Questions Related to Science

The atomicity of Argon is 1.

Argon is a noble gas and exists as a single atom (monatomic) because it has a completely filled outermost electron shell, making it stable and unreactive.

Polyatomic ions are ions that consist of two or more atoms bonded together covalently and carry a net positive or negative charge. These atoms act as a single charged unit in chemical reactions.

Examples:

1. Ammonium ion – NH₄⁺
2. Sulphate ion – SO₄²⁻

The atomic number of magnesium (Mg) is 12.

In a neutral Mg atom, the number of electrons = number of protons = 12.
In a Mg²⁺ ion, the atom loses 2 electrons, so the number of electrons = 12 − 2 = 10.

Therefore:
Mg atom → 12 electrons
Mg²⁺ ion → 10 electrons

The two atoms which can exist independently are Helium (He) and Argon (Ar).

These are noble gases and exist as single atoms because they have a completely filled outermost electron shell, making them stable and unreactive.

The element used as the reference for atomic mass is Carbon.

The isotope Carbon-12 (C-12) is taken as the standard reference, and its atomic mass is defined as exactly 12 atomic mass units (amu)

The organisation that approves the names of elements all over the world is the International Union of Pure and Applied Chemistry (IUPAC).

The symbol of gold is Au

The Law of Constant Proportion states that a chemical compound always contains the same elements in the same fixed ratio by mass, regardless of its source or method of preparation.

Example:
Water (H₂O) always contains hydrogen and oxygen in the ratio of 1:8 by mass, whether it is obtained from a river, rain, or produced in a laboratory.

The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction.

According to this law, the total mass of the reactants is always equal to the total mass of the products.

Example:
When hydrogen reacts with oxygen to form water,
2H₂ + O₂ → 2H₂O
The total mass of hydrogen and oxygen before the reaction is equal to the mass of water formed after the reaction.

The scientist who laid the foundation of chemical sciences was Antoine Lavoisier.

He is known as the Father of Modern Chemistry because he:

• Established the Law of Conservation of Mass, which states that matter can neither be created nor destroyed in a chemical reaction.
• Gave a clear definition of elements and compounds.
• Helped develop a systematic naming system for chemical substances.

(a) Ethyne (C₂H₂)
= (2 × 12) + (2 × 1)
= 24 + 2
= 26 g/mol

(b) Sulphur molecule (S₈)
= 8 × 32
= 256 g/mol

(c) Phosphorus molecule (P₄)
= 4 × 31
= 124 g/mol

(d) Hydrochloric acid (HCl)
= (1 × 1) + (1 × 35.5)
= 1 + 35.5
= 36.5 g/mol

(e) Nitric acid (HNO₃)
= (1 × 1) + (1 × 14) + (3 × 16)
= 1 + 14 + 48
= 63 g/mol

(a) Quick lime (CaO)   ---->   Calcium and Oxygen
(b) Hydrogen bromide (HBr)    ---->   Hydrogen and Bromine
(c) Baking powder (NaHCO₃)    ----->   Sodium, Hydrogen, Carbon, and Oxygen
(d) Potassium sulphate (K₂SO₄)   ------>   Potassium, Sulphur, and Oxygen

(a) Magnesium chloride  ---->  MgCl₂
(b) Calcium oxide  ---->   CaO
(c) Copper nitrate   ----->  Cu(NO₃)₂
(d) Aluminium chloride   ---->  AlCl₃
(e) Calcium carbonate   ------>   CaCO₃

Polyatomic ions are ions that contain two or more atoms joined together by covalent bonds and carry a net positive or negative charge. These atoms act as a single charged unit during chemical reactions.

Examples:

• Ammonium ion (NH₄⁺)
• Hydroxide ion (OH⁻)
• Nitrate ion (NO₃⁻)
• Sulphate ion (SO₄²⁻)
• Carbonate ion (CO₃²⁻)

Thus, polyatomic ions are groups of atoms that behave as one charged particle.

From the data:
3.0 g of carbon + 8.0 g of oxygen   ---->   11.0 g of carbon dioxide

This means 3 g of carbon requires 8 g of oxygen to form 11 g of carbon dioxide.

Now, if 3 g of carbon is burnt in 50 g of oxygen, carbon is the limiting reactant because it can combine only with 8 g of oxygen. The remaining oxygen will be left unused.

So, the amount of carbon dioxide formed will still be 11 g.

Mass of carbon dioxide formed = 11.0 g

Law governing the reaction:
This is governed by the Law of Constant Proportion, which states that a chemical compound always contains the same elements in the same fixed ratio by mass.

Total mass of the compound = 0.24 g
Mass of boron = 0.096 g
Mass of oxygen = 0.144 g

Percentage of boron = (0.096 / 0.24) × 100 = 40%
Percentage of oxygen = (0.144 / 0.24) × 100 = 60%

Therefore, the percentage composition of the compound is:
Boron = 40%
Oxygen = 60%

Density of the packet = Mass / Volume = 500 g / 350 cm³ = 1.43 g/cm³

Since the density of the packet (1.43 g/cm³) is greater than the density of water (1 g/cm³), the packet will sink in water.

Mass of water displaced = Volume of packet × Density of water
= 350 cm³ × 1 g/cm³
= 350 g

Final Answer:
The packet will sink in water, and the mass of water displaced is 350 g.

Density of the substance = Mass / Volume = 50 g / 20 cm³ = 2.5 g/cm³

Since the density of the substance (2.5 g/cm³) is greater than the density of water (1 g/cm³), the substance will sink in water.

A block of plastic comes up to the surface of water because the buoyant force acting on it is greater than its weight. Since the density of plastic is less than the density of water, the upward buoyant force pushes it up until it reaches the surface.

The buoyant force on an object immersed in a liquid acts vertically upward, that is, opposite to the direction of gravity.

(a) Time to rise to top = half the total time = 6/2 = 3 s.
Initial velocity u = g × time = 10 × 3 = 30 m/s.

(b) Maximum height H = u² / (2g) = (30)² / (2 × 10) = 900 / 20 = 45 m.

(c) Position after t = 4 s (take upward as positive, origin at thrower):
y = ut − ½ g t² = 30×4 − ½×10×4² = 120 − 80 = 40 m above the thrower.

Final answers:
(a) 30 m/s upward
(b) 45 m
(c) 40 m above the thrower (after 4 s)