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.
What is the chemical symbol for iron?
Q 2.
Define molecule. What are its important properties?
Q 3.
(i) State the law of constant proportions.
(ii) Show that water illustrates the law of constant proportions.
Q 4.
Fill in the blanks:
The atomic mass of sodium is 23. The gram atomic mass of sodium is _________.
Q 5.
What is gram-atomic mass of an element?
Q 6.
Name the international organization who approves names of elements.
Q 7.
Fill in the blanks:
The chemical symbol of mercury is _________.
Q 8.
Fill in the blanks:
One mole of water contains ______________ molecules.
Q 9.
Fill in the blanks:
The mass of 5 moles of ammonia (NH
3) is __________.
Q 10.
Write the chemical symbols of the following:
(i) Gold
(ii) Iron
(iii) Chlorine
(iv) Mercury
Q 11.
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 12.
Name one element each which forms diatomic and tetra atomic molecule.
Q 13.
Give four examples of diatomic molecules.
Q 14.
Give the names of the elements present in the following compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
Q 15.
Define 'formula mass' of a compound.
Q 16.
How many atoms are there in 0.25 mole of hydrogen ?
Q 17.
Calculate the number of molecules of phosphorus (P
4) present in 31 gram of phosphorus.
Q 18.
Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
Q 19.
Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
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.
What are ionic compounds?
Q 22.
Convert 12 g of oxygen gas into moles.
Q 23.
What is the mass of 5 moles of sodium carbonate (Na
) ?(Atomic masses : Na = 23 u ; C = 12 u ; O = 16 u)
Q 25.
Write down the formulae of
(i) sodium oxide
(ii) aluminium chloride
(iii) sodium suphide
(iv) magnesium hydroxide
Q 26.
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 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 is gram-molecular mass of a substance?
Q 29.
How many moles are there in 4.6 gms of Sodium(Na)?
Q 30.
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 31.
(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 32.
Explain with example that law of conservation of mass is valid for chemical reactions.
Q 33.
Fill in the blanks:
Mass of 2 Hydrogen atoms is __________.
Q 34.
Based on type of substance, how molecules are classified?
Q 35.
Give examples of triatomic molecules.
Q 36.
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 37.
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 38.
Write down the formulae for the following compounds :
(a) Calcium oxide
(b) Magnesium hydroxide
Q 39.
Who established the two important laws of chemical combinations?
Q 40.
What are the postulates of Dalton's atomic theory?
Q 41.
Which element has the smallest atom in size?
Q 43.
Define mole. What is its significance?
Q 44.
If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?
Q 45.
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 46.
Find out number of atoms in 15 moles of He.
Q 47.
Fill in the following blanks with suitable words :
(a) In water, the proportion of oxygen and hydrogen is............by mass.
(b) In a chemical reaction, the sum of the masses of the reactants and the products remains
Q 48.
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 49.
State Law of constant proportions. Explain with an example.
Q 50.
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?
