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.
Fill in the blanks:
The atomic mass of sodium is 23. The gram atomic mass of sodium is _________.
Q 2.
What is the chemical symbol for iron?
Q 3.
What is gram-atomic mass of an element?
Q 4.
Define molecule. What are its important properties?
Q 5.
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
Q 6.
Name the international organization who approves names of elements.
Q 7.
Write the chemical symbols of the following:
(i) Gold
(ii) Iron
(iii) Chlorine
(iv) Mercury
Q 8.
Define 'formula mass' of a compound.
Q 9.
How many atoms are there in 0.25 mole of hydrogen ?
Q 10.
Name one element each which forms diatomic and tetra atomic molecule.
Q 11.
Which of the following statements is NOT true about an atom?
(a) Atoms are the building blocks from which molecules and ions are formed.
(b) Atoms cannot exist independently.
(c) Atoms are neutral in nature
(d) Atoms combine together to form matter that we can see, feel or touch.
Q 12.
Fill in the blanks:
The chemical symbol of mercury is _________.
Q 13.
What is the mass of 5 moles of sodium carbonate (Na
) ?(Atomic masses : Na = 23 u ; C = 12 u ; O = 16 u)
Q 15.
Write down the names of compounds represented by the following formulae:
(i) Al
2(SO
4)
3 (ii) CaCl
2 (iii) K
2SO
4 (iv) KNO
3 (v) CaCO
3
Q 16.
Convert 12 g of oxygen gas into moles.
Q 17.
(i) State the law of constant proportions.
(ii) Show that water illustrates the law of constant proportions.
Q 18.
Fill in the blanks:
The mass of 5 moles of ammonia (NH
3) is __________.
Q 19.
Name the element having following Latin names
(i) Stibium
(ii) Cuprum
(iii) Argentum
(iv) Natrium
(v) Stannum
(vi) Wolfram
(vii) plumbum
(viii) Kalium
Q 20.
Fill in the blanks:
According to law of definite proportions, in a chemical substance the elements are always present in __________ proportions by mass.
Q 21.
Give four examples of diatomic molecules.
Q 22.
What are ionic compounds?
Q 23.
Who established the two important laws of chemical combinations?
Q 24.
Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?
Q 25.
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 26.
What are polyatomic ions? Give examples?
Q 27.
Calculate the number of molecules of phosphorus (P
4) present in 31 gram of phosphorus.
Q 28.
State Law of constant proportions. Explain with an example.
Q 30.
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 31.
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 32.
Convert into moles: (a) 12 g of oxygen gas (b) 20 g of water (c) 22 g of carbon dioxide
Q 34.
Fill in the blanks:
The mass of 1 mole of a substance is called its _____________.
Q 35.
Fill in the blanks:
An element has only one type of __________.
Q 36.
Based on type of substance, how molecules are classified?
Q 37.
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 38.
Give the names of the elements present in the following compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
Q 39.
Name one element which forms diatomic and triatomic molecule.
Q 40.
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 41.
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 42.
Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
Q 43.
Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
Q 44.
Fill in the blanks:
Atomic radius is measured in __________.
Q 45.
How will you define chemical symbol?
Q 46.
What is the significance of a chemical symbol?
Q 47.
(i) What is the chemical formula of Water molecule?
(ii) What is its atomicity?
(iii) Calculate the ratio of masses of atoms of elements present in water molecule.
(iv) Calculate the ratio by number of atoms of elements present in water molecule.
Q 48.
What is gram-molecular mass of a substance?
Q 49.
(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 50.
What is the law of conservation of mass?
