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Atoms and Molecules · Gujarat Board (GSEB) STD 9 (GSEB - English Medium). 75 questions.

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Question 11 Mark
An element Y has a valency of 4. Write the formula for its:
  1. Chloride.
  2. Oxide.
  3. Sulphate.
  4. Carbonate.
  5. Nitrate.
Answer
  1. YCl4
  2. YO0
  3. Y(SO4)2
  4. Y(CO3)2
  5. Y(NO3)4
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Question 21 Mark
Name the elements represented by the following symbols:
Hg, Pb, Au, Ag, Sn.
Answer
Symbols
Elements
Hg
Mercury
Pb
Lead
Au
Silver
Ag
Silver
Sn
Tin
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Question 31 Mark
Work out the formulae for the following compounds:
Sodium oxide.
Answer
Sodium oxide:

$\ \text{Symbols}:\ \ \ \ \ \text{Na}\ \ \ \ \ \ \ \text{O}\\ {\text{Valencies}}:\ +1\ \ \ \ -2$

Thus, the formula of sodium oxide is Na2O

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Question 41 Mark
Write the full form of IUPAC.
Answer
The full form of IUPAC is International Union of Pure and Applied Chemistry.
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Question 61 Mark
Work out the formula for magnesium hydrogencarbonate.
Answer
$\ \text{Symbols}:\ \ \ \ \ \text{Mg}\ \ \ \ \ \ \text{HCO}_3\\ {\text{Valencies}}:\ \ +2\ \ \ \ \ \ \ -1$
Thus, the resulting compound is Mg(HCO3)2
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Question 81 Mark
Give the names and symbols of five familiar substances which you think are elements.
Answer
Elements
Symbols
Hydrogen
H
Helium
He
Lithium
Li
Beryllium
Be
Boron
B
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Question 101 Mark
Write the relation between nanometer and metre.
Answer
$1 \text{ nanometre} = \frac{1}{109}\text{metre}$
or
$1 \text{ nanometre} = 10^{-9}\text{ metre}$
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Question 121 Mark
What is the difference between 2H and H2?
Answer
2H represents two separate atoms of hydrogen and H2 represents one molecule of hydrogen.
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Question 141 Mark
An element Z has a valency of 3. What is the formula of oxide of Z?
Answer
Valency of element Z = 3
Valency of oxygen = 2
So, formula of oxide of element = Z2O3
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Question 161 Mark
If the valency of carbon is 4 and that of sulphur is 2, work out the formula of the compound formed by the combination of carbon with sulphur. What is the name of this compound?
Answer
$\ \text{Symbols}:\ \ \ \ \ \text{C}\ \ \ \ \ \ \ \ \ \text{S}\\ {\text{Valencies}}:\ +4\ \ \ \ -2$
Name and formula of the resulting compound is Carbon disulphide, CS2
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Question 171 Mark
What do we call those particles which are formed:
By the loss of electrons by atoms?
Answer
Cations are formed by the loss of electrons by atoms.
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Question 181 Mark
What is meant by 'a mole of carbon atoms'?
Answer
The amount of substance, whose mass in grams is numerically equal to its atomic mass, is called gram atomic mass of that substance. Gram atomic mass of oxygen is 16g.
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Question 201 Mark
The molecular formula of glucose is C6H12O6. Calculate its molecular mass. (Atomic masses: C = 12 u; H = 1 u; O = 16 u)
Answer
Molecular formula of glucose = C6H12O6
Molecular mass of glucose = (6 × C) + (12 × H) + (6 × O) = 72 + 12 + 96 = 180u
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Question 211 Mark
How many atoms are present in one gram atomic mass of a substance?
Answer
One mole of atoms (6.002 × 1023 atoms)
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Question 221 Mark
Name the scientist who gave:
Law of conservation of mass.
Answer
Law of conservation of mass was proposed by Antoine Lavoisier.
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Question 231 Mark
An element X has a valency of 2. Write the simplest formula for:
oxide of the element.
Answer
Oxide of element: As valency of oxygen is -2 and that of element is +2 so, the resulting compound is XO.
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Question 251 Mark
Name the element used as a standard for atomic mass scale.
Answer
Carbon is used as a standard for atomic mass scale.
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Question 261 Mark
Define atomic mass of an element.
Answer
The atomic mass of an element is the relative mass of its atom as compared with the mass of a carbon-12 atom taken as 12 units.
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Question 281 Mark
Work out the formulae for the following compounds:
Calcium carbonate.
Answer
Calcium carbonate:

$\ \text{Symbols}:\ \ \ \ \ \text{Ca}\ \ \ \ \ \ \ \ \ \text{CO}_3\\ {\text{Valencies}}:\ \ +2\ \ \ \ \ \ \ \ -2$

