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Science Question Answer (2 Marks)

Atoms and Molecules · Maharashtra Board STD 9 (MSBSHSE - English Medium). 55 questions.

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Question Answer (2 Marks)

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Question 12 Marks
What is meant by atomicity? Explain with two examples.
Answer
The number of atoms present in one molecule of an element is called atomicity of that element.
For example: Atomicity of sodium is 1 and that of nitrogen is 2.
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Question 22 Marks
What are (i) ionic compounds, and (ii) molecular compounds? Give two examples of each type of compounds.
Answer
  1. Lonic compounds: The compounds which are formed by combination of metals and non-metals are called ionic compounds. For ex. $CaCl_2$​​​​​​​ and $CaCO_3$​​​​​​​
  2. Molecular compounds: These compounds are formed by the combination between two non-metal elements. For ex. $HCl$ and $H_2S$
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Question 32 Marks
Which postulate of Dalton’s atomic theory is the result of law of conservation of mass given by Lavoisier?
Answer
The postulate of Dalton's atomic theory, which is the result of the law of conservation of mass proposed by Lavoisier, states that, "Atoms can neither be created nor destroyed."
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Question 42 Marks
Calculate the formula masses of the following compounds:
  1. Calcium chloride
  2. Sodium carbonate
(Given: Atomic masses: $Ca = 40\ u; Cl = 35. 5\ u; Na = 23\ u; C = 12\ u; O = 16\ u$)
Answer
Formula Mass of Calcium chloride $(CaCl_2)$
$= 1 \times Ca + 2 \times Cl$
$= (40 + 71)u$
$= 111\ u$
Formula Mass of Sodium carbonate $(Na_2CO_3)$
$= 2 \times Na + 1 \times C + 3 \times O$
$= (2 \times 23 + 1 \times 12 + 3 \times 16)u$
$= 106\ u$
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Question 52 Marks
What is the significance of the formula of a substance?
Answer
Significance of formula of a substance:
  1. Formula represents the name of the substance.
  2. Formula represents one molecule of a substance.
  3. Formula gives the number of atoms of each element present in one molecule.
  4. Formula also represents one mole of molecules of the substance.
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Question 62 Marks
Define ‘molecular mass’ of a substance.
Answer
The molecular mass of a substance is the relative mass of its one molecule as compared with a 126CC612atom. The mass of an atom of 126CC612 is considered to be 12a.m.u.
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Question 72 Marks
What is meant by the ‘molar mass’ of a substance? State the unit in which molar mass is usually expressed.
Answer
The molar mass of the substance is the mass of 1 mole of that substance. Molar mass is generally expressed in grams or 'g'.
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Question 82 Marks
What is the mass of $5$ moles of sodium carbonate $(Na_2CO_3)$?
(Atomic masses: $Na = 23\ u; C = 12\ u; O = 16\ u$)
Answer
$1$ mole of $Na_2CO_3$= $106\ g$
So, $5$  $\times$ mole of $Na_2CO_3$
$= 5 \times 106\ g$
$= 530g$
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Question 92 Marks
How many moles are there in 34.5g of sodium?
(Atomic mass of Na = 23u)
Answer
1 mole of sodium weighs = 23gm
So, 1gm of sodium will have $=\frac{1}{23}\text{ moles of sodium}$
Therefore, 34.5gm of sodium will have $=\frac{34.5}{23}=1.5\text{ moles of sodium}$
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Question 102 Marks
An element B shows valencies of $4$ and $6$. Write the formulae of its two oxides.
Answer
When the valency shown $B$ is $4$, then
$\ \text{Symbols}:\ \ \ \ \ \text{B}\ \ \ \ \ \ \ \ \ \text{O}\\ {\text{Valencies}}:\ +4\ \ \ \ -2$
The resulting compound is $BO_2$
When the valency shown by $B$ is $6$, then
$\ \text{Symbols}:\ \ \ \ \ \text{B}\ \ \ \ \ \ \ \ \ \text{O}\\ {\text{Valencies}}:\ +6\ \ \ \ -2$
The resulting compound is $BO_3$​​​​​​​
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Question 112 Marks
How many moles are represented by $100\ g$ of glucose, $C _6 H _{12} O _6$ ?
