CBSE Class 9 Chemistry Atoms And Molecules Worksheet Set D

Express each of the following in kilograms

Question: 5.84 × 10⁻³ mg
Answer: 5.84 × 10⁻⁹ kg

Question: 58.34 g
Answer: 5.834 × 10⁻² kg

Question: 0.584 g
Answer: 5.84 × 10⁻⁴ kg

Question: 5.873 × 10⁻²¹ g
Answer: 5.873 × 10⁻²⁴ kg

Write the formulae for the following and calculate the molecular mass for each one of them.

Question: Caustic potash
Answer: KOH
(39 + 16 + 1) = 56 g mol⁻¹

Question: Baking powder
Answer: NaHCO₃
23 + 1 + 12 + (3 × 16) = 84 g mol⁻¹

Question: Limestone
Answer: CaCO₃
40 + 12 + (3 × 16) = 100 g mol⁻¹

Question: Caustic soda
Answer: NaOH
23 + 16 + 1 = 40 g mol⁻¹

Question: Ethanol
Answer: C₂H₅OH = C₂H₆O
(2 × 12) + (6 × 1) + 16 = 46 g mol⁻¹

Question: Common salt
Answer: NaCl
23 + 35.5 = 58.5 g mol⁻¹

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

Question: 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
Answer:  (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

Question: Write the molecular formulae of all the compounds that can be formed by the combination of following ions
Cu²⁺, Na⁺ , Fe³⁺ , Cl^– , SO^2-₄ , PO^3-₄
Answer: CuCl₂/ CuSO₄/ Cu₃(PO₄)₂

Question: Classify each of the following on the basis of their atomicity.
(a) F₂ (b) NO₂ (c) N₂O (d) C₂H₆ (e) P₄ (f) H₂O₂ (g) P₄O₁₀ (H) O₃ (i) HCl (j) CH₄ (k) He (l) Ag
Answer:  (a) 2 atom (b) 3 atom (c) 3 atom (d) 8 atom (e) 4 atom (f) 4 atom (g) 14 atom (h) 3 atom (i) 2 atom (j) 5 atom (k) 1 atom (noble gases are always monoatomic) (l) Polyatomic (because metals are bound by metallic bond and any measurable quantity of a metal can contain millions of atoms.)

Question: Give the chemical formulae for the following compounds and compute the ratio by mass of the combining elements in each one of them. (You may use appendix-III).
(a) Ammonia
(b) Carbon monoxide
(c) Hydrogen chloride
(d) Aluminium fluoride
(e) Magnesium sulphide
Answer:  (a) NH3; ratio N : H = 14 : 3
(b) CO; ratio C : O = 3 : 4
(c) HCl; ratio H : Cl = 1 : 35.5 or 2 : 71
(d) AlF3; ratio Al : F = 9 : 19
(e) MgS; ratio Mg : S = 3 : 4

Question: Does the solubility of a substance change with temperature? Explain with the help of an example.
Answer:  Yes, it is a temperature dependent property. The solubility generally, increases with increase in temperature. For example, you can dissolve more sugar in hot water than in cold water

Question: You are provided with a fine white coloured powder which is either sugar or salt.
How would you identify it without tasting?
Answer:  On heating the powder, it will char if it is a sugar.
Alternatively, the powder may be dissolved in water and checked for its conduction of electricity. If it conducts, it is a salt.

Question: Calculate the number of moles of magnesium present in a magnesium ribbon weighing 12 g. Molar atomic mass of magnesium is 24g mol^–1.
Answer: Number of moles = mass/molar mass = 12 g/24 g = 0.5 mol

Question: Raunak took 5 moles of carbon atoms in a container and Krish also took 5 moles of sodium atoms in another container of same weight. (a) Whose container heavier? (b) Whose container has more number of atoms?
Answer: (a) Mass of sodium atoms carried by Krish = (5×23)g =115g
While mass of carbon atom carried by Raunak = (5×12)g = 60g
Thus, Krish’s container is heavy
(b) Both the bags have same number of atoms as they have same number of moles of atoms.

