Question:
State Law of constant proportions. Explain with an example.
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.
Define 'formula mass' of a compound.
Q 2.
(i) State the law of constant proportions.
(ii) Show that water illustrates the law of constant proportions.
Q 3.
What is the chemical symbol for iron?
Q 4.
What is gram-atomic mass of an element?
Q 5.
Write the chemical symbols of the following:
(i) Gold
(ii) Iron
(iii) Chlorine
(iv) Mercury
Q 6.
Name one element each which forms diatomic and tetra atomic molecule.
Q 7.
How many atoms are there in 0.25 mole of hydrogen ?
Q 8.
Fill in the blanks:
The atomic mass of sodium is 23. The gram atomic mass of sodium is _________.
Q 9.
Define molecule. What are its important properties?
Q 10.
What are ionic compounds?
Q 11.
Explain with example that law of conservation of mass is valid for chemical reactions.
Q 12.
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 13.
Fill in the blanks:
The chemical symbol of mercury is _________.
Q 14.
Fill in the blanks:
One mole of water contains ______________ molecules.
Q 15.
Give the names of the elements present in the following compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
Q 16.
What is the molecular mass of a substance?
Q 17.
Find out number of atoms in 15 moles of He.
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.
Convert 12 g of oxygen gas into moles.
Q 20.
Fill in the blanks:
Mass of 2 Hydrogen atoms is __________.
Q 21.
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 22.
How many moles are there in 4.6 gms of Sodium(Na)?
Q 23.
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 24.
State whether the following statement is true or false :The symbol of element cobalt is CO.
Q 25.
What is the mass of 5 moles of sodium carbonate (Na
) ?(Atomic masses : Na = 23 u ; C = 12 u ; O = 16 u)
Q 26.
(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 27.
Who established the two important laws of chemical combinations?
Q 28.
Fill in the blanks:
The mass of 5 moles of ammonia (NH
3) is __________.
Q 29.
Fill in the blanks:
According to law of definite proportions, in a chemical substance the elements are always present in __________ proportions by mass.
Q 30.
What is valency of an element?
Q 31.
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 32.
What is gram-molecular mass of a substance?
Q 33.
Calculate the molecular masses of the following compounds :
(a) Methanol, CH
Q 34.
An element X has a valency of 4 whereas another element Y has a valency of 1. What will be the formula of the compound formed between X and Y ?
Q 35.
If 12 g of carbon is burnt in the presence of 32 g of oxygen, how much carbon dioxide will be formed?
Q 36.
State Law of constant proportions. Explain with an example.
Q 37.
Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
Q 38.
Fill in the blanks:
The mass of 1 mole of a substance is called its _____________.
Q 39.
Name the international organization who approves names of elements.
Q 40.
Which element has the smallest atom in size?
Q 41.
What is the atomic mass unit?
Q 42.
Fill in the blanks:
According to Dalton's atomic theory, atoms of different elements differ in ______, size and chemical properties.
Q 43.
Based on type of substance, how molecules are classified?
Q 44.
Give four examples of diatomic molecules.
Q 45.
Who introduced the term 'mole' in chemistry?
Q 46.
If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?
Q 47.
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 48.
Calculate the molecular mass of chloroform (CHC1
Q 49.
Calculate the molecular mass of hydrogen bromide (HBr).
(Atomic masses: H=1; Br= 80 U)
Q 50.
What do we call those particles which have more or less electrons than the normal atoms ?
