Question:
What is meant by the symbol of an element ? Explain with examples.
Answer:
A 'symbol' is a thing (like a 'sketch' or 'letter', etc.) which represents or stands for something else.When we say 'the symbols of elements', it means 'the symbols of the atoms of the elements' (because elements are made up of atoms).
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 gram-atomic mass of an element?
Q 3.
What is the chemical symbol for iron?
Q 4.
Write the chemical symbols of the following:
(i) Gold
(ii) Iron
(iii) Chlorine
(iv) Mercury
Q 5.
Name the international organization who approves names of elements.
Q 6.
Define 'formula mass' of a compound.
Q 7.
Magnesium and oxygen combine in the ratio of 3 : 2 by mass to form magnesium oxide. How much oxygen is required to react completely with 12 g of magnesium?
Q 8.
How many atoms are there in 0.25 mole of hydrogen ?
Q 9.
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 10.
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 11.
Fill in the blanks:
According to law of definite proportions, in a chemical substance the elements are always present in __________ proportions by mass.
Q 12.
Name one element each which forms diatomic and tetra atomic molecule.
Q 13.
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 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.
State Law of constant proportions. Explain with an example.
Q 17.
What are the postulates of Dalton's atomic theory?
Q 18.
Fill in the blanks:
The mass of 5 moles of ammonia (NH
3) is __________.
Q 19.
Define molecule. What are its important properties?
Q 20.
Fill in the blanks:
The chemical symbol of mercury is _________.
Q 21.
What are polyatomic ions? Give examples?
Q 22.
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 23.
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 24.
Who established the two important laws of chemical combinations?
Q 25.
Fill in the blanks:
Hydrogen and oxygen combines to form H
2O and H
2O
2. These two compounds obey the law of ____________________.
Q 26.
Give four examples of diatomic molecules.
Q 27.
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 28.
What are ionic compounds?
Q 29.
(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 30.
Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
Q 31.
Which element has the smallest atom in size?
Q 32.
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 33.
Find out number of atoms in 15 moles of He.
Q 34.
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 35.
Fill in the blanks:
Atomic radius is measured in __________.
Q 36.
What is the significance of a chemical symbol?
Q 37.
If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?
Q 38.
What name is given to the amount of substance containing 6.022 x 10
Q 40.
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 41.
What is relative atomic mass of an element? How it is related to atomic mass unit?
Q 42.
Fill in the blanks:
An element has only one type of __________.
Q 43.
Fill in the blanks:
The chemical symbol of flourine is ________.
Q 44.
What is valency of an element?
Q 45.
Give the names of the elements present in the following compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
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.
Name one element which forms diatomic and triatomic molecule.
Q 48.
Calculate the number of molecules of sulphur (S
8) present in 16 g of solid sulphur.
Q 49.
Calculate the number of molecules of phosphorus (P
4) present in 31 gram of phosphorus.
Q 50.
Calculate the molecular mass of nitric acid,HNO
