## How many neutrons and protons are there inthe following nuclei?

[question] Question. How many neutrons and protons are there inthe following nuclei? ${ }_{6}^{13} \mathrm{C}$ ${ }_{8}^{16} \mathrm{O}$ ${ }_{12}^{24} \mathrm{Mg}$ ${ }_{26}^{56} \mathrm{Fe}$ ${ }_{38}^{88} \mathrm{Sr}$ [/question] [solution] Solution: ${ }^{13}{ }_{6} \mathrm{C}:$ Atomic mass $=13$ Atomic number $=$ Number of protons $=6$ Number of neutrons $=$ (Atomic mass) $-$ (Atomic number) $=13-6=7$ ${ }_{8}^{16} \bigcirc$ Atomic mass $=16$ Atomic number $=8$ Number of protons $=8$ Number...

## Chlorine is prepared in the laboratory by treating manganese dioxide

[question] Question. Chlorine is prepared in the laboratory by treating manganese dioxide $\left(\mathrm{MnO}_{2}\right)$ with aqueous hydrochloric acid according to the reaction $4 \mathrm{HCl}_{(a q)}+\mathrm{MnO}_{2(s)} \rightarrow 2 \mathrm{H}_{2} \mathrm{O}_{(l)}+\mathrm{MnCl}_{2(a q)}+\mathrm{Cl}_{2(g)}$ How many grams of HCl react with 5.0 g of manganese dioxide? [/question] [solution] Solution: $1 \mathrm{~mol}[55+2 \times 16=87 \mathrm{~g}] \mathrm{MnO}_{2}$ reacts completely with $4 \m...

## Calcium carbonate reacts with aqueous HCl to give

[question] Question. Calcium carbonate reacts with aqueous $\mathrm{HCl}$ to give $\mathrm{CaCl}_{2}$ and $\mathrm{CO}_{2}$ according to the reaction, $\mathrm{CaCO}_{3}(s)+2 \mathrm{HCl}_{(a q)} \rightarrow \mathrm{CaCl}_{2(a q)}+\mathrm{CO}_{2(g)}+\mathrm{H}_{2} \mathrm{O}(l)$ What mass of $\mathrm{CaCO}_{3}$ is required to react completely with $25 \mathrm{~mL}$ of $0.75 \mathrm{M} \mathrm{HCl}$ ? [/question] [solution] Solution: 0.75 M of HCl ≡ 0.75 mol of HCl are present in 1 L of water $\e...

## A welding fuel gas contains carbon and hydrogen only. Burning a small sample

[question] Question. A welding fuel gas contains carbon and hydrogen only. Burning a small sample of it in oxygen gives 3.38 g carbon dioxide, 0.690 g of water and no other products. A volume of 10.0 L (measured at STP) of this welding gas is found to weigh 11.6 g. Calculate (i)empirical formula, (ii)molar mass of the gas, and (iii)molecular formula. [/question] [solution] Solution: (i) 1 mole $(44 \mathrm{~g})$ of $\mathrm{CO}_{2}$ contains $12 \mathrm{~g}$ of carbon. $\therefore 3.38 \mathrm{~...

## Calculate the number of atoms in each of the following

[question] Question. Calculate the number of atoms in each of the following (i)52 moles of Ar (ii)52 u of He (iii)52 g of He. [question] [solution] Solution: (i) 1 mole of $A r=6.022 \times 10^{23}$ atoms of $A r$ $\therefore 52 \mathrm{~mol}$ of $\mathrm{Ar}=52 \times 6.022 \times 10^{23}$ atoms of $\mathrm{Ar}$ $=3.131 \times 10^{25}$ atoms of $A r$ (ii) 1 atom of $\mathrm{He}=4 \mathrm{u}$ of $\mathrm{He}$ Or, 4 u of He = 1 atom of He $1 \mathrm{u}$ of $\mathrm{He}=\frac{1}{4}$ atom of $\math...