$(C=12 u, H=1 u, O=16 u)$
Answer
Molar mass of glucose $=180\ g$
$180\ g$ of glucose has $=1 mol$
So, 100 g of glucose will have $=\frac{1 \times 100}{180}=0.55$ moles
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Question 122 Marks
Calculate the mass of $3.011 \times 10^{24}$​​​​​​​ atoms of carbon.
Answer
Mass of  $6.022 \times 10^{23}$​​​​​​​ atoms of Carbon $= 12g$
So, Mass of $1$ Carbon atom $=\frac{12}{6.022\ \times\ 10^{23}\text{g}}$
Hence, mass of $3.011 \times 10^{24}$​​​​​​​ atoms of Carbon $=\frac{3.011\ \times\ 10^{24}\times12}{6.022\ \times\ 10^{23}}=60\text{g}$
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Question 132 Marks
Which ancient Indian philosopher suggested that all matter is composed of very small particles? What name was given by him to these particles?
Answer
Maharishi Kanada was the ancient Indian philosopher who suggested that matter comprises of very small particles. He named these particles ‘parmanu’.
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Question 142 Marks
What is meant by a chemical formula? Write the formulae of one element and one compound.
Answer
A chemical formula represents the composition of a molecule of the substance in terms of the symbols of the elements present in the molecule. It is also known as molecular formula.
  • Chemical formula of element– $H_2$ for hydrogen.
  • Chemical formula of compound– $H_2O$ for water.
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Question 152 Marks
Calculate the molar masses of the following substances. Write the results with proper units.
  1. Ozone molecule, $O_3$
  2. Ethanoic acid, $CH_3COOH$
Answer
  1. Molar mass of o zone $\left( O _3\right)=3 \times$ gram atomic mass of $O =3 \times 16 g=48 g / mole$
  2. Molar mass of Ethanoic acid $\left( CH _3 COOH \right)=2 \times C +4 \times H +2 \times O$
$= (24 + 4 + 32)u$
$= 60g/ mole$
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Question 162 Marks
What is the number of molecules in $1.5$ moles of ammonia?
Answer
One mole of ammonia contains $=6.022 \times 10^{23}$ molecules of ammonia.
So, 1.5 moles of ammonia contains $=1.5 \times 6.022 \times 10^{23}$
$=9.033 \times 10^{23}$ molecules of ammonia.
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Question 172 Marks
How many moles of calcium carbonate $(CaCO_3)$ are present in $10\ g$ of the substance?
$(Ca = 40u; C = 12u; O = 16u)$
Answer
Give mass of $CaCO_3 = 10\ g$
Molar mass of $CaCO_3 = 1 \times Ca + 1 \times C + 3 \times O$
$= (40 + 12 + 48)gm = 100\ gm$
So, no. of moles of $CaCO_3$ $=\frac{\text{Given mass}}{\text{Molar mass}}=\frac{10}{100}=0.1\text{ moles}$
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Question 182 Marks
Which part of the Dalton’s atomic theory came from the law of constant proportions given by Proust?
Answer
The postulate of Dalton's atomic theory which came from the law of constant proportions proposed by Proust, states that "The 'number' and 'kind' of atoms in a given compound is fixed."
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Question 192 Marks
Calculate the molecular masses of the following compounds:
  1. Methanol,$CH_3OH$
  2. Ethanol, $C_2H_5OH$
Answer
a. Molecular mass of Methanol $\left( CH _3 OH \right)=1 \times C +3 \times H +1 \times O +1 \times H$
$=(12+3+16+1) u=32 u$
b. Molecular mass of Ethanol $\left( C _2 H _5 OH \right)=2 \times C +5 \times H +1 \times O +1 \times H$
$=(24+5+16+1)=46 u$
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Question 202 Marks
Calculate the mole ratio of 240g calcium and 240g of magnesium.
(Ca = 40u; Mg = 24u)
Answer
240g of calcium has $=\frac{240}{40}=6\text{ moles}$
240g of magnesium has $=\frac{240}{24}=10\text{ moles}$
So, required mole ratio = 6 : 10 = 3 : 5
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Question 212 Marks
What do the following abbreviation stand for?
  1. $O$
  2. $2O$
  3. $O_2$
  4. $3O_2$
Answer
  1. $O^-$ One atom of oxygen.
  2. $2O^-$ Two separate atoms of oxygen.
  3. $O_{2^-}$One molecule of oxygen.
  4. $3O_{2^-}$ Three molecules of oxygen.
