Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.
(Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)
 

Answer

mole of aluminium oxide (Al₂O₃) = 2 × 27 + 3 × 16
= 102 g
i.e., 102 g of Al₂O₃= 6.022 × 10²³molecules of Al₂O₃
Then, 0.051 g of Al₂O₃contains = 6.022 × 10²³ / 102 × 0.051 molecules
= 3.011 × 10²⁰ molecules of Al₂O₃

The number of aluminium ions (Al3+) present in one molecule of aluminium oxide is 2.

Therefore, the number of aluminium ions (Al³⁺) present in 3.011 × 10²⁰molecules (0.051 g ) of aluminium oxide (Al₂O₃) = 2 × 3.011 × 10²⁰
= 6.022 × 10²⁰

1. Which of the following correctly represents 360 g of water?
(i) 2 moles of H₂0
(ii) 20 moles of water
(iii) 6.022 � 10²³ molecules of water
(iv) 1.2044�10²⁵ molecules of water
(a) (i)
(b) (i) and (iv)
(c) (ii) and (iii)
(d) (ii) and (iv)
Ans. (d) (ii) and (iv)

2. Which of the following statements is not true about an atom?
(a) Atoms are not able to exist independently
(b) Atoms are the basic units from which molecules and ions are formed
(c) Atoms are always neutral in nature
(d) Atoms aggregate in large numbers to form the matter that we can see, feel or touch
Ans. (a) Atoms are not able to exist independently.
Explanation: Atoms of inert gases exist independently, i.e. as single atom.


3. The chemical symbol for nitrogen gas is
(a) Ni
(b) N₂
(c) N⁺
(d) N
Ans. (b) N₂

4. The chemical symbol for sodium is
(a) So
(b) Sd
(c) NA
(d) Na
Ans. (d) Na

5. Which of the following would weigh the highest?
(a) 0.2 mole of sucrose (C₁₂H₂₂O₁₁)
(b) 2 moles of CO₂
(c) 2 moles of CaCO₃
(d) 10 moles of H₂O
Ans. (c) 2 moles of CaCO₃

Explanation: Weight of 1 mole of each compound can be calculated as follows:

(a) Sucrose =  12 � 12+  22 � 1 +11 � 16 = 144  + 22  +  176 =342 g
Or 0.2 M Sucrose =  342 � 0.2  = 68.4 g
(b) 1 M carbon dioxide =    1 � 12 + 2 � 16  = 44 g
Or 2 M CO₂ = � = 44 2 88 g
(c) 1 M CaCO₃ =    1 � 10 +  1  � 12 + 3 � 16 = 100 g
Or 2 M CaCO₃ =   100 �2 = 200 g
(d) 1 M H₂O=     2 � 1+  1 � 16 = 18g
Or 10 M water =  18 �10 = 180g

6. Which of the following has maximum number of atoms?
(a) 18 g of H₂O
(b) 18 g of O₂
(c) 18 g of CO₂
(d) 18 g of CH₄
Ans. (d) 18 g of CH₄
Explanation: Number of atoms


7. Which of the following contains maximum number of molecules?
(a) 1g CO₂
(b) 1g N₂
(c) 1g H₂
(d) 1g CH₄
Ans. (c) 1g H₂

= 3.011 �10 ²³
Molar mass of other molecules are much higher than given mass, so number of molecules in them will be less than that in hydrogen.

Mass of one atom of oxygen is

3.42 g of sucrose are dissolved in 18 g of water in a beaker. The number of oxygen atoms in the solution are

(a) 6.68 x 10²³
(b) 6.09 x 10²²
(c) 6.022 x 10²³
(d) 6.022 x 10²¹

Ans. (a) 6.68 x 10²³

10. A change in the physical state can be brought about
(a) only when energy is given to the system
(b) only when energy is taken out from the system
(c) when energy is either given to, or taken out from the system
(d) without any energy change
Ans. (c) when energy is either given to, or taken out from the system
Explanation: When a solid changes into liquid, it takes energy. When a liquid changes into solid, it releases energy.

Which of the following represent correct chemical formula? Name it.
(a) CaCl
(b) BiPO₄
(c) NaSO₄
(d) NaS
Ans. (b) In BiPO₄; with ions are trivalent and hence this shows the correct chemical formula.
In option (a); Ca is bivalent while chlorine is monovalent. In options (c) and (d); Na is
monovalent while sulphate and Sulphur are bivalent. 

(a) Copper (II) bromide
Ans. CuBr₂
(b) Aluminium (III) nitrate
Ans. Al(NO₃)₃
(c) Calcium (II) phosphate
Ans. Ca₃(PO4)₂
(d) Iron (III) sulphide
Ans. Fe₂S₃
(e) Mercury (II) chloride
Ans. HgCl₂
(f) Magesium (II) acetate
Ans. Mg(CH₃COO)₂

Write the molecular formulae of all the compounds that can be formed by the combination of following ions
Cu²⁺ , Na⁺, Fe³⁺, Cl^- , SO₄^2- , PO₄ ^3-

Ans. CuCl₂/ CuSO₄/ Cu₃(PO₄)₂
NaCl/ Na₂SO₄/ Na₃PO₄
FeCl₃/ Fe₂(SO₄)3 / FePO₄

Write the cations and anions present (if any) in the following compounds
(a) CH₃COONa
(b) NaCl
(c) H₂
(d) NH₄NO₃

(a) ANION  ->CH₃COO^-  CATIONS ->Na⁺

(b) ANION  ->Cl^- CATIONS ->Na⁺
(c) Its a covalent compund    
(d) ANION  ->NO₃^- CATIONS ->NH₄⁺

(a) Calcium and fluorine
Ans. CaF₂
(b) Hydrogen and sulphur
Ans. H₂S
(c)Nitrogen and hydrogen
Ans. NH₃
(d)Carbon and chlorine
Ans. CCI₄
(e)Sodium and oxygen
Ans. Na₂O
(f) Carbon and oxygen
Ans. CO, CO₂

Which of the following symbols of elements are incorrect? Give their correct
symbols
(a) Cobalt CO
(b) Carbon c
(c) Aluminium AL
(d) Helium He
(e) Sodium So
Ans. (a) Incorrect, the correct symbol of cobalt is Co
(b) Incorrect, the correct symbol of carbon is C
(c) Incorrect, the correct symbol of aluminium is Al
(d) Correct (He)
(e) Incorrect, the correct symbol of sodium is Na