Question:
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?
Answer:
Magnesium (Mg) and Oxygen (O) combine in the ratio = 3:2 to form compound magnesium oxide (MgO). It follows Law of Constant proportions.
⇒ 3g of Mg combines with = 2g of O.
∴ 12g of Mg combines with = 2 × 12/3 = 8g of O.
Atoms and Molecules
Q 1.
What is the chemical symbol for iron?
Q 2.
(i) State the law of constant proportions.
(ii) Show that water illustrates the law of constant proportions.
Q 3.
Fill in the blanks:
The atomic mass of sodium is 23. The gram atomic mass of sodium is _________.
Q 4.
Define molecule. What are its important properties?
Q 5.
Fill in the blanks:
The chemical symbol of mercury is _________.
Q 6.
Fill in the blanks:
One mole of water contains ______________ molecules.
Q 7.
What is gram-atomic mass of an element?
Q 8.
Name the international organization who approves names of elements.
Q 9.
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 10.
Name one element each which forms diatomic and tetra atomic molecule.
Q 11.
Fill in the blanks:
The mass of 5 moles of ammonia (NH
3) is __________.
Q 12.
Write the chemical symbols of the following:
(i) Gold
(ii) Iron
(iii) Chlorine
(iv) Mercury
Q 13.
Give four examples of diatomic molecules.
Q 14.
Define 'formula mass' of a compound.
Q 15.
How many atoms are there in 0.25 mole of hydrogen ?
Q 16.
Give the names of the elements present in the following compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
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 24.
Write down the formulae of
(i) sodium oxide
(ii) aluminium chloride
(iii) sodium suphide
(iv) magnesium hydroxide
Q 25.
What is gram-molecular mass of a substance?
Q 26.
How many moles are there in 4.6 gms of Sodium(Na)?
Q 27.
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 28.
(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 29.
Explain with example that law of conservation of mass is valid for chemical reactions.
Q 30.
Based on type of substance, how molecules are classified?
Q 32.
Give examples of triatomic molecules.
Q 33.
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 34.
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 35.
Write down the formulae for the following compounds :
(a) Calcium oxide
(b) Magnesium hydroxide
Q 36.
Who established the two important laws of chemical combinations?
Q 37.
What are the postulates of Dalton's atomic theory?
Q 38.
Fill in the blanks:
Mass of 2 Hydrogen atoms is __________.
Q 39.
Which element has the smallest atom in size?
Q 41.
Define mole. What is its significance?
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 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 44.
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 45.
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 46.
State Law of constant proportions. Explain with an example.
Q 47.
(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 48.
What are polyatomic ions? Give examples?
Q 49.
What is valency of an element?
Q 50.
How many atoms are present in a
(i) H
2S molecule and
(ii) PO
43- ion?