## Use the data given in the following table to calculate the molar mass of naturally occurring argon isotopes

[question] Question. Use the data given in the following table to calculate the molar mass of naturally occurring argon isotopes [/question] [solution] Solution: Molar mass of argon $=39.947 \mathrm{gmol}^{-1}$ [/solution]...

## How many significant figures should be present in the answer of the following calculations

[question] Question. How many significant figures should be present in the answer of the following calculations? (i) $\frac{0.02856 \times 298.15 \times 0.112}{0.5785}$ (ii) $5 \times 5.364$ (iii) $0.0125+0.7864+0.0215$ [/question] [solution] Solution: (i) $\frac{0.02856 \times 298.15 \times 0.112}{0.5785}$ Least precise number of calculation = 0.112 Number of significant figures in the answer = Number of significant figures in the least precise number = 3 (ii) $5 \times 5.364$ Least precise num...

## What will be the mass of one ${ }^{12} \mathrm{C}$ atom in $\mathrm{g}$ ?

[question] Question. What will be the mass of one ${ }^{12} \mathrm{C}$ atom in $\mathrm{g}$ ? [/question] [solution] Solution: 1 mole of carbon atoms $=6.023 \times 10^{23}$ atoms of carbon $=12 \mathrm{~g}$ of carbon $\therefore$ Mass of one ${ }^{12} \mathrm{C}$ atom $=\frac{12 \mathrm{~g}}{6.022 \times 10^{23}}$ $=1.993 \times 10^{-23} \mathrm{~g}$ [/solution]...

## Calculate the molarity of a solution of ethanol in water

[question] Question. Calculate the molarity of a solution of ethanol in water in which the mole fraction of ethanol is 0.040 (assume the density of water to be one) [/question] [solution] Solution: Mole fraction of $\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}=\frac{\text { Number of moles of } \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}}{\text { Number of moles of solution }}$ $0.040=\frac{n_{\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{OH}}}{n_{\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{OH}}+n_{\mathrm{H}_{2} ...

## Which one of the following will have largest number of atoms

[question] Question. Which one of the following will have largest number of atoms? (i)1 g Au (s) (ii)1 g Na (s) (iii)1 g Li (s) (iv)1 g of Cl2(g) [/question] [solution] Solution: (i). $1 \mathrm{~g}$ of $\mathrm{Au}(\mathrm{s})=\frac{1}{197} \mathrm{~mol}$ of $\mathrm{Au}(\mathrm{s})$ $=\frac{6.022 \times 10^{23}}{197}$ atoms of $A u(s)$ $=3.06 \times 10^{21}$ atoms of $A u(s)$ (ii). $1 \mathrm{~g}$ of $\mathrm{Na}(\mathrm{s})=\frac{1}{23}$ $\mathrm{mol}$ of $\mathrm{Na}(\mathrm{s})$ $=\frac{6.0...

## Convert the following into basic units

[question] Question. Convert the following into basic units (i)28.7 pm (ii)15.15 pm (iii)25365 mg [/question] [solution] Solution: (i) $28.7 \mathrm{pm}$ : $1 \mathrm{pm}=10^{-12} \mathrm{~m}$ $\therefore 28.7 \mathrm{pm}=28.7 \times 10^{-12} \mathrm{~m}$ $=2.87 \times 10^{-11} \mathrm{~m}$ (ii) $15.15 \mathrm{pm}$ : $1 \mathrm{pm}=10^{-12} \mathrm{~m}$ $\therefore 15.15 \mathrm{pm}=15.15 \times 10^{-12} \mathrm{~m}$ $=1.515 \times 10^{-12} \mathrm{~m}$ (iii) $25365 \mathrm{mg}$ : $1 \mathrm{mg}...

## If ten volumes of dihydrogen gas react with five volumes of dioxygen gas,

[question] Question. If ten volumes of dihydrogen gas react with five volumes of dioxygen gas, how many volumes of water vapour would be produced? [/question] [solution] Solution: Reaction of dihydrogen with dioxygen can be written as $2 \mathrm{H}_{2(x)}+\mathrm{O}_{2(x)} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}_{(g)}$ Now, two volumes of dihydrogen react with one volume of dihydrogen to produce two volumes of water vapour. Hence, ten volumes of dihydrogen will react with five volumes of dio...