Question: Verify by calculating that
(a) 5 moles of CO₂ and 5 moles of H₂O do not have the same mass.
(b) 240 g of calcium and 240 g magnesium elements have a mole ratio of 3:5.
Answer: 
(A) CO₂ has molar mass = 44g mol^–1
5 moles of CO₂ have molar mass = 44 × 5= 220 g
H₂O has molar mass = 18 g mol^–1
5 moles of H₂O have mass = 18 × 5 g= 90 g
(B) Number of moles in 240g Ca metal = 240/40 = 6
Number of moles in 240g of Mg metal = 240/24 = 10
Ratio 6: 10 = 3: 5

Question: Calcium chloride when dissolved in water dissociates into its ions according to the following equation.
CaCl₂ (aq) → Ca²⁺ (aq) + 2Cl^– (aq)
Calculate the number of ions obtained from CaCl₂ when 222 g of it is dissolved in water.
Answer:  1 mole of calcium chloride = 111g
∴ 222g of CaCl₂ is equivalent to 2 moles of CaCl₂
Since 1 formula unit CaCl₂ gives 3 ions, therefore, 1 mol of CaCl₂ will give 3 moles of ions 2 moles of CaCl₂ would give 3×2 = 6 moles of ions.
No. of ions = No. of moles of ions × Avogadro number
= 6 × 6.022 × 10²³
= 36.132 × 10²³
= 3.6132 × 10²⁴ ions

Question: Cinnabar (HgS) is a prominent ore of mercury. How many grams of mercury are present in 225 g of pure HgS? Molar mass of Hg and S are 200.6 g mol^-1 and 32 g mol^-1 respectively.
Answer:  Molar mass of HgS = 200.6 + 32 = 232.6 g mol^-1
Mass of Hg in 232.6 g of HgS = 200.6 g
Mass of Hg in 225 g of HgS = 200,6/232.6 X 225 = 194.04g

Question: What is the SI prefix for each of the following multiples and submultiples of a unit?
(a) 10³ (b) 10^-1 (c) 10^-2 (d) 10^-6 (e) 10^-9 (f) 10^-12
Answer:  (a) kilo (b) deci (c) centi (d) micro (e) nano (f) pico

Question: A sample of vitamin C is known to contain 24 2.58 ×10²⁴ oxygen atoms. How many moles of oxygen atoms are present in the sample?
Answer:  1 mole of oxygen atoms = 23 6.023×10²⁴ atoms
∴ Number or moles of oxygen atoms = 2.58 X 10²⁴/6.022 X 10²³ = 4.28 mol 4.28 moles of oxygen atoms.

Question: What is the fraction of the mass of water due to neutrons?
Answer:  Mass of one mole (Avogadro Number) of neutrons ~ 1g

There are 8 neutrons in one atom of oxygen
Mass of 8 neutrons = 8/NA
Fraction of mass of water due to neutrons  8/18

Question: Fill in the missing data in the Table 3.1

Answer:

Question: The visible universe is estimated to contain 10²² stars. How many moles of stars are present in the visible universe?
Answer:  Number of moles of stars = 1022/6.023 X 10²³ = 0.0166 moles

Question: Compute the difference in masses of 103 moles each of magnesium atoms and magnesium ions. (Mass of an electron = 9.1×10^-31 kg)
Answer:  A Mg²⁺ ion and Mg atom differ by two electrons.
10³ moles of Mg²⁺ and Mg atoms would differ by
10³×2 moles of electrons
Mass of 2×10³ moles of electrons 2 x 10³ x 6.023 x 10²³ x 9.1 x 10^-31 Kg
2 x 6.022 x 9.1 x 10^-5 Kg
109.6004 x 10^-5 Kg
1.096 x 10^-3 Kg

Question: A gold sample contains 90% of gold and the rest copper. How many atoms of gold are present in one gram of this sample of gold?
Answer:  One gram of gold sample will contain 90/100 = 0.9g of gold
Number of moles of gold = mass of gold/atomic mass of gold
= 0.9/197 = 0.0046
One moles of gold contains NA atoms 23 = 6.022×10²³
∴ 0.0046 mole of gold will contain 23 = 0.0046×6.022×10²³
21 = 2.77×10²¹

Question: Which has more number of atoms?
(i) 100g of N₂
Answer:  100 g of N₂ = 100/28, moles

(ii)100 g of NH₃
Answer:  100 g of NH₃ = 100/17 moles = 100/17 x 6.022 x 10²³ molecules
= 100/17 x 6.022 x 10²³ x 4 atoms
=141.69×10²³
NH₃ would have more atoms.

Question: The mass of one steel screw is 4.11g. Find the mass of one mole of these steel screws. Compare this value with the mass of the Earth (5.98 x 10²⁴kg). Which one of the two is heavier and by how many times?
Answer: One mole of screws weigh = 2.475 × 10²⁴ g = 2.475 × 10²¹ kg

Mass of earth is 3 2.4×10 times the mass of screws
The earth is 2400 times heavier than one mole of screws.

