Question 12 Marks
What is meant by atomicity? Explain with two examples.
AnswerThe 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 $2.$
View full question & answer→Question 22 Marks
What are $(i)$ ionic compounds, and $(ii)$ molecular compounds? Give two examples of each type of compounds.
Answer
- Lonic compounds: The compounds which are formed by combination of metals and non-metals are called ionic compounds. For ex. $CaCl_2$ and $CaCO_3$
- Molecular compounds: These compounds are formed by the combination between two non-metal elements. For ex. $HCl$ and $H_2S$
View full question & answer→Question 32 Marks
Which postulate of Dalton’s atomic theory is the result of law of conservation of mass given by Lavoisier?
AnswerThe 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."
View full question & answer→Question 42 Marks
Calculate the formula masses of the following compounds:
- Calcium chloride
- Sodium carbonate
(Given: Atomic masses: $Ca = 40u; Cl = 35. 5u; Na = 23u; C = 12u; O = 16u)$ Answer
- Formula Mass of Calcium chloride $(CaCl_2)$
$= 1 \times Ca + 2 \times Cl$
$= (40 + 71)u$
$= 111u$
- Formula Mass of Sodium carbonate $(Na_2CO_3)$
$= 2 \times Na + 1 \times C + 3 \times O$
$= (2 \times 23 + 1 \times 12 + 3 \times 16)u$
$= 106u$ View full question & answer→Question 52 Marks
What is the significance of the formula of a substance?
AnswerSignificance of formula of a substance:
- Formula represents the name of the substance.
- Formula represents one molecule of a substance.
- Formula gives the number of atoms of each element present in one molecule.
- Formula also represents one mole of molecules of the substance.
View full question & answer→Question 62 Marks
Define ‘molecular mass’ of a substance.
AnswerThe molecular mass of a substance is the relative mass of its one molecule as compared with a $126CC612$atom. The mass of an atom of $126CC612$ is considered to be $12a.m.u.$
View full question & answer→Question 72 Marks
What is meant by the ‘molar mass’ of a substance? State the unit in which molar mass is usually expressed.
AnswerThe molar mass of the substance is the mass of $1$ mole of that substance. Molar mass is generally expressed in grams or $'g'.$
View full question & answer→Question 82 Marks
What is the mass of $5$ moles of sodium carbonate $(Na_2CO_3)? $(Atomic masses: $Na = 23u; C = 12u; O = 16u)$
Answer$1$ mole of $Na_2CO_3 = 106g$ So, $5 \times$ mole of N$a_2CO_3 = 5 \times 106g = 530g$
View full question & answer→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 $=\frac{34.5}{23}=1.5\text{ moles of sodium}$
View full question & answer→Question 102 Marks
An element $B$ shows valencies of $4$ and $6$. Write the formulae of its two oxides.
AnswerWhen 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 $BO_3$
View full question & answer→Question 112 Marks
How many moles are represented by $100g$ of glucose, $C_6H_{12}O_6? (C = 12u, H = 1u, O = 16u)$
AnswerMolar mass of glucose $= 180g$ 180g of glucose has $= 1 $ mol So, $100g$ of glucose will have $=\frac{1\ \times\ 100}{180}=0.55\text{ moles}$
View full question & answer→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 $= 12\ g$ 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 $=\frac{3.011\ \times\ 10^{24}\times12}{6.022\ \times\ 10^{23}}=60\text{g}$
View full question & answer→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?
AnswerMaharishi Kanada was the ancient Indian philosopher who suggested that matter comprises of very small particles. He named these particles ‘parmanu’.
View full question & answer→Question 142 Marks
What is meant by a chemical formula? Write the formulae of one element and one compound.
AnswerA 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.
View full question & answer→Question 152 Marks
Calculate the molar masses of the following substances. Write the results with proper units.
- Ozone molecule, $O_3$
- Ethanoic acid, $CH_3COOH$
Answer
- Molar mass of o zone $(O_3) = 3 \times $ gram atomic mass of $O = 3 \times 16g = 48$g/ mole
- Molar mass of Ethanoic acid $(CH_3COOH) = 2 \times C + 4 \times H + 2 \times O$
$= (24 + 4 + 32)u$
$= 60$g/ mole View full question & answer→Question 162 Marks
What is the number of molecules in $1.5$ moles of ammonia?
