9th Class (CBSE) Science: Atoms and Molecules

Question: Calculate the formula unit mass of NaCl and CaCl₂.
(Na = 23, Cl = 35.5, Ca = 40)

Answer: Formula unit mass of NaCl = 23 + 35.5
= 58.5 u

Formula unit mass of CaCl₂ = 40 + (2 x 35.5)
= 40 + 71 = 111 u

Question: Write down the chemical formula for the following compounds:
(a) Aluminium carbonate
(b) Calcium sulphide
(c) Zinc carbonate
(d) Copper phosphate
(e) Magnesium bicarbonate
(f) Aluminium hydroxide.

Answer: The chemical formula are:

Question: Give the atomicity of the following compounds:
(a) Ca(OH)₂ (d) H₂S0₄
(b) Mg(HC0₃)₂ (e) Al₂(S0₄)₃
(c) Cu₂0. (f) MgCl₂

Answer: The atomicity of the molecules are:

Question: Explain the difference between 20, 0₂ and 0₃.

Answer: 2O —> It represents 2 atoms of oxygen (cannot exist independently).
O₂ —> It represents one molecule of oxygen (made up of 2 atom) can exist freely.
O₃ —> It represents one molecule of ozone (made up of 3 atoms) it can exist independently.

Question: Explain the difference between 2n and n2

Answer:

1. 2N means two molecules of Nitrogen atom.
   N₂ means two atoms of nitrogen in its one molecules.
2. N₂ shows that two nitrogen are bonded with covalent bond, and it become nitrogen gas whereas 2N does not show any bond, it just show two number of nitrogen.
3. N₂ is stable molecule, while 2N is nascent hydrogen with no stability.

Question:  What is the difference between 2H and H2 ?

Answer: H2 is Molecular hydrogen. It is a molecule of hydrogen that consists of two hydrogen atoms bonded together by one single bond (also known as a sigma bond). A subscript also denotes how many of a particular atom is in a molecule, thus H₂ has two hydrogens.

2H, on the other hand, denotes two moles of elemental hydrogen. It should be noted that elemental hydrogen is not bonded to anything.

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

Answer:

Question: 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?

Answer: Ratio of H : O by mass in water is:

Hydrogen : Oxygen —> H₂O
∴ 1 : 8 = 3 : x
x = 8 x 3
x = 24 g
∴ 24 g of oxygen gas would be required to react completely with 3 g of hydrogen gas.

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

Answer: The postulate of Dalton’s atomic theory that is the result of the law of conservation of mass is—the relative number and kinds of atoms are constant in a given compound. Atoms cannot be created nor destroyed in a chemical reaction.