Question: Compute the number of ions present in 5.85 g of sodium chloride.
Answer: 5.85 g of NaCl = 5.85/58.5 = 0.1 moles
Or 0.1 moles of NaCl particle
Each NaCl particle is equivalent to one Na⁺ one CI^–
⇒ 2 ions
⇒ Total moles of ions = 0.1 × 2
⇒2 moles
No. of ions = 0.2×6.022×10²³
23 ⇒1.2042×10²³ ions

Question: Compute the difference in masses of one mole each of aluminium atoms and one mole of its ions. (Mass of an electron is 9.1 x 10^-28 g). Which one is heavier?
Answer: Mass of 1 mole of aluminium atom = the molar mass of aluminium = 27 g mo^l-1 An aluminium atom needs to lose three electrons to become an ion, AI³⁺ For one mole of AI³⁺ ion, three moles of electrons are to best lost.
The mass of three moles of electrons = 3 x (9.1 x 10^-23) x 6.022 x 10²³g
= 27.3 x 6.022 x 10^-5g
= 164.400 x 10^-5g
= 0.00164 g
Molar mass of Al3+ = (27 – 0.00164) g mol^-1 = 26.998g mol⁻¹
Difference = 27 – 26.9984 = 0.0016 g

Question: What are ionic and molecular compounds? Give examples.
Answer: Atoms of different elements join together in definite proportions to form molecules of compounds. Examples— water, ammonia, carbondioxide. Compounds composed of metals and non-metals contain charged species. The charged species are known as ions. An ion is a charged particle and can be negatively or positively charged. A negatively charged ion is called an anion and the positively charged ion is called cation. Examples— sodium chloride,
calcium oxide.

Question: A silver ornament of mass ‘m’ gram is polished with gold equivalent to 1% of the mass of silver. Compute the ratio of the number of atoms of gold and silver in the ornament.
Answer: Mass of silver = m g
Mass of gold = m/100 g

=108:100×197
=108:19700
=1:182.41

Question: A sample of ethane (C₂H₆) gas has the same mass as 20 1.5×10²⁰ molecules of methane (CH₄). How many C₂H₆ molecules does the sample of gas contain?
Answer: Mass of 1 molecules of CH₄ = 16g/N_A

Question: Complete the following crossword puzzle (Fig. 3.1) by using the name of the chemical elements. Use the data given in Table 3.2.

Answer: (1) Silver (2) Gold (3) iron (4) Copper (5) Phosphorus (6) Mercury (7) Hydrogen (8) Lead

Question: (a) In this crossword puzzle (Fig 3.2), names of 11 elements are hidden Symbols of these are given below. Complete the puzzle. 1. Cl, 2. H, 3. Ar, 4. O, 5. Xe, 6. N, 7. He, 8. F, 9. Kr, 10. Rn, 11. Ne
(b) Identify the total number of inert gases their names and symbols from this cross-word puzzle.

Answer: (a)

(b) Insert Gases: There are total six insert six inert gases present in this crossword, i.e.
helium (He), Argon (Ar), Xenon (Xe), Krypton (Kr), Radon (Rn) and Neon (Ne).

Question: The difference in the mass of 100 moles each of sodium atoms and sodium ions is 5.48002 g. Compute the mass of an electron.
Answer: A sodium atom and ion, differ by one electron. For 100 moles each of sodium atoms and ions there would be a difference of 100 moles of electrons.
Mass of 100 moles of electrons= 5.48002 g
Mass of 1 mole of electron = (5.48002/100) g
Mass of one electron = 5.48002/100 X 6.022 X 10²³ = 9.1 X 10^-23 g = 9.1 X 10^-31 kg

Question: In photosynthesis, 6 molecules of carbon dioxide combine with an equal number of water molecules through a complex series of reactions to give a molecule of glucose having a molecular formula C₆H₁₂O⁶. How many grams of water would be required to produce 18 g of glucose? Compute the volume of water so consumed assuming the density of water to be 1 g cm^-3.
Answer: 6CO₂ + 6H₂O Chlorophyll/sunlight C₆H₁₂ O₆ + 6O₂ 1 mole of glucose needs 6 moles of water 180 g of glucose needs (6 × 18) g of water 1 g of glucose will need 108/180 g of water.