AnswerOne 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.
View full question & answer→Question 172 Marks
How many moles of calcium carbonate $\left( CaCO _3\right)$ are present in $10 g$ of the substance? $( Ca =40 u ; C =12 u ; O =16 u )$
AnswerGive mass of $CaCO_3 = 10g$
Molar mass of $CaCO_3 = 1 \times Ca + 1 \times C + 3 \times O = (40 + 12 + 48)gm = 100gm$
So, no. of moles of $CaCO_3 .$
$=\frac{\text{Given mass}}{\text{Molar mass}}=\frac{10}{100}=0.1\text{ moles}$
View full question & answer→Question 182 Marks
Which part of the Dalton’s atomic theory came from the law of constant proportions given by Proust?
AnswerThe 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."
View full question & answer→Question 192 Marks
Calculate the molecular masses of the following compounds:
- Methanol, $CH_3OH$
- Ethanol, $C_2H_5OH$
Answer
- Molecular mass of Methanol $(CH_3OH) = 1 \times C + 3 \times H + 1 \times O + 1 \times H$
$= (12 + 3 + 16 + 1)u = 32u$
- Molecular mass of Ethanol $(C_2H_5OH) = 2 \times C + 5 \times H + 1 \times O + 1 \times H$
$= (24 + 5 + 16 + 1) = 46u$ View full question & answer→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 $= 6 : 10 = 3 : 5$
View full question & answer→Question 212 Marks
What do the following abbreviation stand for?
- $O$
- $2O$
- $O_2$
- $3O_2$
Answer
- $O^- $ One atom of oxygen.
- $2O^-$ Two separate atoms of oxygen.
- $O_{2^-}$ One molecule of oxygen.
- $3O_{2^-}$Three molecules of oxygen.
View full question & answer→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
- $\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.
- $\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. View full question & answer→Question 232 Marks
Calculate the mass of $12.044 \times 10^{25}$ molecules of oxygen$(O_2).$
AnswerOne 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 $= 6400\ gm = 6.4\ Kg$
View full question & answer→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
- 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.
- 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 $Cl.$ View full question & answer→Question 252 Marks
What is the significance of the formula $H_2O?$
AnswerSignificance of the formula $H_2O:$
- $H_2O$ represents water.
- It represents one molecule of water.
- $H_2O$ also represents $6.022 \times 10^{23}$ molecules of water.
- It represents $18\ gm$ of water.
View full question & answer→Question 262 Marks
Which postulate of Dalton’s atomic theory can explain the law of constant proportions?
AnswerThe 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.
View full question & answer→Question 272 Marks
What is a molecule? Explain with an example.
AnswerA 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.
View full question & answer→Question 282 Marks
What is meant by saying that ‘the molecular mass of oxygen is $32’?$
AnswerThe statement, "the molecular mass of oxygen is $32"$, means that a molecule of oxygen is $32$ times heavier than $1 / 12^{\text {th }}$ of the mass of a $126 CC612$atom.
View full question & answer→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:
- Sodium sulphide.
- Copper nitrate.
Answeri. $\mathrm{Na}_2 \mathrm{~S}$
ii. $\mathrm{Cu}\left(\mathrm{NO}_3\right)_2$
View full question & answer→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.
View full question & answer→Question 312 Marks
If sulphur exists as $S_8$ molecules, calculate the number of moles in $100g$ of sulphur. $(S = 32u)$
AnswerGiven mass of sulphur $= 100\ g$
Molar mass of $S_8 = 32 \times 8\ g = 256\ g$
No. of moles $=\frac{\text{Given mass}}{\text{Molar mass}}$
$=\frac{100}{256}=0.39\text{ moles}$
View full question & answer→Question 322 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 $\mathrm{Mg}^{+2}$
- An anion is formed by the gain of one or more electrons by an atom. For Ex. Chlorine loses one electron to form $\mathrm{Cl}^{-}$
View full question & answer→Question 332 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
$=\frac{28}{6.022\ \times\ 10^{23}}\text{ grams of N}_2$
$=4.648\times10^{23}\text{grams of N}_2$
View full question & answer→Question 342 Marks
What is meant by the symbol of an element? Explain with examples.
