Question

Water is a ____ made up of hydrogen and oxygen combined in the ratio ____ by volume or _____ by mass.
1.Compound, $3:2$, $1:1$ respectively.
2.Element, $1:2$, $3:1$ respectively.
3.Compound, $2:1$, $1:8$ respectively.
4.Molecule, $1:1$, $1:16$ respectively.

Answer 1

Hint: Molecule: The substance which is formed by same type of elements i.e. same metals or same non-metal.
Compound: The substance which is formed by two or more different elements are known as compounds.

Complete step by step solution:
Let us first talk about elements, molecules and compounds.
Element: It is defined as the smallest part of any substance. They are made of the same type of atoms. They are basically of two types i.e. metals and non-metals. For example: hydrogen, carbon, oxygen, etc.
Molecule: The substance which is formed by the same type of elements i.e. same metals or same non-metal. For example: hydrogen molecule i.e. $H_2$ (which is made of two hydrogen atom), oxygen molecule i.e. $O_2$ (which is made of two oxygen atoms), etc.
Compound: The substance which is formed by two or more different elements are known as compounds. For example: water i.e. $H_{2}O$ (which is made of two hydrogen elements and one oxygen element), carbon dioxide i.e. $C{O}_2$ (which is made of two oxygen and one carbon atom), etc.
Now we know that the molecular formula (which tells the composition of elements in the compound) of water is $H_{2}O$. By the formula it is clear that in one molecule of water there are two hydrogen atoms and one oxygen atom. So the ratio of hydrogen and oxygen by volume is $2:1$.
Now molar mass is defined as the mass of one atom of the element.
We know that the molar mass of hydrogen is $2$ and molar mass of oxygen is $16$. So the ratio of hydrogen and oxygen by mass is as $2:16=1:8$.
Hence, water is a compound made up of hydrogen and oxygen combined in the ratio $2:1$ by volume or $1:8$ by mass.

So option C is correct.

Note:
Atomic mass: The mass of an atom is known as atomic mass. It is represented by the symbol $\text{A}$.
Atomic number: It is defined as the number of protons in an atom. It is represented by the symbol $Z$.