Chemistry

Mole Concept and Stoichiometry

354 Questions

The mole concept is a core area of chemistry focusing on the quantitative relationships between reactants and products in chemical reactions. Questions cover molecular mass calculations, stoichiometric conversions, and determining the number of molecules. This topic is essential for most science entrance and competitive examinations.

molecular mass calculationsstoichiometric conversionsmole fraction problemsheavy water reactionsoxidation calculations

Mole Concept and Stoichiometry Questions

Multiple choice chemistry substances in common use percent composition water of crystallisation percentage composition and empirical formula

The number of molecules of water of crystallisation present in one molecule of ferrous sulphate is_______.

  1. $5$
  2. $7$
  3. $6$
  4. $10$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

When water is used in the formation of crystals knows as water of crystallization. 'OR' 

Water of crystallization is the total weight of water in a substance at a given temperature and is mostly present in a definite ratio. 
On heating, iron(II) sulfate first loses its water of crystallization and the original green crystals are converted into a brown-colored anhydrous solid. 


$FeSO _4.7H _2O$ the heptahydrate in solution (water as solvent) transforms to both heptahydrate and tetrahydrate when the temperature reaches $56.6 ^\circ C$.


Hence, option $B$ is correct.

Multiple choice chemistry introduction to analytical chemistry percent composition water of crystallisation percentage composition and empirical formula

What is the percent of composition of the compound that forms when $222.7g$ of N combines compleletly with $77.4g$ of O?

  1. $70%, 30%$
  2. $80%, 20%$
  3. $74.2%, 25.8%$
  4. $72.2%, 27.8%$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The mass composition$:$
Total mass$: 222.7+77.4=300.1$
Mass percentage of N$:$
$222.7300.1=74.25$
Mass percentage of O$:$
$100-74.2=25.8%$

Multiple choice chemistry introduction to analytical chemistry percent composition water of crystallisation percentage composition and empirical formula

The formula for % composition of a compound is:

  1. molar mass of a compound/mass due to specific component $\times$ 100
  2. mass due to specific component $\times$ 100
  3. mass due to specific component/molar mass of a compound $\times$ 100
  4. molar mass of a specific component/temperature $\times$ 100
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

To calculate the per cent composition of a component in a compound: Find the molar mass of the compound by adding up the masses of each atom in the compound using the periodic table or a molecular mass calculator. Calculate the mass due to the component in the compound you are for which you are solving by adding up the mass of these atoms. Divide the mass due to the component by the total molar mass of the compound and multiply by 100.

% $\text{composition of a compound}=\cfrac{\text{ Mass due to specific component}}{\text{molar mass of a compound}}\times 100$

Hence, the correct option is $\text{C}$

Multiple choice chemistry introduction to analytical chemistry percent composition water of crystallisation percentage composition and empirical formula

% composition requires ................ of the compound :

  1. molar mass

  2. temperature

  3. atmospheric pressure

  4. both a and b
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The present composition (by mass) of a compound can be calculated by dividing the mass of each element by the total mass of the compound. i.e. by calculating the $molar$ $mass$ of the compound.

Multiple choice salts salts and their classification acids, bases and salts chemistry

The "alum" used in cooking is potassium aluminum sulfate hydrate, $KAl(SO _{3}) _{2}\cdot xH _{2}O$. To find the value of x, a sample of the compound is heated. The mass of the empty crucible is $20.01\ g$.
The alum hydrate was added to the crucible until the total mass of the crucible and hydrate was $24.75\ g$. The sample was heated in the crucible until the final mass of the crucible and anhydrous product was $22.5\ g$.
What is the value of x?

