Question:
What is valency of an element?
Answer:
The valency of an element is defined as its capacity to combine with other elements. It can be be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound.
Elements may have valency: 1(monovalent), 2 (divalent), 3(trivalent) and 4 (tetravalent) respectively. Some elements show variable valency in different compounds.
e.g. In red oxide of Copper called Copper (I) Oxide (Cu2O), Cu has valency=1. While in black oxide of Copper i.e. Copper(II) Oxide (CuO), Cu is divalent (valency=2).
Atoms and Molecules
Q 1.
What is the chemical symbol for iron?
Q 2.
(i) State the law of constant proportions.
(ii) Show that water illustrates the law of constant proportions.
Q 3.
Name one element each which forms diatomic and tetra atomic molecule.
Q 4.
Define 'formula mass' of a compound.
Q 5.
Explain with example that law of conservation of mass is valid for chemical reactions.
Q 6.
Define molecule. What are its important properties?
Q 7.
What is gram-atomic mass of an element?
Q 8.
Fill in the blanks:
One mole of water contains ______________ molecules.
Q 9.
Fill in the blanks:
The atomic mass of sodium is 23. The gram atomic mass of sodium is _________.
Q 10.
Give the names of the elements present in the following compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
Q 11.
Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
Q 12.
Magnesium is two times heavier than C-12 atom, what shall be the mass of Mg atom in terms of atomic mass units? (Given mass of C-12 atom = 12u)
Q 13.
What is the mass of
(a) 1 mole of nitrogen atoms?
(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?
(c) 10 moles of sodium sulphite (Na
2SO
3)?
Q 14.
Convert 12 g of oxygen gas into moles.
Q 15.
What is the mass of 5 moles of sodium carbonate (Na
) ?(Atomic masses : Na = 23 u ; C = 12 u ; O = 16 u)
Q 16.
Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
Q 17.
Fill in the blanks:
According to law of definite proportions, in a chemical substance the elements are always present in __________ proportions by mass.
Q 18.
Fill in the blanks:
The chemical symbol of flourine is ________.
Q 19.
What is the molecular mass of a substance?
Q 20.
What are ionic compounds?
Q 21.
Name one element which forms diatomic and triatomic molecule.
Q 22.
How many atoms are there in 0.25 mole of hydrogen ?
Q 23.
Fill in the blanks:
Atoms can be observed using ____________ Microscope.
Q 24.
What is the atomic mass unit?
Q 25.
Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23 u, Fe = 56 u)?
Q 26.
Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.
(
Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)
Q 27.
Fill in the blanks:
The chemical symbol of mercury is _________.
Q 28.
What is the law of conservation of mass?
Q 29.
What are the postulates of Dalton's atomic theory?
Q 30.
How will you define chemical symbol?
Q 31.
Based on type of substance, how molecules are classified?
Q 32.
Give examples of triatomic molecules.
Q 33.
What is gram-molecular mass of a substance?
Q 34.
Convert into moles: (a) 12 g of oxygen gas (b) 20 g of water (c) 22 g of carbon dioxide
Q 35.
Find out number of atoms in 15 moles of He.
Q 36.
What do we call those particles which have more or less electrons than the normal atoms ?
Q 37.
(a) What is meant by the 'molar mass'of a substance ? State the unit in which molar mass usually expressed.
(b) Calulate the molar masses of the following substances. Write the results with proper units
(1) Ozone molecule, O
Q 38.
Who established the two important laws of chemical combinations?
Q 39.
State Law of constant proportions. Explain with an example.
Q 41.
Write the chemical formulae of the following. Also identify the ions present.
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate.
Q 42.
Fill in the blanks:
The mass of 5 moles of ammonia (NH
3) is __________.
Q 43.
Fill in the blanks:
The mass of 1 mole of a substance is called its _____________.
Q 44.
Write the chemical symbols of the following:
(i) Gold
(ii) Iron
(iii) Chlorine
(iv) Mercury
Q 45.
Fill in the blanks:
The atomicity of sulphur S
8 is ________.
Q 46.
Calculate the formula unit masses of ZnO, Na
2O, K
2CO
3, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.
Q 47.
What is the mass of:
(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?
Q 48.
Calculate the number of molecules of sulphur (S
8) present in 16 g of solid sulphur.
Q 49.
Give one major drawback of Dalton's atomic theory of matter.
Q 50.
Write the formulae of the following compounds. Also name the elements present in them.
(a) Water
(b) Ammonia
(c) Methane
(d) Sulphur dioxide
(f) Ethanol
