NCERT Solutions for Class 9 Science Chapter 3 Atoms and Molecules

When electric current is passed through the solution of a salt like sodium chloride (NaCl), the positive ion (Na+) migrates towards cathode (negative electrode). It is therefore, called cation. Remember this:
(i) Positive ion migrating towards cathode on passing electric current is known as cation.
(ii) Negative ion migrating towards anode on passing electric current is known as anion.

If the atomic mass of an element is in fraction, this means that it is existed in the form of isotopes. The atomic mass is the average atomic mass and it is generally fractional.

In all chemical reactions, there is only exchange of reactants taking place when products are formed. Since there is no loss or gain of mass, the chemical reactions are according to law of conservation of mass.

The law of constant composition does not hold when different isotopes of an element take part in the formation of a particular compound. For example,
In carbon dioxide (CO2)
By using C-12 isotopes, the ratio of C : O is 12 : 32.
By using C-14 isotopes, the ratio of C : O is 14 : 32.
This means that the two ratios are not the same.

Molecules can exist independently. However, the atoms of noble gases (He, Ne, Ar, Kr, Xe) can also exist independently.

Carbon + Oxygen → Carbon dioxide 3g of carbon reacts with 8 g of oxygen to produce 11g of carbon dioxide. If 3g of carbon is burnt in 50g of oxygen, then 3g of carbon will react with 8 g of oxygen. The remaining 42 g of oxygen will be left un-reactive. In this case also, only 11g of carbon dioxide will be formed. The above answer is governed by the law of constant proportions.

The mass of 4 moles of aluminium atoms is (4 × 27)g = 108g

32 g of oxygen gas = 1 mole Then, 12g of oxygen gas = 12/32 mole = 0.375 mole

Mass of one mole of oxygen atoms = 16g Then, mass of 0.2 mole of oxygen atoms = 0.2 × 16g = 3.2g