  1. $2$
  2. $3$
  3. $12$
  4. $18$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Mass of crucible Hydrate $=24.75$

Mass of hydrate $=24.75-20.01=4.74$ $g$
Mass of anhydrate $=22.5-20.01=2.49$ $g$
$KAl(SO _3) _2\cdot xH _2O\longrightarrow KAl(SO _3) _2+xH _2O$
Mass: $(18x+226)$ $g$          Mass $=226$ $g$
$(226+18x)$ $g$ $\overset {gives}{\longrightarrow}$ $226$ $g$
$4.74$ $g$ $\overset {gives}{\longrightarrow}$ $2.49$ $g$
$\cfrac{226+18x}{4.74g}=\cfrac{226}{2.49}$
$x\approx 12$

Multiple choice chemistry quantitative chemistry molecular mass relative formula mass masses of atoms and molecules

What is the mass in grams of $6.022\times 10^{23}$ atoms of oxygen?

  1. 16

  2. 8

  3. 32

  4. 4
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
The mass in grams of $6.022\times 10^{23}$ atoms of oxygen is 16 grams. Because $6.022\times 10^{23}$ is avagadro number. Avagadro number of particles are equals to the weight of its molecular weight. So atomic weight of $6.022\times 10^{23}$ atoms of oxygen is  16 which is the atomic weight of oxygen.
Hence option B is correct.
Multiple choice chemistry quantitative chemistry molecular mass relative formula mass masses of atoms and molecules

Sulphur trioxide is prepared by the following two reactions:
$S _8(s)+8O _2(g)\rightarrow 8SO _2(g)$
$2SO _2(g)+O _2(g)\rightarrow 2SO _3(g)$
How many grams of $SO _3$ are produced from $1$ mole of $S _8$?

  1. $1280.0$
  2. $640.0$
  3. $960.0$
  4. $320.0$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

${S _8} _{(s)}+{8O _2} _{(g)} \longrightarrow {8SO _2} _{(g)}$

${2SO _2} _{(g)}+{O _2} _{(g)}\longrightarrow {2SO _3} _{(g)}$
By stoichiometry, $1$ mole of ${S _8} _{(s)}$ produces $8$ mole ${SO _2} _{(g)}$
Also, $2$ mole of ${SO _2} _{(g)}$ produces $2$ mole ${SO _3} _{(g)}$
Therefore, $8$ mole ${SO _2} _{(g)}$ produces $8$ moles ${SO _3} _{(g)}$
i.e. $1$ mole ${S _8} _{(s)}$ produces $8$ moles ${SO _3} _{(g)}$
                                         i.e. $8 \times 80g$ of $SO _3$
                                         i.e $640g$ of $SO _3$

Multiple choice chemistry quantitative chemistry molecular mass relative formula mass masses of atoms and molecules

Consider the following reaction sequence:


${ S } _{ 8 }(s)+{ 80 } _{ 2 }(g)\rightarrow { 8SO } _{ 2 }(g)$

${ 2SO } _{ 8 }(g)+{ O } _{ 2 }(g)\rightarrow { 2SO } _{ 3}(g)$

How many grams of ${ SO } _{ 3 }$ are produced from $1$ mole ${ SO } _{ 8 }$?

  1. $1280 g$
  2. $690 g$
  3. $640 g$
  4. $320 g$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
${{S} _{8}} _{\left( s \right)} + 8 {{O} _{2}} _{\left( g \right)} \longrightarrow 8 {S{O} _{2}} _{\left( g \right)}$

As we have $1$ mole of ${S} _{8}$

According to reaction, one mole of $S{O} _{2}$ produces 8 mles of $S{O} _{2}$

Further,

$2 {S{O} _{8}} _{\left( g \right)} + {{O} _{2}} _{\left( g \right)} \longrightarrow 2 {S{O} _{3}} _{\left( g \right)}$

As per the reaction,

No. of moles of $S{O} _{3}$ produced by $2$ moles of $S{O} _{2} = 2 \text{ moles}$

$\therefore$ No. of moles of $S{O} _{3}$ produced by $8$ moles of $S{O} _{2} = 8 \text{ moles}$

As we know that,

$\text{Wt. of compound} = \text{no. of moles} \times \text{molar mass}$

$\therefore$ Weight of $S{O} _{3}$ in 8 moles $= 8 \times 80 = 640 g \; \left[ \because \text{Molar mass of } S{O} _{3} = 80 g \right]$

Hence, $640$ grams of $S{O} _{3}$ are produced by $1$ mole of ${S} _{8}$.