## How are $0.50 \mathrm{~mol} \mathrm{Na}_{2} \mathrm{CO}_{3}$ and $0.50 \mathrm{M} \mathrm{Na}_{2} \mathrm{CO}_{3}$ different?

[question] Question. How are $0.50 \mathrm{~mol} \mathrm{Na}_{2} \mathrm{CO}_{3}$ and $0.50 \mathrm{M} \mathrm{Na}_{2} \mathrm{CO}_{3}$ different? [/question] [solution] Solution: Molar mass of $\mathrm{Na}_{2} \mathrm{CO}_{3}=(2 \times 23)+12.00+(3 \times 16)=106 \mathrm{~g} \mathrm{~mol}^{-1}$ Now, 1 mole of $\mathrm{Na}_{2} \mathrm{CO}_{3}$ means $106 \mathrm{~g}$ of $\mathrm{Na}_{2} \mathrm{CO}_{3}$. $\therefore 0.5 \mathrm{~mol}$ of $\mathrm{Na}_{2} \mathrm{CO}_{3} \quad=\frac{106 \mathrm{~...

## Dinitrogen and dihydrogen react with each other

[question] Question. Dinitrogen and dihydrogen react with each other to produce ammonia according to the following chemical equation $\mathrm{N}_{2}(g)+\mathrm{H}_{2}(g) \rightarrow 2 \mathrm{NH}_{3}(g)$ (i) Calculate the mass of ammonia produced if $2.00 \times 10^{3} \mathrm{~g}$ dinitrogen reacts with $1.00$ $\times 10^{3} \mathrm{~g}$ of dihydrogen. (ii) Will any of the two reactants remain unreacted? (iii) If yes, which one and what would be its mass? [/question] [solution] Solution: (i)Bal...

## In a reaction

[question] Question. In a reaction $A+B_{2} \rightarrow A B_{2}$ Identify the limiting reagent, if any, in the following reaction mixtures (i) 300 atoms of $A+200$ molecules of $B$ (ii) $2 \mathrm{~mol} A+3 \mathrm{~mol} B$ (iii) 100 atoms of $A+100$ molecules of $B$ (iv) $5 \mathrm{~mol} \mathrm{~A}+2.5 \mathrm{~mol} \mathrm{~B}$ (v) $2.5 \mathrm{~mol} \mathrm{~A}+5 \mathrm{~mol} \mathrm{~B}$ [/question] [solution] Solution: A limiting reagent determines the extent of a reaction. It is the reac...

## If the speed of light is $3.0 \times 10^{8} \mathrm{~m} \mathrm{~s}^{-1}$,

[question] Question. If the speed of light is $3.0 \times 10^{8} \mathrm{~m} \mathrm{~s}^{-1}$, calculate the distance covered by light in $2.00 \mathrm{~ns}$. [/question] [solution] Solution: According to the question: Time taken to cover the distance $=2.00 \mathrm{~ns}$ $=2.00 \times 10^{-9} \mathrm{~s}$ Speed of light $=3.0 \times 10^{8} \mathrm{~ms}^{-1}$ Distance travelled by light in $2.00 \mathrm{~ns}$ $=$ Speed of light $\times$ Time taken $=\left(3.0 \times 10^{8} \mathrm{~ms}^{-1}\rig...

## The following data are obtained when dinitrogen and dioxygen react together to form different compounds

[question] Question. The following data are obtained when dinitrogen and dioxygen react together to form different compounds (a)Which law of chemical combination is obeyed by the above experimental data?Give its statement (b)Fill in the blanks in the following conversions (i) $\quad 1 \mathrm{~km}=\ldots \ldots \ldots \ldots \ldots \ldots . \mathrm{mm}=\ldots \ldots \ldots \ldots \ldots \ldots . \mathrm{pm}$ (ii) $1 \mathrm{mg}=\ldots \ldots \ldots \ldots \ldots \ldots \ldots \mathrm{kg}=\ldots ...

