Vidaara.orgClass 9 · Chemistry
CodeVID-C9-03-T1-01
Assignment — Laws of Chemical Combination & Dalton's Theory
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Show all working for Sections B, C and D.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
The law of constant proportions was proposed by:
- A.Lavoisier
- B.Proust
- C.Dalton
- D.Bohr
2.
In carbon dioxide, carbon and oxygen are present in the mass ratio:
- A.1 : 8
- B.3 : 8
- C.8 : 3
- D.1 : 2
3.
According to Dalton, atoms of the same element are:
- A.different in mass
- B.identical in mass and properties
- C.always charged
- D.divisible
4.
A balanced chemical equation is a direct consequence of the law of:
- A.constant proportions
- B.conservation of mass
- C.multiple proportions
- D.definite volumes
5.
Atoms combine to form compounds in ratios that are:
- A.large fractions
- B.random
- C.small whole numbers
- D.always 1 : 1
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
State the law of conservation of mass and the law of constant proportions.
7.
Hydrogen and oxygen combine in the ratio 1 : 8 by mass. How much oxygen is needed to react with 3 g of hydrogen?
8.
List any two postulates of Dalton's atomic theory.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
10.6 g of sodium carbonate reacts with 12.0 g of ethanoic acid to give 4.4 g of carbon dioxide, 1.8 g of water and a salt. Find the mass of the salt using the law of conservation of mass.
10.
A compound of magnesium and oxygen contains Mg and O in the ratio 3 : 2 by mass. Find the masses of Mg and O in 20 g of the compound.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
State Dalton's atomic theory and explain how it accounts for the law of conservation of mass and the law of constant proportions.
Answer Key
Section A — Multiple Choice Questions
- (B) Proust
- (B) 3 : 8
- (B) identical in mass and properties
- (B) conservation of mass
- (C) small whole numbers
Section B — Short Answer (2 marks)
- Conservation of mass: total mass of reactants equals total mass of products. Constant proportions: a pure compound always has the same elements in the same fixed mass ratio.
- 1 g H needs 8 g O, so 3 g H needs 3 × 8 = 24 g of oxygen.
- Matter is made of indivisible atoms; atoms of a given element are identical in mass and properties (any two valid postulates accepted).
Section C — Short Answer (3 marks)
- Mass of reactants = 10.6 + 12.0 = 22.6 g. Mass of salt = 22.6 − (4.4 + 1.8) = 22.6 − 6.2 = 16.4 g.
- Total parts = 3 + 2 = 5. Mg = (3/5) × 20 = 12 g; O = (2/5) × 20 = 8 g.
Section D — Long Answer (5 marks)
- Dalton's theory: all matter is made of indivisible atoms; atoms cannot be created or destroyed in a reaction; all atoms of one element are identical; atoms of different elements differ; atoms combine in small whole-number ratios; a compound has a fixed number and kind of atoms. Conservation of mass follows because atoms are only rearranged, never created or destroyed, so the total mass is unchanged. Constant proportions follow because a compound always contains the same atoms in the same fixed whole-number ratio, which fixes the mass ratio of its elements regardless of source.
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