The correct option is C.
Multiple choice chemistry quantitative chemistry molecular mass relative formula mass masses of atoms and molecules

How many grams of $H _{2}SO _{4}$ are present in $0.25\ g$ mole of $H _{2}SO _{4}$?

  1. $2.45$
  2. $24.5$
  3. $0.25$
  4. $245$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
$Moles\quad of\quad { H } _{ 2 }{ SO } _{ 4 }=0.25\\ Molecular\quad mass(M)\quad of\quad { H } _{ 2 }{ SO } _{ 4 }=2+32+16\times 4\\ \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad =34+64\\ \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad =98\quad amu\\ no.\quad of\quad grams(w)\quad =\quad ?\\ we\quad have,\quad no.\quad of\quad moles\quad =\frac { w }{ M } \\ \qquad \qquad \qquad \qquad \qquad W=\quad no.\quad of\quad moles\times M\\ \qquad \qquad \qquad \qquad \qquad \quad \quad =0.25\times 98\\ \qquad \qquad \qquad \qquad \qquad \quad =24.5g\quad of\quad { H } _{ 2 }{ SO } _{ 4 }$
Multiple choice chemistry quantitative chemistry molecular mass relative formula mass masses of atoms and molecules

What weight of $SO _2$ can be made by burning sulphur in $5.0$ moles of oxygen?

  1. $640$ grams
  2. $160$ grams
  3. $80$ grams
  4. $320$ grams
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$S+O _2\rightarrow SO _2$


$1$ mole of $O _2$ gives $1$ mole of $SO _2$


Thus $5$ mole of $O _2$ will give 5 mole of $SO _2$

Thus amount of $SO _2$ in 5 mole $SO _2$ is 

$m=5mole\times 64 g/mole\\m=320g$

Thus $5$ mole of $O _2$ will obtain $320g$ of $SO _2$


Hence, the correct option is D.

Multiple choice chemistry quantitative chemistry molecular mass relative formula mass masses of atoms and molecules

A sample of impure cuprite, $Cu _2O$, contains 66.6% copper. What is the percentage of pure $Cu _2 O$ in the sample: 

  1. 75%

  2. 25%

  3. 60%

  4. 80%
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Number of moles of $Cu$ in $66.6g= \cfrac {66.6 g}{63.5}=1.05$ moles

We have $1$ mole of oxygen per $2$ mole of $Cu$

So, moles of oxygen is $0.525$.
Weight of oxygen present= $0.525 mol\times 16g/mol= 8.4g$

So, we have $(66.6+8.4)g=75g$

Our sample is $75$% pure $Cu _2O$.

Multiple choice chemistry quantitative chemistry molecular mass relative formula mass masses of atoms and molecules

The number of electrons in 1.8 ml of $H _{2}O$ will be:

  1. $0.1N _{A}$
  2. $0.2N _{A}$
  3. $0.3N _{A}$
  4. $N _{A}$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation
There is a total of 10 electrons in $ H _{2}O$ molecule.

moles of $H _{2}O(n)=\dfrac{mass\, of\, substance}{molar\, mass}$

Therefore, 1.8 ml of $H _{2}O$ means 0.1 mole
of $H _{2}O$

Number of atoms = $n\times $ Avogrdro Number

Number of atom = $0.1\times 6.022\times 10^{23}$

$= 0.6022\times 10^{23}$

Number of electron of $H _{2}O$ = $0.6022\times 10^{23}\times 10$

$= 6.022\times 10^{23}$ = Avogard's number

So, the correct option is $D$