Question 12 Marks
The difference in the mass of $100$ moles each of sodium atoms and sodium
ions is $5.48002\ g$. Compute the mass of an electron.
AnswerMass of $100$ mole of electrons $= 5.48002\ g$
$\therefore$ Mass of $1$ mole of electron
$=\frac{5.48002}{100}\text{g}$
$\therefore$ Mass of $1$ electron
$=\frac{5.48002}{100\times6.022\times10^{23}}=9.1\times10^{-28}\text{g}=9.1\times10^{-31}\text{kg}$
View full question & answer→Question 22 Marks
Define the atomic mass unit.
AnswerOne atomic mass unit is equal to exactly one-twelfth $(1/12th)$ the mass of one atom of carbon $-12$. The relative atomic masses of all elements have been found with respect to an atom of carbon $-12$.
View full question & answer→Question 32 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 42 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 nonmetals are called ionic compounds. For ex. $\mathrm{CaCl}_2$ and $\mathrm{CaCO}_3$
- Molecular compounds: These compounds are formed by the combination between two nonmetal elements. For ex. $HCl$ and $\mathrm{H}_2 \mathrm{S}$
View full question & answer→Question 52 Marks
Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
Answer The postulate of Dalton’s atomic theory which is a result of the law of conservation of mass is: Atoms are indivisible particles, which can neither be created nor destroyed in a chemical reaction.
View full question & answer→Question 62 Marks
The mass of $1.0505 \times 10^{23}$ molecules of carbon dioxide $\left(\mathrm{CO}_2\right)$,
AnswerMolecular mass of $\mathrm{CO}_2=(12)+2(16)=44 \mathrm{~g} / \mathrm{mol}$
$1$ mole of $\mathrm{CO}_2=6.022 \times 10^{23}$ molecules
Since $6.022 \times 10^{23}$ molecules has mass $=44 \mathrm{~g} / \mathrm{mol}$
$1.0505 \times 10^{23}$ molecules will have mass $=44 \times 1.0505 \times 10236.022 \times 1023=7.6755 \mathrm{~g}$
View full question & answer→Question 72 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 82 Marks
What is the role of valency in the combination of atoms?
Answer Valency is the combining power of an element. Elements in the same group of the periodic table have the samevalency. The valency of an element is related to how many electrons are in the outer shell.
View full question & answer→Question 92 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 102 Marks
What is the significance of the formula of a substance?
Answer Significance 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 112 Marks
Calculate the number of moles of magnesium present in a magnesium ribbon weighing $12\ g$. Molar atomic mass of magnesium is $24g\ mol^{-1}.$
AnswerGiven that molar atomic mass of $Mg = 24g\ mol^{-1}$
$\because 24g$ of $Mg = 1\ mol$
$\therefore$ $12\ g$ of $Mg$ $=\frac{1\times12}{24}=\frac{1}{2}=0.5\text{mol}$
Alternative Method Number of moles of $Mg$ $=\frac{\text{given mass}}{\text{molar mass}}=\frac{12\text{g}}{24\text{g mol}^{-1}}=0.5\text{mol}$
View full question & answer→Question 122 Marks
Predict the total number of atoms in case:
Hydrogen sulphide.
AnswerHydrogen sulphide $\left(\mathrm{H}_2 \mathrm{S}\right)$ :
There are two hydrogen atoms and one sulphur atom.
Total number of atoms $=2 \mathrm{H}+1 \mathrm{~S}$
Hence, there are total $3$ atoms.
View full question & answer→Question 132 Marks
Predict the mass possessed by certain number of oxygen atoms which is equal to the number of molecules present $1g$ of hydrogen gas?
AnswerMass of oxygen atoms = number of molecules present $1g$ of hydrogen gas Number of molecules in $1\text{g} \ \text {H}_2=\frac{1}{2\ }\text{mole}$ $\Rightarrow\frac{1}{2}\times6.022\times10^{23}\text{atoms}$
$\Rightarrow3.01\times10^{23}$
Thus, the mass possessed by certain number of oxygen atoms is $3.01\times10^{23}$
View full question & answer→Question 142 Marks
Predict the total number of atoms in case:
Sodium oxide.
AnswerSodium oxide $\left(\mathrm{Na}_2 \mathrm{O}\right)$ :
There are two sodium atoms and one oxygen atom.
Total number of atoms $=2 \mathrm{Na}+10$
Hence, there are total $3$ atoms.
View full question & answer→Question 152 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 $12$a.m.u.
