CBSE Class 9 Science Atoms and Molecules Worksheet Set 07

SHORT ANSWER QUESTIONS
 

Question. How many moles are contained in 180 g of water.
Answer: Molar mass of water = \( 18\text{ g} \)
No of moles contained in 180 g = given mass/ molar mass
                                                    \( = 180/18 = 10\text{ moles} \)

Question. Define atomicity. Give examples
Answer: It is the number of atoms present in one molecule of a substance is called its atomicity. For example, atomicity of Ozone (\( \text{O}_3 \)) is 3, Phosphorus (\( \text{P}_4 \)) is 4, Sulphur (\( \text{S}_8 \)) is 8

Question. In a chemical reaction 0.096 g boron combines with 0.144 g of oxygen to form a compound. Calculate the mass of the compound formed.
Answer:
Boron + Oxygen \( \rightarrow \) Compound of Boron and Oxygen
\( 0.096 + 0.144 \rightarrow 0.240\text{ g} \)
This can be accounted on the basis of Law of Conservation of mass

Question. What is a mole? What is the unit of mole? How many molecules are there in a certain mass of a substance?
Answer: A mole is the amount of a substance which contains the same number of entities (atoms, molecules or ions) as there are atoms in exactly 12 g of carbon-12. The unit of mole is given by the symbol ‘mol’.
Avogadro number \( N_{\text{A}} = 6.022 \times 10^{23} \)
Number of molecules in a certain mass = \( (\text{mass of the substance} \times N_{\text{A}}) / \text{molar mass} \)

Question. Calculate the formula mass of sodium carbonate (\( \text{Na}_2\text{CO}_3\cdot10\text{H}_2\text{O} \)).
Answer: Formula mass of sodium carbonate
\( = (2 \times \text{atomic mass of Na}) + (1 \times \text{atomic mass of C}) + (3 \times \text{atomic mass of O}) + 10 \, [(2 \times \text{atomic mass of H}) + (1 \times \text{atomic mass of O})] \)
\( = 2 \times 23 + 1 \times 12 + 3 \times 16 + 10 \, [(2 \times 1) + (1 \times 16)] \)
\( = 46 + 12 + 48 + 180 = 286\text{ u} \)

Question. Calculate the mass of carbon present in 2 g \( \text{CO}_2 \).
Answer: \( 44\text{ g } \text{CO}_2 \) contains \( 12\text{ g C} \) in it
Hence, \( 2\text{ g } \text{CO}_2 \) contains \( (12/44) \times 2 = 0.545\text{ g} \)

Question. Calculate the molecular masses of (i) \( \text{C}_{12}\text{H}_{22}\text{O}_{11} \) (ii) \( \text{Al}_2(\text{SO}_4)_3 \)
Answer:
(i) \( \text{C}_{12}\text{H}_{22}\text{O}_{11} = 12 \times 12 + 22 \times 1 + 16 \times 11 = 342\text{ u} \)
(ii) (ii) \( \text{Al}_2(\text{SO}_4)_3 = 2 \times 27 + 3 \times (32 + 4 \times 16) = 342\text{ u} \)

Question. What is an ion? Give one example.
Answer: The negatively and positively charged particles are called ions. Negatively charged ion is called anion and positively charged ion is called cation
For example: \( \text{Cl}^- \), \( \text{SO}_4^{2-} \), \( \text{H}^+ \), \( \text{Pb}^{2+} \), etc

Question. Which of the following are tri-atomic and tetra-atomic molecules?
\( \text{CH}_3\text{Cl} \), \( \text{CaCl}_2 \), \( \text{NH}_3 \), \( \text{PCl}_3 \), \( \text{P}_2\text{O}_5 \), \( \text{H}_2\text{O} \)
Answer:
(i) Tri-atomic molecules are \( \text{CaCl}_2 \), \( \text{H}_2\text{O} \).
(ii) Tetra-atomic molecules are \( \text{NH}_3 \), \( \text{PCl}_3 \).

