Science and Technology Class 9 Question Answers Chapter 4 Maharashtra Board

Measurement of Matter

1. Give examples.

a. Positive radicals

Answer : H⁺, Na⁺, K⁺, Ag⁺, Cu⁺, Hg⁺

b. Basic radicals

Answer : Na⁺ – Sodium ion, K⁺ – Potassium ion, Ag⁺ – Silver ion

c. Composite radicals

Answer :

d. Metals with variable valency

Answer:

(a) Iron (Ferrum)
(i) Fe²⁺ – Ferrous [Iron – II]
(ii) Fe³⁺ – Ferric [Iron – III]

(b) Copper (Cuprum)
(i) Cu⁺ – Cuprous [Copper -1]
(ii) Cu²⁺ – Cupric [Copper – II]

(c) Mercury (Hydragyrum)
(i) Hg⁺ – Mercurous [Mercury -1]
(ii) Hg²⁺ – Mercuric [Mercury – II]

e. Bivalent acidic radicals

Answer:

S^2- ,O^2- ,Se^2-

f. Trivalent basic radicals

Answer:
Al³⁺ – Aluminium, Cr³⁺ – Chromium, Fe³⁺ – Ferric.

2. Write symbols of the following elements and the radicals obtained from them, and indicate the charge on the radicals.

Mercury, potassium, nitrogen, copper, sulphur, carbon, chlorine, oxygen

Answer :

3. Write the steps in deducing the chemical formulae of the following compounds.

Sodium sulphate, potassium nitrate, ferric phosphate, calcium oxide, aluminium hydroxide

Answer:

In order to write the chemical formulae of compounds, it is necessary to know the symbols and valency of various radicals.

1. Sodium Sulphate:

Step – 1 : To write the symbols of the radicals (Basic radicals on the left and acidic radicals on the right)

Na₂ SO₄

Step – 2 : To write the valency below the respective radical.

Step – 3: To cross-multiply as shown by arrows the number of radicals.

Step – 4 : To write down the chemical formula of the compound.

Na₂ SO₄

(Sodium sulphate)

2. Potassium Nitrate:

The Symbols and Valencies of Potassium and Nitrate.

Cross-multiply the valencies

3. Ferric phosphate:

Step 1: To write the symbols of the radicals (Basic radicals on the left and acidic radicals on the right) Fe PO₄

Step 2: To write the valency below the respective radical.

Step 3: To cross-multiply as shown by arrows the number of radicals.

Step 4: To write down the chemical formula of the compound.
FePO₄

4. Calcium oxide:

The Symbols and Valencies of Calcium and Oxygen.

Symbol	Ca	O
Valency	2	2

Cross-multiply the valencies

The chemical formula of Calcium oxide is CaO.

5. Aluminium hydroxide:

The Symbols and Valencies of Aluminium and Hydroxide.

Symbol	Al	OH
Valency	3	1

Cross-multiply the valencies

The chemical formula of Aluminium hydroxide is Al(OH)₃.

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4. Write answers to the following questions and explain your answers.

a. Explain how the element sodium is monovalent.

Answer:

1. The number of protons or electrons (atomic number) in Sodium (Na) atom is 11. Therefore the electronic configuration of sodium atom is (2, 8,1).
2. In chemical reaction, sodium atom has the capacity to give away le_ from its outermost orbit to form Na+ ion with stable electronic configuration (2, 8).
3. As sodium atom gives away le- and a cation of sodium is formed, hence the valency of sodium is 1 and therefore, the element sodium is monovalent.

b. M is a bivalent metal. Write down the steps to find the chemical formulae of its compounds formed with the radicals, sulphate and phosphate.

Answer:

M is a bivalent metal. Following are the steps to find the chemical formulae of its compounds formed with the radicals, sulphate and phosphate:

The symbols and valencies of M and sulphate:

symbols	M	SO4
valencies	2	2

Cross-multiply the valencies

The chemical formula is MSO₄.

The symbols and valencies of M and phosphate:

symbols	M	PO4
valencies	2	3

Cross-multiply the valencies

The chemical formula is M₃(PO₄)₂.

c. Explain the need for a reference atom for atomic mass. Give some information about two reference atoms.

Answer:

• The mass of an atom is concentrated in its nucleus and it is due to the protons (p) and neutrons (n) in it.
• Since an atom is very very tiny, it was not possible to measure atomic mass accurately. Therefore, the concept of relative mass of an atom was formed.
• To express relative mass of an atom, reference of atom is considered. The two reference atoms were as follows:

(a) Hydrogen (H) atom: The hydrogen atom is the lightest. The relative mass of a hydrogen atom is 1 which has only 1 proton in its nucleus. On this scale, the relative atomic mass of many elements comes out to be fractional. Therefore, carbon was selected as a reference atom.

