9th Class (CBSE) Science: Atoms and Molecules

Question: Showing results for the molecular formula of glucose is C₆H₁₂O₆. Calculate its molecular mass. (Atomic masses: C = 12 u; H = 1 u; O = 16 u)

Answer: It is very simple to calculate molecular weight of any compound when you know atomic Number of elements.
Glucose = C₆H₁₂O₆
C (atomic Number)=6
H (atomic Number)=1
O (atomic Number)=8
Multiply by 2 in each elements then you will get molecules weight of each elements
C=6×2=12
H=1×1=1
O=8×2=16
Then multiply by Number of elements by molecules weight of C, H and O
C6H12O6 = C(6×12)+H(12×1)+O(6×16)=180 Molecules weight of glucose

This is the simple method to calculate molecular weight of any compound if you know atomic Number of elements and just multiply by 2 in atomic Number you will get molecules weight

Question: Find the number of moles in the following:

1. 50 g of H₂O
2. 7 g of Na

Answer: Number of moles in

1. Molar mass of H₂O = 18 g
   Given mass of H₂O = 50 g

Question: Find the number of atoms in the following:

1. 0.5 mole of C atom
2. 2 mole of N atom

Answer:

1. 0.5 mole of C atom:
   Number of atoms in 1 mole of C atom = 6.022 x 10²³ atoms
   Number of atoms in 0.5 mole of C atom = 6.022 x 10²³ x 0.5
   = 3.011 x 10²³ atoms
2. 2 mole of N atom:
   Number of atoms in 1 mole of N atom = 6.022 x 10²³ atoms
   Number of atoms in 2 mole of N atom = 6.022 x 2 x 10²³
   = 1.2044 x 10²⁴ atoms

Question: Find the mass of the following:

1. 6.022 x 10²³ number of O₂ molecules
2. 1.5 mole of CO₂ molecule

Answer:

1. 6.022 x 10²³ number of 02 molecules:
   Mass of 1 mole of O₂ molecule = 6.022 x 10²³ molecules = 32 g
2. 1.5 mole of CO₂ molecule:
   Mass of 1 mole of C0₂ molecule = 6.022 x 10²³ molecules = 44 g
   Mass of 1.5 mole C0₂ molecule = 44 x 1.5 = 66 g

Question: Show the relationship between mole, Avogadro number and mass.

Answer:

Question: What are the rules for writing the symbol of an element?

Answer: IUPAC —> International Union of Pure and Applied Chemistry approves name of elements.
Symbols are the first one or two letters of the element’s name in English. The first letter of a symbol is always written as a capital letter (upper case) and the second letter as a small letter (lower case).
e.g., Hydrogen —> H Helium —> He
Some symbols are taken from the names of elements in Latin, German or Greek.
e.g., Symbol of iron is Fe, its Latin name is Ferrum.
Symbol of sodium is Na, its Latin name is Natrium.

Question: Explain relative atomic mass and relative molecular mass.

Answer: Relative atomic mass: It can be defined as the number of times one atom of given element is heavier than 1/12 th of the mass of an atom of carbon-12.

Relative Molecular Mass: It is defined as the number of times one molecule of a substance or given element is heavier than 1/12 th of the mass of one atom of carbon-12.

Question: The formula of carbon-dioxide is CO₂. What information do you get from this formula?

Answer:

1. CO₂ represents carbon-dioxide.
2. CO₂ is one molecule of carbon-dioxide.
3. CO₂ is one mole of carbon-dioxide i.e., it contains 6.022 x 10²³ molecules of carbon dioxide.
4. CO₂ contains 1 atom of carbon and two atoms of oxygen.
5. CO₂ represents 44 g of molar mass.

Question: State 3 points of difference between an atom and an ion.

Answer:

Atom:

1. An atom has no charge.
2. Number of electrons = number of protons.
3. Atom is reactive.

Ion:

1. An ion has either positive or negative charge.
2. Number of electrons ≠ number of protons.
3. Ion is stable.