Question

State the law of conservation of mass. Give one example to illustrate this law.

Answer 1

Part 1: Law of conservation of mass:

• The law of conservation of mass states that mass cannot be created nor be destroyed in a chemical reaction.
• Thus, the mass remains constant in any chemical reaction.

Part 2: Example:

• Consider a chemical reaction, where carbon and oxygen react to produce Carbon dioxide.
• $\mathrm{C}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{CO}}_2\left(\mathrm{g}\right)$
• The atomic mass of carbon i.e. $\mathrm{C}$ is $12\mathrm{u}$ and the molar mass of oxygen i.e. ${\mathrm{O}}_2$ is $32\mathrm{u}$. Thus, the total mass of reactants is $=12\mathrm{u}+32\mathrm{u}=44\mathrm{u}$.
• The molar mass of Carbon dioxide i.e. ${\mathrm{CO}}_2$ is $12+2\times 16=12+32=44\mathrm{u}$.
• Thus, the sum of masses of carbon and oxygen is equal to the mass of Carbon dioxide.

Hence, the mass is same before and after the reaction which shows that mass is conserved.