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Question 222 Marks
Explain the formation of (i) sodium ion, and (ii) chloride ion, form their respective atoms giving the number of protons and number of electrons in each one of them. What is the reason for positive charge on a sodium ion and a negative charge on a chloride ion?
Answer
  1. $\text{Na (11 protons, 11 electrons)}\xrightarrow{\ \ \ \ \ {-1\text{ electron}}\ \ \ \ \ }\text{Na}^+\text{(11 protons, 10 electrons)}$
The reason for positive charge on sodium is the loss of electron.
  1. $\text{Cl (17 protons, 17 electrons)}\xrightarrow{\ \ \ \ \ {+1\text{ electron}}\ \ \ \ \ }\text{Cl}^-\text{(17 protons, 18 electrons)}$
The reason for negative charge on chlorine is the gain of electron.
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Question 232 Marks
Calculate the mass of $12.044 \times 10^{25}$ molecules of oxygen $(O_2).$
Answer
One mole of $O_2 = 32\ gm$
$6.022 \times 10^{23}$ molecules of $O_2$ have mass = $32\ gm$
So, $12.044 \times 10^{25}$ molecules of $O_2$ will have mass = $6400\ gm = 6.4\ Kg$
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Question 242 Marks
What is the difference between the molecule of an element and the molecule of a compound? Give one example of each.
Answer
  1. The molecule of an element contains two or more similar atoms chemically bonded together.
For example: A molecule of hydrogen element consists of 2 hydrogen atoms combined together.
  1. Whereas the molecule of compound contains two or more different type of atoms chemically bonded together.
For example: The molecule of hydrogen chloride (HCl) contains two different type of atoms, i.e. H and Cl.
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Question 252 Marks
What is the significance of the formula $H_2O$?
Answer
Significance of the formula $H_2O$:
  1. $H_2O$ represents water.
  2. It represents one molecule of water.
  3. $H_2O$ also represents $6.022 \times 10^{23}$ molecules of water.
  4. It represents $18\ gm$ of water.
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Question 262 Marks
Which postulate of Dalton’s atomic theory can explain the law of constant proportions?
Answer
The postulate "The elements consist of atoms having fixed mass, and that the number and kind of atoms of each element in a given compound is fixed" can be used to explain the law of constant proportions.
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Question 272 Marks
What is a molecule? Explain with an example.
Answer
A molecule is an electrically neutral group of two or more atoms chemically bonded together.
For example: Ozone gas has three oxygen atoms combined together, so ozone exists in the form of $O_3$ molecule.
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Question 282 Marks
What is meant by saying that ‘the molecular mass of oxygen is $32$’?
Answer
The statement, "the molecular mass of oxygen is $32$", means that a molecule of oxygen is $32$ times heavier than $1/ 12^{th}$ of the mass of a $126CC612$atom.
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Question 292 Marks
The valencies (or charges) of some of the ions are given below:
Ion
Valency(Charges)
Ion
Valency(Charge)
Sodium ion
1+
 Nitrate ion
1-
Copper ion
2+
Sulphide ion
2-
Using this information, write down the formulae of:
  1. Sodium sulphide.
  2. Copper nitrate.
Answer
  1. $Na_2S$
  2. $Cu(NO_3)_2$
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Question 302 Marks
How many atoms are there in $0.25$ mole of hydrogen?
Answer
$1$ mole of hydrogen contains $= 6.022 \times 10^{23}$​​​​​​​ atoms of hydrogen
So, $0.25$ moles of hydrogen will have $= 6.022 \times 10^{23} \times 0.25 = 1.50 \times 10^{23}$​​​​​​​ atoms of hydrogen.
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MCQ 312 Marks
What do the following denote?
  • A
    $N$
     
  • B
    $2N$
     
  • C
    $N_2$
     
  • D
    $2N_2$
Answer
  1. $N$– One atom of nitrogen.
  2. $2N$– Two separate atoms of nitrogen.
  3. $N_2$– One molecule of nitrogen.
  4. $2N_2$​​​​​​​– Two molecules of nitrogen.
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Question 322 Marks
If sulphur exists as $S_8$​​​​​​​ molecules, calculate the number of moles in $100\ g$ of sulphur.
$(S = 32u)$
Answer
Given mass of sulphur = $100\ g$
Molar mass of $S_8 = 32 \times 8 g$ = $256\ g$
No. of moles $=\frac{\text{Given mass}}{\text{Molar mass}}$
$=\frac{100}{256}=0.39\text{ moles}$
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Question 332 Marks
Name any two laws of chemical combination.