## Round up the following upto three significant figures

[question] Question. Round up the following upto three significant figures (i)34.216 (ii)10.4107 (iii)0.04597 (iv)2808 [/question] [solution] Solution: (i)34.2 (ii)10.4 (iii)0.0460 (iv)2810 [/solution]...

## How many significant figures are present in the following

[question] Question. How many significant figures are present in the following (i) $0.0025$ (ii) 208 (iii) 5005 (iv) 126,000 (v) $500.0$ (vi) $2.0034$ [/question] [solution] Solution: (i)0.0025 There are 2 significant figures. (ii)208 There are 3 significant figures. (iii)5005 There are 4 significant figures. (iv)126,000 There are 3 significant figures. (v)500.0 There are 4 significant figures. (vi)2.0034 There are 5 significant figure [/solution]...

## Express the following in the scientific notation

[question] Question. Express the following in the scientific notation (i) $0.0048$ (ii) 234,000 (iii) 8008 (iv) $500.0$ (v) $6.0012$ [/question] [solution] Solution: (i) $0.0048=4.8 \times 10^{-3}$ (ii) $234,000=2.34 \times 10^{5}$ (iii) $8008=8.008 \times 10^{3}$ (iv) $500.0=5.000 \times 10^{2}$ (v) $6.0012=6.0012$ [/solution]...

## A sample of drinking water was foundto be severely contaminated with chloroform,

[question] Question. A sample of drinking water was found to be severely contaminated with chloroform, $\mathrm{CHCl}_{3}$, supposed to be carcinogenic in nature. The level of contamination was 15 ppm (by mass) (i)Express this in percent by mass. (ii)Determine the molality of chloroform in the water sample. [/question] [solution] Solution: (i) $1 \mathrm{ppm}$ is equivalent to 1 part out of 1 million $\left(10^{6}\right)$ parts. Mass percent of 15 ppm chloroform in water $=\frac{15}{10^{6}} \tim...

## What do you mean by significant figures

[question] Question. What do you mean by significant figures [/question] [solution] Solution: Significant figures are those meaningful digits that are known with certainty. They indicate uncertainty in an experiment or calculated value. For example, if 15.6 mL is the result of an experiment, then 15 is certain while 6 is uncertain, and the total number of significant figures are 3. Hence, significant figures are defined as the total number of digits in a number including the last digit that repr...

## Match the following prefixes with their multiples

[question] Question. Match the following prefixes with their multiples [/question] [solution] Solution: [/solution]...

## What is the SI unit of mass? How is it defined?

[question] Question. What is the SI unit of mass? How is it defined? [/question] [solution] Solution: The SI unit of mass is kilogram (kg). 1 Kilogram is defined as the mass equal to the mass of the international prototype of kilogram [/solution]...

## Pressure is determined as force per unit area of the surface.

[question] Question. Pressure is determined as force per unit area of the surface. The SI unit of pressure, Pascal is as shown below $1 \mathrm{~Pa}=1 \mathrm{~N} \mathrm{~m}^{-2}$ If mass of air at sea level is $1034 \mathrm{~g} \mathrm{~cm}^{-2}$, calculate the pressure in Pascal. [/question] [solution] Solution: Pressure is defined as force acting per unit area of the surface. $=1.01332 \times 10^{5} \mathrm{~kg} \mathrm{~m}^{-1} \mathrm{~s}^{-2}$ We know $1 \mathrm{~N}=1 \mathrm{~kg} \mathrm...

