Answer:
V = 5 cm³
ρ = 4 g/cm³
Explanation:
Step 1: Calculate the volume (V)
We have a wooden cuboid of dimensions 5 cm × 1 cm × 1 cm. We can calculate its volume using the following expression.
V = 5 cm × 1 cm × 1 cm
V = 5 cm³
Step 2: Calculate the density (ρ)
The density is equal to the mass divided by the volume.
ρ = m / V
ρ = 20 g / 5 cm³
ρ = 4 g/cm³
Number of moles of Hydrogen gas produced = 2.5 moles
Given the balanced chemical equation,

This means, 1 mole of Mg and 2 moles of HCl gives 1 mole of Hydrogen gas.
Given that 2.5 moles of Magnesium is taken and an excess of Hydrochloric acid.
The mole ratio of Magnesium and Hydrogen = 1:1
Thus 2.5 moles of Magnesium with an excess of Hydrochloric acid will yield 2.5 moles of Hydrogen gas.
[By referring excess of HCl, we may assume that it contains at least 5 moles of HCl. Then only 2.5 moles of Mg will produce 2.5 moles of Hydrogen gas].
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Answer: The correct option is 2.
Explanation: Group 15 elements are Nitrogen, phosphorous, Arsenic, Antimony, Bismuth.
To know whether an element exits as a diatomic molecule or not, we look at the atomicity of an element.
Atomicity is the number of atoms of an element combining to form a molecule.
Atomicity of Nitrogen = 2
Atomicity of Phosphorous = 4
Atomicity of Bismuth = 1
Atomicity of Arsenic = 1
The element having atomicity 2 will exist as a diatomic molecule.
Hence, Nitrogen is a diatomic molecule 
There are there elements are important in organic compounds
carbon
hydrogen
oxygen :/example
glucose c6h12o6
The will be c. Covalent bond and