View full question & answer→Question 162 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 172 Marks
What is the mass of $5$ moles of sodium carbonate $\left(\mathrm{Na}_2 \mathrm{CO}_3\right)$ ? (Atomic masses: $\left.\mathrm{Na}=23 \mathrm{u} ; \mathrm{C}=12 \mathrm{u} ; \mathrm{O}=16 \mathrm{u}\right)$
Answer$1$ mole of $\mathrm{Na}_2 \mathrm{CO}_3=106 \mathrm{~g}$ So, $5 \times$ mole of $\mathrm{Na}_2 \mathrm{CO}_3=5 \times 106 \mathrm{~g}=530 \mathrm{~g}$
View full question & answer→Question 182 Marks
Predict the total number of atoms in case:
Calcium phosphate.
AnswerCalcium phosphate $\left[\mathrm{Ca}_3\left(\mathrm{PO}_4\right) 2\right]$ :
There are three calcium atoms, two phosphorus atoms and eight oxygen atoms.
Total number of atoms $=2 \mathrm{Ca}+2 \mathrm{P}+8 \mathrm{O}$
Hence, there are total $12$ atoms.
View full question & answer→Question 192 Marks
Write the atomicity of the following:
$\mathrm{I}_2$
Answer$\mathrm{I}_2$ : The Atomicity of $\mathrm{I}_2$ is 2 because it contains two atoms in its molecule.
View full question & answer→Question 202 Marks
How many moles are there in $34.5\ g$ of sodium? (Atomic mass of $Na = 23\ u$)
Answer$1$ mole of sodium weighs $= 23\ gm$
So, $1\ gm$ of sodium will have $=\frac{1}{23}\text{ moles of sodium}$
Therefore, $34.5\ gm$ of sodium will have $=\frac{34.5}{23}=1.5\text{ moles of sodium}$
View full question & answer→Question 212 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 $\mathrm{BO}_2$
When the valency shown by $B$ is $6$, then
$\ \text{Symbols}:\ \ \ \ \ \text{B}\ \ \ \ \ \ \ \ \ \text{O}\\ {\text{Valencies}}:\ +6\ \ \ \ -2$
The resulting compound is $\mathrm{BO}_3$
View full question & answer→Question 222 Marks
How many moles are represented by $100\ g$ of glucose, $\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6$ ?
$(\mathrm{C}=12 \mathrm{u}, \mathrm{H}=1 \mathrm{u}, \mathrm{O}=16 \mathrm{u})$
AnswerMolar mass of glucose $= 180\ g$
$180\ g$ of glucose has $= 1\ mol$
So, $100\ g$ of glucose will have $=\frac{1\ \times\ 100}{180}=0.55\text{ moles}$
View full question & answer→Question 232 Marks
Does the solubility of a substance change with temperature? Explain with the help of an example.
AnswerYes, solubility of a substance depends on temperature. The solubility generally * increases with increase in temperature. For example, one can dissolve more sugar in hot water than in cold water. *This may not be true for some solutes.
View full question & answer→Question 242 Marks
If one mole of carbon atoms weighs $12$grams, what is the mass (in grams) of $1$ atom of carbon?
AnswerOne mole of carbon atoms weighs $12\ g$ (Given)
i.e., mass of $1$ mole of carbon atoms $=12 \mathrm{~g}$
Then, mass of $6.022 \times 10^{23}$ number of carbon atoms $=12 \mathrm{~g}$
Therefore, mass of 1 atom of carbon $=12 \div\left(6.022 \times 10^{23}\right)$
$=1.9926 \times 10^{-23} \mathrm{~g}$
View full question & answer→Question 252 Marks
Calculate the mass of $3.011 \times 10^{24}$ atoms of carbon.
AnswerMass of $6.022 \times 10^{23}$ atoms of Carbon $=12 \mathrm{~g}$
So, Mass of $1$ Carbon atom $=\frac{12}{6.022 \times 10^{23} \mathrm{~g}}$
Hence, mass of $3.011 \times 10^{24}$ atoms of Carbon $=\frac{3.011 \times 10^{24} \times 12}{6.022 \times 10^{23}}=60 \mathrm{~g}$
View full question & answer→Question 262 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 272 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– $\mathrm{H}_2$ for hydrogen. for hydrogen.
- Chemical formula of compound– $\mathrm{H}_2 \mathrm{O}$ for water.
View full question & answer→Question 282 Marks
What are polyatomic ions? Give examples.
