is a compound.
Elements are made up of only one kind of atoms. Since, oxygen $\left({\text{O}}_2\right)$, nitrogen $\left({\text{N}}_2\right)$ and chlorine $\left({\text{Cl}}_2\right)$ are made of only one type of atoms i.e., oxygen, nitrogen and chlorine respectively, hence they are classified as elements.

Answer: Option A. -> Ammonium
:
A and D
$\bullet$ A monoatomic ion is an ion consisting exactly one atom. While a polyatomic ion is an ion consisting more than one atom.
$$\begin{array}{ccc}\text{Ion}& \text{Formula}& \text{No. of Atoms}\\ \text{Ammonium}& N{H}_4^{+}& \text{1 N, 4H}\\ \text{Hydride}& H^{-}& \text{1 H}\\ \text{Oxide}& O^{2-}& \text{2 O}\\ \text{Hydroxide}& O{H}^{-}& \text{1 H, 1 O}\end{array}$$
$\bullet$ Thus, $O{H}^{-}$ and $N{H}_4^{+}$ are polyatomic ions and $O^{2-}$ and  $H^{-}$ are monoatomic ions.

Answer: Option C. -> triatomic
:
C

• Atomicity is defined as the number of atoms present in a single molecule.


• One molecule of water $\left(H_{2}O\right)$ contains two atoms of hydrogen and one atom of oxygen.


• This means a water molecule contains three atoms in total.


• Hence, the atomicity of water is three i.e. water is triatomic.

Answer: Option A. -> 1440 g
:
A

Number of moles of water $=\frac{\text{Total number of molecules}}{\text{Avogadro constant}}=\frac{4.82\times {10}^{25}}{6.022\times {10}^{23}}=80$
Therefore, mass of water = $\text{Number of moles}\times \text{Molecular Mass}$
$=80\times 18=1440.71$ g

Answer: Option B. -> $\left(\frac{1}{12}\right)$${}^{\text{th}}$ of the mass of one atom of carbon 12.
:
B

Carbon -12 isotope is used as the standard reference for measuring atomic masses.
1 atomic mass unit is a mass unit equal to exactly $\left(\frac{1}{12}\right)$${}^{\text{th}}$ the mass of one atom of carbon 12.
It is expressed as u or amu.