AnswerThe 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 $“Ca”.$
View full question & answer→Question 352 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 $=\frac{12.044\ \times\ 10^{25}}{6.022\ \times\ 10^{23} }=200\text{ moles}$
View full question & answer→Question 362 Marks
What is an atom? How do atoms usually exist?
AnswerAn 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.
View full question & answer→Question 372 Marks
Define ‘formula unit’ of an ionic compound. What is the formula unit of:
- Sodium chloride
- Magnesium chloride?
AnswerThe simplest combination of ions that produces an electrically neutral unit, is called 'formula unit' of the ionic compound.
- Formula unit of sodium chloride: $\mathrm{NaCl}$
- Formula unit of magnesium chloride: $\mathrm{MgCl}_2$
View full question & answer→Question 382 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?
AnswerAccording 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.
View full question & answer→Question 392 Marks
Calculate the molecular masses of the following compounds:
- Hydrogen sulphide, $H_2S$
- Carbon disulphide, $CS_2$
Answer
- Molecular mass of hydrogen sulphide $(H_2S)$
$= 2 \times H + 1 \times S$
$= (2 + 32)u$
$= 34u$
- Molecular mass of Carbon disulphide $(CS_2)$
$= 1 \times C + 2 \times S$
$= (12 + 2 \times 32)u$
$= 76u$ View full question & answer→Question 402 Marks
What weight of each element is present in $1.5$ moles of sodium sulphite, $Na_2SO_3$? (Atomic masses: $Na = 23u; S = 32u; O = 16u)$
Answer$1.5$ moles of $Na _2 SO _3$ 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 $=4.5 \times 16 g=72 g$
View full question & answer→Question 412 Marks
How many moles of $O_2$ are there in $1.20 \times 10^{22}$ oxygen molecules$?$
AnswerOne 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}}\text{ moles of O}_2 $
Therefore, $1.02 \times 10^{23}$ molecules of $O_2$ will have
$=\frac{1.2\ \times\ 10^{22}}{6.022\ \times\ 10^{23}}\text{ moles of O}_2$
$=0.0199\text{ moles of O}_2$
View full question & answer→Question 422 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.
View full question & answer→Question 432 Marks
Define gram molecular mass of a substance. How much is the gram molecular mass of oxygen?
AnswerThe 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 $32\ g.$
View full question & answer→Question 442 Marks
Name the elements water is made of. What are the valencies of these elements$?$ Work out the chemical formula for water.
AnswerWater is made up of Hydrogen and oxygen. Valency of hydrogen is $+1;$ Valency of oxygen is $-2.$ Chemical formula of water is $H_2O.$
View full question & answer→Question 452 Marks
Why is it not possible to see an atom even with the most powerful microscope?
AnswerAtoms 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.
View full question & answer→Question 462 Marks
Define mole. What are the two things that a mole represents?
AnswerA 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.
View full question & answer→Question 472 Marks
Is it possible to see atoms these days? Explain your answer.
AnswerYes, $‘THE \ SCANNING \ TUNNELLING \ MICROSCOPE’$ 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.
View full question & answer→Question 482 Marks
What is an ion? How is an ion formed? Explain with the help of two examples of different ions.
AnswerAn 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, $\mathrm{Na}^{+}$, formed by loss of one electron.
- Chloride ion, $\mathrm{Cl}_{-2}$, formed by gain of one electron.
View full question & answer→Question 492 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?
AnswerIf $100$ 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.
View full question & answer→Question 502 Marks
What is the atomicity of the following?
- Oxygen
- Ozone
- Neon
- Sulphur
- Phosphorus
- Sodium
Answer
- Oxygen $= 2$
- Ozone $= 3$
- Neon $= 1$
- Sulphur $= 8$
- Phosphorus $= 4$
- Sodium $= 1$
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