AnswerA polyatomic ion is a group of atoms carrying a charge (positive or negative). For example, ammonium ion $\big(\text{NH}_{4}^{+}\big),$ hydroxide ion $\big(\text{OH}^-\big),$ carbonate ion $\big(\text{CO}_{3}^{2-}\big),$ sulphate ion $\big(\text{SO}_{4}^{2-}\big).$
View full question & answer→Question 292 Marks
Which one is a larger quantity molar mass or molecular mass? Explain.
AnswerMolar mass is the mass of $1$ mole of molecules of that element. For example: Molar mass of hydrogen is $1\ g$, This means that $1$ mole of hydrogen atoms will have mass around $1\ g$.
View full question & answer→Question 302 Marks
Cinnabar $(HgS)$ is a prominent ore of mercury. How many grams of mercury are present in $225\ g$ of pure $HgS?$ Molar mass of Hg and S are $200.6g\ mol^{–1}$ and $32g\ mol^{–1}$ respectively.
AnswerMolar mass $HgS = 200.6 + 32 = 232.6g\ mole^{-1}$ Mass of Hg in $232.6g$ of $HgS =200.6g$
$\therefore$ Mass of $Hg$ in $225\ g$ of $HgS$
$=\frac{200.6}{232.6}\times225=194.04\text{g}$
View full question & answer→Question 312 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 322 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 332 Marks
How many moles of calcium carbonate $(CaCO_3)$ are present in $10\ g$ of the substance$?$
$(Ca = 40u; C = 12u; O = 16u)$
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 342 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."
View full question & answer→Question 352 Marks
Predict the total number of atoms in case:
Magnesium bromide.
AnswerMagnesium bromide $\left(\mathrm{M}_{\mathrm{g}} \mathrm{Br}_2\right)$ :
There are two bromine atoms and one magnesium atom.
Total number of atoms $=2 \mathrm{Br}+1 \mathrm{Mg}$
Hence, there are total $3$ atoms.
View full question & answer→Question 362 Marks
What is the fraction of the mass of water due to neutrons?
AnswerMass of one mole (Avogadro number) of neutrons ~$1g$ Mass of one neutron $=\frac{1}{\text{Avogadro Number (N)}_\text{A}}\text{g}$
Mass of one molecule of water $=\frac{\text{molar mass}}{\text{N}_\text{A}}=\frac{18}{\text{N}_\text{A}}\text{g}$
There are $8$ neutrons in one atom of oxygen, Mass of one neutrons $=\frac{8}{\text{N}_\text{A}}$ Fraction of mass of water due to netrons $\sim\frac{8}{18}$
View full question & answer→Question 372 Marks
Name any two monovalent cations, divalent cations and trivalent cations. Also name any one compound each one of them make.
Answer$\mathrm{Na}^{+}, \mathrm{K}^{+} \mathrm{NaCl}, \mathrm{KCl} \mathrm{~Ca}^{2+}, \mathrm{Mg}^{2+} \mathrm{CaCl}_2, \mathrm{MgF}_2 \mathrm{Al}^{3+}, \mathrm{Fe}^{3+} \mathrm{AlCl}_3, \mathrm{Fe}_2 \mathrm{O}_3$
View full question & answer→Question 382 Marks
Give postulates of Dalton's atomic theory.
Answer Postulates of Dalton's Atomic Theory This can also be stated as all the atoms of an element have identical mass while theatoms of different elements have different masses. Atoms of different elements combine in fixed ratios to form compounds. Atoms are neither created nor destroyed.
View full question & answer→Question 392 Marks
Predict the total number of atoms in case:
Aluminium hydroxide.
AnswerAluminium hydroxide $\left[\mathrm{Al}(\mathrm{OH})_3\right]$ :
There are three hydrogen and oxygen atoms and one atom of aluminum.
Total number of atoms $=3 \mathrm{H}+3 \mathrm{O}+1 \mathrm{Al}$
Hence, there are total $7$ atoms.
View full question & answer→Question 402 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 412 Marks
A $0.24\ g$ sample of compound of oxygen and boron was found by analysis to contain $0.096\ g$ of boron and $0.144\ g$ of oxygen. Calculate the percentage composition of the compound by weight.
AnswerMass of the given sample compound $= 0.24\ g$
Mass of boron in the given sample compound $= 0.096\ g$
Mass of oxygen in the given sample compound $= 0.144\ g \%$
composition of compound = $ \%$ of boron and $ \%$ of oxygen
Therefore % of boron $= \frac{\text{mass of boron}}{\text{mass of the sample compound}} \times 100$
$= \frac{0.096}{0.24}\times 100 = 40$
Therefore $ \%$ of oxygen $= \frac{\text{mass of oxygen}}{\text{mass of the sample compound}} \times 100$ $= \frac{0.144}{0.24}\times 100 = 60$
View full question & answer→Question 422 Marks
Calculate the mole ratio of $240\ g$ calcium and $240\ g$ of magnesium.
