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 mass of:
(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?
Q 2.
Write the full form of IUPAC.
Q 3.
An element Z has a valency of 3. What is the formula of oxide of Z ?
Q 4.
What is the numerical value of Avogadro number ?
Q 5.
How many moles are there in 4.6 gms of Sodium(Na)?
Q 6.
What name is given to the number 6.022 x 10
Q 7.
How many moles of calcium carbonate (CaCO
Q 8.
If one mole of nitrogen molecules weighs 28 g, calculate mass of one molecule of nitrogen in grams.
Q 9.
Fill in the blanks:
The _______________ of a compound is a symbolic representation of its composition.
Q 10.
The molecular formula of glucose is C
Q 11.
What is an ion ? How is an ion formed ? Explain with the help of two examples of different ions.
Q 12.
What is the difference between a cation and an anion ? Explain with examples. Using this information, write down the formulae of:
(i) Sodium sulphide
(ii) Copper nitrate
Q 13.
How many moles are 3.6 g of water ?
Q 14.
What is the mass of 0.2 mole of oxygen atoms ?
Q 15.
Is there any exception to law of conservation of mass?
Q 16.
What is relative atomic mass of an element? How it is related to atomic mass unit?
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.
What is meant by atomicity ? Explain with two
Q 19.
What do we call those particles which are formed :
(a)by the gain of electrons by atoms ?
(b)by the loss of electrons by atoms ?
Q 20.
How many moles are there in 34.5 g of sodium ? (Atomic mass of Na = 23 u)
Q 21.
If 16 g of oxygen contains 1 mole of oxygen atoms, calculate the mass of one atom of oxygen.
Q 22.
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 23.
Dalton's atomic theory says that atoms are indivisible. Is this statement still valid ? Give reasons for answer.
Q 24.
Calculate the number of molecules in 4 g of oxygen.
Q 25.
Calculate the mass in grams of 0.17 mole of hydrogen sulphide, H
Q 26.
Show by means of calculations that 5 moles of CO, and 5 moles of H
do not have the same mass. How much is the difference in their masses ?
Q 27.
(a) What is meant by 'a mole of carbon atoms' ?
(b)(b) Which has more atoms, 50 g of aluminium or 50 g of iron ? Illustrate your answer with the help of calculations.
(Atomic masses : A1 = 27 u ; Fe = 56 u)
Q 28.
(a) Define gram atomic mass of a substance.How much is the gram atomic mass of oxygen ?
(b) How many moles of oxygen atoms are present in one mole of the following compounds ?
(i)Al
(ii) co
2(iii) C1
(iv) H
(p)A1
Q 29.
Explain with example that law of conservation of mass is valid for chemical reactions.
Q 30.
State Law of constant proportions. Explain with an example.
Q 31.
Give four examples of diatomic molecules.
Q 32.
Calculate the molecular mass of ethanoic acid, CH
Q 33.
Calculate the molecular mass of chloroform (CHC1
Q 34.
Name the elements water is made of. What are the valencies of these elements ? Work out the chemical formula for water.
Q 35.
An element X has a valency of 4 whereas another element Y has a valency of 1. What will be the formula of the compound formed between X and Y ?
Q 36.
Name the following compounds. Also write the symbols/formulae of the ions present in them :
(a) CuSO
Q 37.
Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
Q 38.
Fill in the blanks:
Atoms can be observed using ____________ Microscope.
Q 39.
How will you define chemical symbol?
Q 40.
Fill in the blanks:
Hydrogen and oxygen combines to form H
2O and H
2O
2. These two compounds obey the law of ____________________.
Q 41.
When 'mole' was chosen internationally standard way to express larger number of chemical units?
Q 42.
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 43.
What are the building blocks of matter ?
Q 44.
Calculate the molecular masses of the following :
(a) Hydrogen, H
Q 45.
State whether the following statements are true or false :
(a)A sodium ion has positive charge because it has more protons than a neutral atom
(b)A chloride ion has negative charge because it has more electrons than a neutral atom.
Q 46.
Work out the formulae for the following compounds :
(a) Sodium oxide
Q 47.
What are the postulates of Dalton's atomic theory?
Q 48.
What is the atomic mass unit?
Q 49.
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 50.
What is the molecular mass of a substance?