Question. Write the cations and anions present (if any) in the following compounds:
(a) \( \text{CH}_3\text{COONa} \)
(b) \( \text{NaCl} \)
Answer:
Cation: \( \text{Na}^+ \) (in both a and b)
Anion: \( \text{CH}_3\text{COO}^- \), \( \text{Cl}^- \)

Question. What is the mass of (i) 0.5 mol of \( \text{CO}_2 \) (ii) one atom of silver ( At mass Ag = 108 u)
Answer:
(i) Mass of 1 mole \( \text{CO}_2 = 44\text{g} \)
So, mass of 0.5 mol \( \text{CO}_2 = 44 \times 0.5 = 22\text{ g} \)
(ii) Mass of one mole Ag atoms = 108 g
i.e., \( 6.022 \times 10^{23} \) atoms of Ag weigh 108 g
So, 1 atom of Ag weighs \( 108 / (6.022 \times 10^{23}) = 1.793 \times 10^{-22}\text{ g} \)

Question. What are monoatomic and poly atomic ions? Give examples
Answer: Ions consisting of only single atoms are called monoatomic and those consisting of group of atoms are called polyatomic ions. Eg: \( \text{Cl}^- \), \( \text{H}^+ \) are monoatomic, \( \text{SO}_4^{2-} \), \( \text{PO}_4^{3-} \) are polyatomic ions

Question. Define valency of an element
Answer: Valency of an element is defined as its combining capacity. In the case of ions, valency is defined as the number of units of charge present on the ion

Question. Write chemical formula for the following compounds:
(i) Potassium nitrate
(ii) Sodium sulphate
Answer:
(i) Potassium nitrate
K      \( \text{NO}_3 \)
1+    1-                  Formula = \( \text{KNO}_3 \)

(ii) Sodium sulphate
Na      \( \text{SO}_4 \)
1+     2-                  Formula = \( \text{Na}_2\text{SO}_4 \)

Question. An element M forms the oxide \( \text{M}_2\text{O}_3 \). What will be the formula of its phosphate?
Answer:
M      O                  Formula : \( \text{M}_2\text{O}_3 \)
3      2-
Valency of M = +3
M      \( \text{PO}_4 \)              Formula: \( \text{MPO}_4 \)
3      3-
Formula of phosphate \( \text{MPO}_4 \)

Question. An element E is trivalent. Write the formula of its oxide, chloride and sulphide.
Answer:
Symbol      E      O                  Formula \( \text{E}_2\text{O}_3 \)
Valency     3      2

Symbol      E      Cl                 Formula \( \text{ECl}_3 \)
Valency     3      1

Symbol      E      S                  Formula \( \text{E}_2\text{S}_3 \)
Valency     3      2

Question. An element X shows a variable valency of 3 and 5. What are the formulae of the oxides formed by it?
Answer:
Symbol      X      O                  Formula \( \text{X}_2\text{O}_3 \)
Valency     3      2

Symbol      X      O                  Formula \( \text{X}_2\text{O}_5 \)
Valency     5      2

Question. A metal carbonate has the formula \( \text{MCO}_3 \). What is the valency of the metal?
Answer: \( \text{CO}_3^{2-} \) is carbonate ion, has valency -2. So valency of metal is +2

Question. Derive the formula of the following compounds
(i) Aluminium oxide
(ii) Calcium phosphate
Answer:
(i) Symbol      Al      O                  Formula \( \text{Al}_2\text{O}_3 \)
    Charge      3      2

(ii) Symbol/Formula      Ca      \( \text{PO}_4 \)
      Charge                  2+      3-                  Formula \( \text{Ca}_3(\text{PO}_4)_2 \)

Question. What is variable valency? Give two examples of metal atoms which show variable valency
Answer: Some elements show more than one valency. This is called variable valency.
Copper shows +1 and +2
Iron shows +2 and +3

Question. Define molar mass.
Answer: The mass of one mole of the substance is called its molar mass. Its unit is g/mol. This can be equal to gram atomic mass or gram molecular mass depending upon whether the substance is atomic or molecular

Question. If the formula of a metal sulphate is \( \text{MSO}_4 \), what is the formula of the nitrate of the same metal?
Answer: Valency of metal = 2+
Hence metal nitrate is \( \text{M}(\text{NO}_3)_2 \)

Question. Define atomic mass unit
Answer: Atomic mass unit is defined as the mass equal to exactly one twelfth (1/12 th) of the mass of an atom of C-12

Question. Hydrogen and oxygen combine in the ratio 1:8 by mass to form water. What weight of oxygen gas would be required to react completely with 3 g of hydrogen gas?
Answer: As the ratio is 1:8, means 1 g hydrogen combines with 8 g of oxygen.
3 g hydrogen will react with \( 3 \times 8 = 24\text{ g} \) of oxygen

Question. Write the names of the compounds represented by the following formulae: AlN, \( \text{CaSO}_4 \)
Answer:
AlN: Aluminium nitride
\( \text{CaSO}_4 \): Calcium sulphate