Answer
Two laws of chemical combination are:
  1. Law of conservation of mass.
  2. Law of multiple proportions.
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Question 342 Marks
What is the difference between a cation and an anion? Explain with examples.
Answer
  • A cation is formed by the loss of one or more electrons by an atom. For ex. Magnesium loses $2$ electron to form $Mg^{+2}$
  • An anion is formed by the gain of one or more electrons by an atom. For Ex. Chlorine loses one electron to form $Cl^-$
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Question 352 Marks
If one mole of nitrogen molecules weights $28\ g$, calculate the mass of one molecule of nitrogen in grams.
Answer
$6.022 \times 10^{23}$ molecules of $N_2$ weigh =$28\ gm$
So, $1$ molecule of $N_2$​​​​​​​ will weigh $=\frac{28}{6.022\ \times\ 10^{23}}\text{ grams of N}_2$
$=4.648\times10^{23}\text{grams of N}_2$
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Question 362 Marks
What is meant by the symbol of an element? Explain with examples.
Answer
The symbol of element is the “first letter” or “first letter and another letter” of the English name or Latin name of the element.
For example: symbol of Hydrogen is “H” and symbol of Calcium is “Ca”.
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Question 372 Marks
Calculate the number of moles in $12.044 \times 10^{25}$ atoms of phosphorus.
Answer
$6.022 \times 10^{23}$ atoms of phosphorus contains = $1$ mole of phosphorus
So, $12.044 \times 10^{25}$ atoms of phosphorus will have $=\frac{12.044 \times 10^{25}}{6.022 \times 10^{23}}=200$ moles
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Question 382 Marks
What is an atom? How do atoms usually exist?
Answer
An atom is the smallest particle of an element that can take part in a chemical reaction. They usually exist in combination with the atoms of same element or another element.
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Question 392 Marks
Define ‘formula unit’ of an ionic compound. What is the formula unit of:
  1. Sodium chloride
  2. Magnesium chloride?
Answer
The simplest combination of ions that produces an electrically neutral unit, is called 'formula unit' of the ionic compound.
  1. Formula unit of sodium chloride: $NaCl$
  2. Formula unit of magnesium chloride: $MgCl_2$
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Question 402 Marks
‘If 100 grams of pure water taken from different sources is decomposed by passing electricity, 11 grams of hydrogen and 89 grams of oxygen are always obtained’. Which chemical law is illustrated by this statement?
Answer
According to the given information, water has been taken from different sources and still decomposes into same amount of oxygen and hydrogen, therefore, this is an example of the law of constant proportions.
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Question 412 Marks
Calculate the molecular masses of the following compounds:
  1. Hydrogen sulphide, $H_2S$
  2. Carbon disulphide, $CS_2$
Answer
Molecular mass of hydrogen sulphide $(H_2S)$$= 2 × H + 1 × S$
$= (2 + 32)u$
$= 34u$
Molecular mass of Carbon disulphide $(CS_2)$ $= 1 \times C + 2 \times S$
$= (12 + 2 \times 32)u$
$= 76u$
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Question 422 Marks
What weight of each element is present in $1.5$ moles of sodium sulphite, $Na_2SO_3$?
(Atomic masses: $Na = 23\ u; S = 32\ u; O = 16\ u$)
Answer
$1.5$ moles of $Na_2SO_3$ has $3$ moles of Na, $1.5$ mole of $S$ and $4.5$ moles of $O.$
Thus, mass of sodium $= 3 \times 23 g = 69\ g$
Mass of sulphur $= 1.5 \times 32 = 48\ g$
Mass of oxygen $= 4.5 \times 16 g = 72\ g$
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Question 432 Marks
How many moles of $O_2$ are there in $1.20 \times 10^{22}$ oxygen molecules?
Answer
One mole of $O _2$ contains $=6.022 \times 10^{23}$ molecules of oxygen
So, $1$ molecule of $O _2$ has $=\frac{1}{6.022 \times 10^{23}}$ moles of $O _2$
Therefore, $1.02 \times 10^{23}$ molecules of $O _2$ will have $=\frac{1.2 \times 10^{22}}{6.022 \times 10^{23}}$ moles of $O _2$ $=0.0199$ moles of $O _2$
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Question 442 Marks
Which postulate of Dalton’s atomic theory can explain the law of conservation of mass?
