Question:
(i) State the law of constant proportions.
(ii) Show that water illustrates the law of constant proportions.
Answer:
In a chemical substance the elements are always present in definite proportions by
mass, no matter how that chemical compound is prepared.
Example: Water always contains Hydrogen and Oxygen in the same proportion i.e. the ratio of
the mass of hydrogen to the mass of oxygen is always 1:8, whatever the source of water. Whether water comes from melting of ice or by condensation of steam (physical changes) or by it is produced through chemical reactions.
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.
Define molecule. What are its important properties?
Q 5.
Name one element each which forms diatomic and tetra atomic molecule.
Q 6.
Define 'formula mass' of a compound.
Q 7.
Name the international organization who approves names of elements.
Q 8.
Write the chemical symbols of the following:
(i) Gold
(ii) Iron
(iii) Chlorine
(iv) Mercury
Q 9.
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 10.
Fill in the blanks:
According to law of definite proportions, in a chemical substance the elements are always present in __________ proportions by mass.
Q 11.
Fill in the blanks:
The chemical symbol of mercury is _________.
Q 12.
Give four examples of diatomic molecules.
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 14.
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 15.
Fill in the blanks:
The mass of 5 moles of ammonia (NH
3) is __________.
Q 17.
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 18.
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 19.
How many atoms are there in 0.25 mole of hydrogen ?
Q 20.
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 21.
Based on type of substance, how molecules are classified?
Q 22.
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 23.
Convert 12 g of oxygen gas into moles.
Q 24.
Who established the two important laws of chemical combinations?
Q 25.
(i) State the law of constant proportions.
(ii) Show that water illustrates the law of constant proportions.
Q 26.
Fill in the blanks:
The mass of 1 mole of a substance is called its _____________.
Q 27.
Fill in the blanks:
_________ are building block of all matter.
Q 28.
Name the element having following Latin names
(i) Stibium
(ii) Cuprum
(iii) Argentum
(iv) Natrium
(v) Stannum
(vi) Wolfram
(vii) plumbum
(viii) Kalium
Q 29.
How will you define chemical symbol?
Q 30.
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 31.
Fill in the blanks:
An element has only one type of __________.
Q 32.
Fill in the blanks:
Hydrogen and oxygen combines to form H
2O and H
2O
2. These two compounds obey the law of ____________________.
Q 33.
(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 34.
What are polyatomic ions? Give examples?
Q 35.
Calculate the number of molecules of phosphorus (P
4) present in 31 gram of phosphorus.
Q 36.
State Law of constant proportions. Explain with an example.
Q 37.
What is the significance of a chemical symbol?
Q 38.
What is relative atomic mass of an element? How it is related to atomic mass unit?
Q 39.
Fill in the blanks:
The chemical symbol of flourine is ________.
Q 40.
What is valency of an element?
Q 41.
Write down the formulae of
(i) sodium oxide
(ii) aluminium chloride
(iii) sodium suphide
(iv) magnesium hydroxide
Q 42.
Define mole. What is its significance?
Q 43.
How many moles are there in 4.6 gms of Sodium(Na)?
Q 44.
If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?
Q 45.
Find out number of atoms in 15 moles of He.
Q 46.
Calculate the number of molecules of sulphur (S
8) present in 16 g of solid sulphur.
Q 47.
Dalton's atomic theory says that atoms are indivisible. Is this statement still valid ? Give reasons for answer.
Q 48.
Calculate the molecular masses of the following compounds :
(a) Methanol, CH
Q 49.
Write down the formulae for the following compounds :
(a) Calcium oxide
(b) Magnesium hydroxide
Q 50.
An element Z has a valency of 3. What is the formula of oxide of Z ?
