# Atoms and Molecules

## Science

### NCERT

1   In a reaction, $5.3g$ of sodium carbonate reacted with $6g$ of ethanoic acid. The products were $2.2g$ of carbon dioxide, $0.9g$ water and $8.2g$ of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass. Sodium carbonate + ethanoic acid $\to$ sodium ethanoate + carbon dioxide + water

In the given reaction, sodium carbonate reacts with ethanoic acid to produce sodium ethanoate, carbon dioxide, and water.$\\$ Sodium carbonate + Ethanoic acid $\to$ Sodium ethanoate + Carbon diaoxide + Water$\\$ $Na_2 CO_3 + CH_3 COOH \to CH_3 COONa + CO_2 + H_2 O$$\\ Mass of sodium carbonate = 5.3 g (Given)\\ Mass of ethanoic acid = 6 g (Given)\\ Mass of sodium ethanoate = 8.2 g (Given)\\ Mass of carbon dioxide = 2.2 g (Given)\\ Mass of water = 0.9 g (Given)\\ Now, total mass before the reaction = (5.3 + 6) g = 11.3 g$$\\$ And, total mass after the reaction $= (8.2 + 2.2 + 0.9) g = 11.3 g$$\\ \therefore Total mass before the reaction = Total mass after the reaction\\ Hence, the given observations are in agreement with the law of conservation of mass. 2 Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas? ##### Solution : It is given that the ratio of hydrogen and oxygen by mass to form water is 1:8.$$\\$ Then, the mass of oxygen gas required to react completely with $1 g$ of hydrogen gas is $8 g.$$\\$ Therefore, the mass of oxygen gas required to react completely with $3 g$ of hydrogen gas is$\\$ $8 * 3 g = 24 g.$

3   Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?

##### Solution :

The postulate of Dalton’s atomic theory which is a result of the law of conservation of mass is:$\\$ Atoms are indivisible particles, which can neither be created nor destroyed in a chemical reaction.

4   Which postulate of Dalton’s atomic theory can explain the law of definite proportions?

##### Solution :

The postulate of Dalton’s atomic theory which can explain the law of definite proportion is: The relative number and kind of atoms in a given compound remains constant.

5   Define atomic mass unit.

##### Solution :

Mass unit equal to exactly one-twelfth $(\dfrac{1}{12^{th}})$ the mass of one atom of carbon$-12$ is called one atomic mass unit. It is written as $‘u’.$