Answer
The postulate "The elements consists of atoms and that atoms can neither be created nor destroyed" can be used to explain the law of conservation of mass.
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Question 452 Marks
Define gram molecular mass of a substance. How much is the gram molecular mass of oxygen?
Answer
The amount of substance whose mass in grams is numerically equal to its molecular mass is called gram molecular mass of that substance. Gram molecular mass of the oxygen is 32g.
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Question 462 Marks
Name the elements water is made of. What are the valencies of these elements? Work out the chemical formula for water.
Answer
Water is made up of Hydrogen and oxygen.
Valency of hydrogen is $+1;$
Valency of oxygen is $-2.$
Chemical formula of water is $H_2O.$
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Question 472 Marks
Why is it not possible to see an atom even with the most powerful microscope?
Answer
Atoms are the building blocks of nature. They are tiniest particles in nature and optical microscopes cannot magnify the image of such a tiny object, so that becomes visible to us. It is possible to watch an atom under an electron microscope.
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Question 482 Marks
Define mole. What are the two things that a mole represents?
Answer
A group of $6.022 \times 10^{23}$ particles (atoms, molecules or ions) of a substance is called a mole of that substance.
One mole represents the amount of a substance equal to its 'GRAM ATOMIC MASS' or 'GRAM MOLECULAR MASS' and $6.022 \times 10^{23}$ no.of particles of the substance.
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Question 492 Marks
Is it possible to see atoms these days? Explain your answer.
Answer
Yes, ‘THE SCANNING TUNNELLING MICROSCOPE’ enables people to see atoms. This microscope can produce computer generated images of the surface of elements which show the individual atoms. The atoms show up as blurred images.
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Question 502 Marks
What is an ion? How is an ion formed? Explain with the help of two examples of different ions.
Answer
An ion is a positively or negatively charged atom (or group of atoms). An ion is formed by the loss or gain of an electrons by an atom, so it contains an unequal number of protons and electrons.
Example:
  • Sodium ion, $Na^+$, formed by loss of one electron.
  • Chloride ion, $Cl_{-2}$, formed by gain of one electron.
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Question 512 Marks
‘If 100 grams of calcium carbonate (whether in the form of marble or chalk) are decomposed completely, then 56 grams of calcium oxide and 44 grams of carbon dioxide are obtained’. Which law of chemical combination is illustrated by this statement?
Answer
If 100 grams of calcium carbonate is decomposed in any form (marble or chalk), 56 grams of calcium oxide and 44 grams of carbon dioxide are obtained. This reaction illustrates the law of conservation of mass.
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Question 522 Marks
What is the atomicity of the following?
  1. Oxygen
  2. Ozone
  3. Neon
  4. Sulphur
  5. Phosphorus
  6. Sodium
Answer
  1. Oxygen = 2
  2. Ozone = 3
  3. Neon = 1
  4. Sulphur = 8
  5. Phosphorus = 4
  6. Sodium = 1
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Question 532 Marks
The valencies (or charges) of some of the ions are given below:
Ion
Valency(Charges)
Ion
Valency(Charge)
Sodium ion
1+
Bromide ion
1-
Ammonium ion
1+
Hydroxide ion
1-
Calcium ion
2+
Sulphate ion
2-
Lead ion
2+
Phosphate ion
3-
Using this information, write down the formulate of the following compounds:
  1. Sodium phosphate.
  2. Ammonium sulphate.
  3. Calcium hydroxide.
  4. Lead bromide.
Answer
  1. Sodium phosphate: $Na_3PO_4$
  2. Ammonium sulphate: $(NH4)_2SO_4$
  3. Calcium Hydroxide: $Ca(OH)_2$
  4. Lead bromide: $PbBr_2$
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Question 542 Marks
If $6\ g$ of oxygen contains $1$ mole of oxygen atoms, calculate the mass of one atom of oxygen.
Answer
$6.022 \times 10^{23}$​​​​​​​ atoms of Oxygen weigh $= 16\ g$
So, mass of $1$ atom of Oxygen $=\frac{16}{6.022\ \times\ 10^{23}}=2.656\times10^{-23}\text{g}$
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Question 552 Marks
Explain the significance of the symbol H.
Answer
Significance of symbol H:
  1. It represents Hydrogen element.
  2. It represents one atom of Hydrogen element.
  3. It represents one mole of Hydrogen atoms.
  4. It represents 2gms of Hydrogen.
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