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 5 moles of sodium carbonate (Na
) ?(Atomic masses : Na = 23 u ; C = 12 u ; O = 16 u)
Q 2.
Fill in the blanks:
According to law of definite proportions, in a chemical substance the elements are always present in __________ proportions by mass.
Q 3.
What is gram-atomic mass of an element?
Q 4.
Define 'formula mass' of a compound.
Q 5.
Fill in the blanks:
The atomic mass of sodium is 23. The gram atomic mass of sodium is _________.
Q 6.
(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 7.
Give the names of the elements present in the following compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
Q 8.
(i) State the law of constant proportions.
(ii) Show that water illustrates the law of constant proportions.
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.
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 12.
What is the chemical symbol for iron?
Q 13.
Fill in the blanks:
The atomicity of sulphur S
8 is ________.
Q 14.
Name the unit in which the radius of an atom is usually expressed.
Q 15.
Name the following compounds. Also write the symbols/formulae of the ions present in them :
(a) CuSO
Q 16.
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 17.
Fill in the following blanks :
(a)1 mole Contains...........atoms- molecules or ions of a substance
(b)A mole represents an..........number of Particles of a substance.
(c)60 g of carbon element are...........moles of carbon atoms.
(d) 0.5 mole of calcium element has a mass of..........
(e) 64 g of oxygen gas contains..........moles of oxygen atoms.
Q 18.
How many moles of calcium carbonate (CaCO
Q 19.
If one mole of nitrogen molecules weighs 28 g, calculate mass of one molecule of nitrogen in grams.
Q 20.
If 16 g of oxygen contains 1 mole of oxygen atoms, calculate the mass of one atom of oxygen.
Q 21.
Who proposed Law of Definite Proportions (or Law of Constant Composition)?
Q 22.
When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?
Q 23.
Fill in the blanks:
The mass of 1 mole of a substance is called its _____________.
Q 24.
Name the element having following Latin names
(i) Stibium
(ii) Cuprum
(iii) Argentum
(iv) Natrium
(v) Stannum
(vi) Wolfram
(vii) plumbum
(viii) Kalium
Q 25.
Define molecule. What are its important properties?
Q 26.
(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 28.
Write down the formulae of
(i) sodium oxide
(ii) aluminium chloride
(iii) sodium suphide
(iv) magnesium hydroxide
Q 29.
How do we know the presence of atoms if they do not exist independently for most of the elements?
Q 30.
What is gram-molecular mass of a substance?
Q 31.
What are the building blocks of matter ?
Q 32.
What is meant by the symbol of an element ? Explain with examples.
Q 33.
Calculate the molecular mass of hydrogen bromide (HBr).
(Atomic masses: H=1; Br= 80 U)
Q 34.
Work out the formulae for the following compounds :
(a) Sodium oxide
Q 35.
What are (ionic compounds, and (ii) molecular compounds ? Give two examples of each type of compounds.
Q 36.
What name is given to the number 6.022 x 10
Q 37.
Calculate the number of moles in 12.044 x 10
atoms of phosphorus
Q 38.
Calculate the mass m grams of 0.17 mole of hydrogen sulphide, H,S.(Atomic masses : H = 1 U/ S = 32 u)
Q 39.
Calculate the mole ratio of 240 g of calcium and 240 g of magnesium. (Ca = 40 u ; Mg = 24 u)
Q 40.
Explain with example that law of conservation of mass is valid for chemical reactions.
Q 41.
What are the postulates of Dalton's atomic theory?
Q 42.
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 43.
Who proposed the chemical notation based on first two letters of the name of the element?
Q 44.
How will you define chemical symbol?
Q 45.
Fill in the blanks:
Hydrogen and oxygen combines to form H
2O and H
2O
2. These two compounds obey the law of ____________________.
Q 46.
Based on atomicity, how molecules are categorized?
Q 47.
Give four examples of diatomic molecules.
Q 48.
What is meant by the term chemical formula?
Q 49.
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 50.
What is the molecular mass of a substance?