(b) Carbon (C) atom: The carbon atom is selected as reference atom. In this scale, the relative mass of a carbon atom is accepted as 12.

• The relative atomic mass of 1 hydrogen (H) atom compared to the carbon (C) atom becomes

d. What is meant by Unified Atomic Mass.

Answer:

• During earlier time, relative mass of an atom was considered for measuring the mass of an atom directly. But since the founding of unified mass, relative mass is not accepted henceforth.
• Unified atomic mass is the unit of atomic mass called as Dalton.
• Its symbol is ‘u’. lu = 1.66053904 x 10^-27 kg.

e. Explain with examples what is meant by a ‘mole’ of a substance.

Answer:

• A mole is that quantity of a substance whose mass in grams is equal in magnitude to the molecular mass of that substance in Daltons.
• For example: Atomic mass of oxygen atom (O) is 16u. Thus, the molecular mass of oxygen molecule (O₂) is 16 x 2 = 32u. Therefore, 32 g of oxygen is 1 mole of oxygen.

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5. Write the names of the following compounds and deduce their molecular masses.

Na₂SO₄, K₂CO₃, CO₂, MgCl₂, NaOH, AlPO₄, NaHCO₃

Answer:

6. Two samples ‘m’ and ‘n’ of slaked lime were obtained from two different reactions. The details about their composition are as follows:

Number of moles in water = Mass of water in gramsMolecular mass of water

${\displaystyle \frac{\text{Mass of water in grams}}{\text{Molecular mass of water}}}$

=

${\displaystyle \frac{90}{18}}$

= 5 moles

∴ 5 moles of H₂O contains = 5 × 6.022 × 10²³ molecules = 30.11 × 10²³ molecules.

8.8g carbon dioxide

Number of moles in CO₂ = Mass of COin gramsMolecular mass of CO

${\displaystyle \frac{{\text{Mass of CO}}_2\text{in grams}}{{\text{Molecular mass of CO}}_2}}$

=

${\displaystyle \frac{8.8}{44}}$

= 0.2 mole

∴ 0.2 moles of CO₂ contains = 0.2 × 6.022 × 10²³ molecules = 1.2044 × 10²³ molecules.

∴ The number of molecules in 8.8 grams of carbon dioxide is 1.2044 × 10²³.

The number of molecules in 90 grams of water is 3.01 × 10²⁴.

7.1g chlorine

No. of moles in chlorine = Mass of chlorine in gramsMolecular mass of chlorine

${\displaystyle \frac{\text{Mass of chlorine in grams}}{\text{Molecular mass of chlorine}}}$

=

${\displaystyle \frac{7.1}{71}}$

= 0.1 mole

∴ 0.1 moles of Cl₂ contains = 0.1 × 6.022 × 10²³ molecules = 0.6022 × 10²³ molecules.

∴ Number of molecules in 7.1 grams of chlorine is 6.022 × 10²².

8. If 0.2 mol of the following substances are required how many grams of those substances should be taken?

Sodium chloride

We know that,

Number of moles of a substance = Mass of the substance in gramsMolecular mass of the substance

${\displaystyle \frac{\text{Mass of the substance in grams}}{\text{Molecular mass of the substance}}}$

Molar mass = sum of constituent atomic masses

Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol

Number of moles of a substance = Mass of the substance in gramsMolecular mass of the substance

${\displaystyle \frac{\text{Mass of the substance in grams}}{\text{Molecular mass of the substance}}}$

Let Mass of the substance in grams be x,

${\displaystyle 0.2=\frac{\text{x}}{58.5}=11.7}$

g

We need, 11.7 g of NaCl for obtaining 0.2 moles of NaCl.

magnesium oxide

Molar mass of MgO = 24 + 16 = 40 g/mol

Number of moles of a substance = Mass of the substance in gramsMolecular mass of the substance

${\displaystyle \frac{\text{Mass of the substance in grams}}{\text{Molecular mass of the substance}}}$

Let Mass of the substance in grams be x,
x

${\displaystyle 0.2=\frac{\text{x}}{40}=8}$

g

we need, 8g of MgO to obtain 0.2 moles of MgO.

calcium carbonate

Molar mass of CaCO₃ = 40 + 12  + 3(16) = 100 g/mol

Number of moles of a substance = Mass of the substance in gramsMolecular mass of the substance

${\displaystyle \frac{\text{Mass of the substance in grams}}{\text{Molecular mass of the substance}}}$

Let Mass of the substance in grams be x,

x

${\displaystyle 0.2=\frac{\text{x}}{100}=20}$

g

we need, 20 g of CaCO₃ for obtaining 0.2 moles of CaCO₃.