## If the density of methanol is $0.793 \mathrm{~kg} \mathrm{~L}^{-1}$,

[question] Question. If the density of methanol is $0.793 \mathrm{~kg} \mathrm{~L}^{-1}$, what is its volume needed for making $2.5 \mathrm{~L}$ of its 0.25 M solution? [/question] [solution] Solution: Molar mass of methanol $\left(\mathrm{CH}_{3} \mathrm{OH}\right)=(1 \times 12)+(4 \times 1)+(1 \times 16)$ $=32 \mathrm{~g} \mathrm{~mol}^{-1}$ $=0.032 \mathrm{~kg} \mathrm{~mol}^{-1}$ Molarity of methanol solution $=\frac{0.793 \mathrm{~kg} \mathrm{~L}^{-1}}{0.032 \mathrm{~kg} \mathrm{~mol}^{-1}}...

## What is the concentration of sugar $\left(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\right)$ in $\mathrm{mol} \mathrm{L}^{-1}$ if its $20 \mathrm{~g}$ are dissolved in enough

[question] Question. What is the concentration of sugar $\left(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\right)$ in $\mathrm{mol} \mathrm{L}^{-1}$ if its $20 \mathrm{~g}$ are dissolved in enough water to make a final volume up to 2 L? [/question] [solution] Solution: Molarity (M) of a solution is given by $=0.02925 \mathrm{~mol} \mathrm{~L}^{-1}$ $\therefore$ Molar concentration of sugar $=0.02925 \mathrm{~mol} \mathrm{~L}^{-1}$ [/solution]...

## In three moles of ethane $\left(\mathrm{C}_{2} \mathrm{H}_{6}\right)$, calculate the following:

[question] Question. In three moles of ethane $\left(\mathrm{C}_{2} \mathrm{H}_{6}\right)$, calculate the following: (i)Number of moles of carbon atoms. (ii)Number of moles of hydrogen atoms. (iii)Number of molecules of ethane [/question] [solution] Solution: (i) 1 mole of $\mathrm{C}_{2} \mathrm{H}_{6}$ contains 2 moles of carbon atoms. $\therefore$ Number of moles of carbon atoms in 3 moles of $\mathrm{C}_{2} \mathrm{H}_{6}$ = 2 × 3 = 6 (ii) 1 mole of $\mathrm{C}_{2} \mathrm{H}_{6}$ contains 6...

## Calculate the atomic mass (average) of chlorine using the following data

[question] Question. Calculate the atomic mass (average) of chlorine using the following data [/question] [solution] Solution: The average atomic mass of chlorine = 26.4959 + 8.9568 = 35.4527 u The average atomic mass of chlorine = 35.4527 u [/solution]...

## Determine the molecular formula of an oxide of iron in which the mass per cent of iron

[question] Question. Determine the molecular formula of an oxide of iron in which the mass per cent of iron and oxygen are $69.9$ and $30.1$ respectively. Given that the molar mass of the oxide is $159.69 \mathrm{~g} \mathrm{~mol}^{-1}$. [/question] [solution] Solution: Mass percent of iron (Fe) = 69.9% (Given) Mass percent of oxygen (O) = 30.1% (Given) Number of moles of iron present in the oxide $=\frac{69.90}{55.85}$ = 1.25 Number of moles of oxygen present in the oxide $=\frac{30.1}{16.0}$ =...

## How much copper can be obtained from 100 g of copper sulphate (CuSO4)?

[question] Question. How much copper can be obtained from 100 g of copper sulphate (CuSO4)? [/question] [solution] Solution: 1 mole of $\mathrm{CuSO}_{4}$ contains 1 mole of copper. Molar mass of CuSO4= (63.5) + (32.00) + 4(16.00) = 63.5 + 32.00 + 64.00 = 159.5 g 159.5 g of CuSO4contains 63.5 g of copper $\Rightarrow 100 \mathrm{~g}$ of $\mathrm{CuSO}_{4}$ will contain $\frac{63.5 \times 100 \mathrm{~g}}{159.5}$ of copper. $\therefore$ Amount of copper that can be obtained from $100 \mathrm{~g} ...