Thus, the resulting compound is CaCO3

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Question 291 Mark
Stat the chemical symbols for the following elements:
Sodium, Potassium, Iron, Copper, Mercury, Silver.
Answer
Elements
Symbols
Sodium
Na
Potassium
K
Iron
Fe
Copper
Cu
Mercury
Hg
Silver
Ag
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Question 301 Mark
Explain the difference between 2N and N2.
Answer
2N represents two separate atoms of nitrogen and N2 represents one molecule of nitrogen.
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Question 311 Mark
What is the mass of 0.2 mole of oxygen atoms?
Answer
Mass of 0.2 moles of oxygen atoms = 0.2 × 16 = 3.2g
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Question 321 Mark
What is meant by ‘a mole of carbon atoms’?
Answer
A mole of carbon atoms means a carbon sample weighing 12g and containing 6.022 × 1023 carbon atoms.
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Question 331 Mark
  1. In what form does oxygen gas occur in nature?
  2. In what form do noble gases occur in nature?
Answer
  1. Oxygen gas occurs as a diatomic molecule in nature.
  2. Noble gases occur as mono atomic gases in nature.
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Question 341 Mark
Calculate the molecular mass of chloroform (CHCl3).
(Atomic masses: C = 12u; H = 1u; Cl = 35.5u)
Answer
Molecular mass of chloroform (CHCl3)
= 1 × C + 1 × H + 3 × Cl
= (12 + 1 + 106.5)u
= 119.5u
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Question 361 Mark
Name the law of chemical combination:
Which was given by Lavoisier.
Answer
The law of chemical combination proposed by Lavoisier is well known as the law of conservation of mass.
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Question 371 Mark
Convert 12g of oxygen gas into moles.
Answer
Given mass of oxygen = 12g
Molar mass of oxygen = 32g
No. of moles $=\frac{\text{Given mass}}{\text{Molar mass}}=\frac{12\text{g}}{32\text{g}}=0.375$
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Question 381 Mark
An element X of valency 3 combines with another element Y of valency 2. What will be the formula of the compound formed?
Answer
$\ \text{Symbols}:\ \ \ \ \ \text{X}\ \ \ \ \ \ \text{ Y}\\ {\text{Valencies}}:\ \ \ \ \ 3\ \ \ \ \ \ \ 2$
Thus, the resulting compound is X2Y3
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Question 391 Mark
Calculate the molecular mass of ethanoic acid, CH3COOH.
(Atomic masses: C = 12u; H = 1u; O = 16u)
Answer
Molecular mass of ethanoic acid (CH3COOH)
= 1 × C + 3 × H + 1 × C + 2 × O + 1 × H
= 12 + 3 + 12 + 32 + 1
= 60u
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Question 401 Mark
Write down the formulae for the following compounds:
Magnesium hydroxide
Answer
Magnesium hydroxide: Mg(OH)2
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Question 411 Mark
Calculate the formulae masses of the following compounds:
Aluminium oxide, Al2O3
(Given: Atomic masses: Na = 23u; O = 16u; Al = 27u)
Answer
Molecular Mass of Al2O3 = (2 × Al) + (3 × O)
= (2 × 27) + (3 × 16)
= 102u
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Question 421 Mark
Calculate the molecular mass of nitric acid, HNO3.
(Atomic masses: H = 1u; N = 14u; O = 16u)
Answer
Molecular mass of Nitric acid (HNO3)
= 1 × H + 1 × N + 3 × O
= (1 + 14 + 48)u
= 63u
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Question 431 Mark
Define atomic mass unit. What is its symbol?
Answer
One atomic mass unit is defined as exactly one-twelfth the mass of an atom of carbon-12. Its symbol is 'u'.
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Question 441 Mark
Name the law of chemical combination:
Which was given by Proust.
Answer
The law of chemical combination proposed by Proust is well known as the law of constant proportions.
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Question 451 Mark
What is meant by saying that ‘the atomic mass of oxygen is 16’?
Answer
It means that one atom of oxygen is 16 times heavier than 1/ 12 of a C-12 atom.
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Question 461 Mark
Calculate the molecular mass of hydrogen bromide (HBr).
(Atomic masses: H = 1u; Br = 80u)
Answer
Molecular mass of hydrogen bromide (HBr)
= 1 × H + 1 × Br
= (1 + 80)u
= 81u
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Question 481 Mark
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?
Answer
As the valency of element X is 4 and that of Y is 1, so the resulting formula is XY4.
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Question 491 Mark
Calculate the formulae masses of the following compounds:
Sodium oxide, Na2O
(Given: Atomic masses: Na = 23u; O = 16u; Al = 27u)
Answer
Molecular mass of Na2O = (2 × Na) + (1 × O)
= (2 × 23) + (1 × 16)
= 62u
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Question 501 Mark
Name the scientist who gave:
Law of constant proportions.
Answer
Law of constant proportions was proposed by Joseph Proust.
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