$(Ca = 40u; Mg = 24u)$
Answer$240\ g$ of calcium has $=\frac{240}{40}=6\text{ moles}$
$240\ g$ of magnesium has $=\frac{240}{24}=10\text{ moles}$
So, required mole ratio $= 6 : 10 = 3 : 5$
View full question & answer→Question 432 Marks
What do the following abbreviation stand for?
a. $O$
b. $20$
c. $\mathrm{O}_2$
d. $3 \mathrm{O}_2$
Answera. $\mathrm{O}$ - One atom of oxygen.
b. $20$ - Two separate atoms of oxygen.
c. $\mathrm{O}_2$ - One molecule of oxygen.
d. $3 \mathrm{O}_2$ - Three molecules of oxygen.
View full question & answer→Question 442 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 452 Marks
Verify by calculating that:
$240\ g$ of calcium and $240\ g$ magnesium elements have a mole ratio of $3 : 5$.
Answer$240\ g$ of calcium and $240\ g$ magnesium elements have a mole ratio of $3 : 5$.
Number of moles in $240\ g$ $Ca$ metal,
$=\frac{240}{40}=6$
Number of moles in $240\ g$ $Mg$ metal,
$=\frac{240}{24}=10$
$\therefore$ Mole Ratio $= 6 : 10 = 3 : 5$
View full question & answer→Question 462 Marks
Verify by calculating that:
$5$ moles of $\mathrm{CO}_2$ and $5$ moles of $\mathrm{H}_2 \mathrm{O}$ do not have the same mass.
AnswerMolar mass of $\mathrm{CO}_2=44 \mathrm{~g} \mathrm{~mol}^{-1}$
Molar mass of $5$ moles of $\mathrm{CO}_2=5 \times 44 \mathrm{~g}=220 \mathrm{~g}$
Molar mass of $\mathrm{H}_2 \mathrm{O}=18 \mathrm{~g} \mathrm{~mol}^{-1}$
Molar mass of $5$ moles of $\mathrm{H}_2 \mathrm{O}=5 \times 18 \mathrm{~g}=90 \mathrm{~g}$
$5$ moles of $\mathrm{CO}_2$ have a mass of $220$ $g$ and $5$ moles of $\mathrm{H}_2 \mathrm{O}$ have a mass of $90$ $g$ .
Hence, they do not have the same mass.
View full question & answer→Question 472 Marks
Which law states, that in a chemical compound, elements always combine in a fixed proportion?
Answer In chemistry, the law of definite proportion, sometimes called Proust's law or the law of definite composition, or law of constant composition states that a given chemical compound always contains its component elements in fixed ratio (by mass) and does not depend on its source and method of preparation.
View full question & answer→Question 482 Marks
The mass of one steel screw is $4.11g$. Find the mass of one mole of these steel screws. Compare this value with the mass of the earth ($5.98 \times 10^{24}kg$). Which one of the two is heavier and by how many times?
AnswerOne mole of screws weigh$ = 4.11 \times (6.022 \times 10^{23}) = 2.475 \times 10^{24}g = 2.475 \times 10^{21}$$kg$$\frac{\text{mass of the Earth}}{\text{mole of $1$ mole of screws}}=\frac{5.98\times10^{24}\text{kg}}{2.475\times10^{21}\text{kg}}=2.4\times10^3$
The mass of the Earth is $ 2.4 \times 10^3$ times the mass of one mole of steel screws.
$\therefore$ The earth is $2400$ times heavier than one mole of steel screws.
View full question & answer→Question 492 Marks
Calculate the mass of $12.044 \times 10^{25}$ molecules of oxygen($\mathrm{O}_2$).
AnswerOne mole of $\mathrm{O}_2=32 \mathrm{gm}$
$6.022 \times 10^{23}$ molecules of $\mathrm{O}_2$ have mass $=32 \mathrm{gm}$
So, $12.044 \times 10^{25}$ molecules of $\mathrm{O}_2$ will have mass $=6400 \mathrm{gm}=6.4 \mathrm{Kg}$
View full question & answer→Question 502 Marks
Why the symbols of few elements, like sodium, do not start with the initial letter of the name?
AnswerIn many cases they do start with the initial letter if you’re speaking a Romance language. Sodium, specifically, has the symbol Na from the Latin word “natrium” for “soda”. Sir Humphrey Davy discovered potassium, named K after “Kalium”.
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