## Calculate the concentration of nitric acid in moles

[question] Question. Calculate the concentration of nitric acid in moles per litre in a sample which has a density, $1.41 \mathrm{~g} \mathrm{~mL}^{-1}$ and the mass per cent of nitric acid in it being $69 \%$. [/question] [solution] Solution: Mass percent of nitric acid in the sample = 69 % [Given] Thus, 100 g of nitric acid contains 69 g of nitric acid by mass. Molar mass of nitric acid (HNO3) $=\{1+14+3(16)\} \mathrm{g} \mathrm{mol}^{-1}$ = 1 + 14 + 48 $=63 \mathrm{~g} \mathrm{~mol}^{-1}$ Num...

## Calculate the mass of sodium acetate $\left(\mathrm{CH}_{3} \mathrm{COONa}\right)$ required to make

[question] Question. Calculate the mass of sodium acetate $\left(\mathrm{CH}_{3} \mathrm{COONa}\right)$ required to make $500 \mathrm{~mL}$ of $0.375$ molar aqueous solution. Molar mass of sodium acetate is $82.0245 \mathrm{~g} \mathrm{~mol}^{-1}$ [/question] [solution] Solution: 0.375 M aqueous solution of sodium acetate ≡ 1000 mL of solution containing 0.375 moles of sodium acetate Number of moles of sodium acetate in 500 mL Molar mass of sodium acetate $=82.0245 \mathrm{~g}$ mole $^{-1}$ (Giv...

## Calculate the amount of carbon dioxide that could be produced when

[question] Question. Calculate the amount of carbon dioxide that could be produced when (i)1 mole of carbon is burnt in air. (ii)1 mole of carbon is burnt in 16 g of dioxygen. (iii)2 moles of carbon are burnt in 16 g of dioxygen [/question] [solution] Solution: The balanced reaction of combustion of carbon can be written as: (i) As per the balanced equation, 1 mole of carbon burns in1 mole of dioxygen (air) to produce1 mole of carbon dioxide. (ii) According to the question, only 16 g of dioxygen...

## Determine the empirical formula of an oxide of iron which has 69.9%

[question] Question. Determine the empirical formula of an oxide of iron which has 69.9% iron and 30.1% dioxygen by mass [/question] [solution] Solution: % of iron by mass = 69.9 % [Given] % of oxygen by mass = 30.1 % [Given] Relative moles of iron in iron oxide Relative moles of oxygen in iron oxide Simplest molar ratio of iron to oxygen = 1.25: 1.88 = 1: 1.5 = 2: 3 $\therefore$ The empirical formula of the iron oxide is $\mathrm{Fe}_{2} \mathrm{O}_{3}$. [/solution]...

## Calculate the mass percent of different elements

[question] Question. Calculate the mass percent of different elements present in sodium sulphate $\left(\mathrm{Na}_{2} \mathrm{SO}_{4}\right)$. [/question] [solution] Solution: The molecular formula of sodium sulphate is $\mathrm{Na}_{2} \mathrm{SO}_{4}$. Molar mass of $\mathrm{Na}_{2} \mathrm{SO}_{4}=[(2 \times 23.0)+(32.066)+4(16.00)]$ $=142.066 \mathrm{~g}$ Mass percent of an element $=\frac{\text { Mass of that element in the compound }}{\text { Molar mass of the compound }} \times 100$ Mas...

## Calculate the molecular mass of the following

[question] Question. Calculate the molecular mass of the following (i) $\mathrm{H}_{2} \mathrm{O}$ (ii) $\mathrm{CO}_{2}$ (iii) $\mathrm{CH}_{4}$ [/question] [solution] Solution: (i) $\mathrm{H}_{2} \mathrm{O}:$ The molecular mass of water, $\mathrm{H}_{2} \mathrm{O}$ = (2 × Atomic mass of hydrogen) + (1 × Atomic mass of oxygen) = [2(1.0084) + 1(16.00 u)] = 2.016 u + 16.00 u = 18.016 = 18.02 u (ii) $\mathrm{CO}_{2}$ : The molecular mass of carbon dioxide, $\mathrm{CO}_{2}$ = (1 × Atomic mass of ...