Tag: masses of atoms and molecules

Questions Related to masses of atoms and molecules

How many grams of phosphoric acid $(H _2PO _4)$ would be needed to neutralise $100$g of magnesium hydroxide $(Mg(OH) _2)$?

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

  1. $66.7$ g

  2. $252$

  3. $112.6$ g

  4. $168$ g

Show answer
Correct Option: C
Explanation:
Apply the formula
${\left( {\dfrac{W}{Equivalent\ wt.}} \right) _{{H _3}P{O _4}}} = \left( {\dfrac{W}{Equivalent\ wt.}} \right) _{Mg{\left( {OH} \right) _2}}$
Hence,
${\dfrac{W}{{98 \times 3}} = \dfrac{{100}}{{58 \times 2}}}$
$\therefore{W = 112.6{\text{ }}gram}$

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

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

  1. 16

  2. 8

  3. 32

  4. 4

Show answer
Correct Option: A
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.

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 $S _8$?

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

  1. $1280.0$

  2. $640.0$

  3. $960.0$

  4. $320.0$

Show answer
Correct Option: B
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 $SO _3$

Calculate the total volume of $0.1$ molar $KMn{O _4}$ solution that is needed to oxidized 100 mg of each furious oxalate and furious sulphate in a mixture in acidic medium. 

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

  1. 1.096 ml

  2. 1.32 ml

  3. 5.48 ml

  4. None of these

Show answer
Correct Option: C

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 ${ SO } _{ 8 }$?

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

  1. $1280 g$

  2. $690 g$

  3. $640 g$

  4. $320 g$

Show answer
Correct Option: C
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.

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

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

  1. $2.45$

  2. $24.5$

  3. $0.25$

  4. $245$

Show answer
Correct Option: B
Explanation:
$Moles\quad of\quad { H } _{ 2 }{ SO } _{ 4 }=0.25\\ Molecular\quad mass(M)\quad of\quad { H } _{ 2 }{ SO } _{ 4 }=2+32+16\times 4\\ \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad =34+64\\ \qquad \qquad \qquad \qquad \qquad \qquad \qquad \qquad =98\quad amu\\ no.\quad of\quad grams(w)\quad =\quad ?\\ we\quad have,\quad no.\quad of\quad moles\quad =\frac { w }{ M } \\ \qquad \qquad \qquad \qquad \qquad W=\quad no.\quad of\quad moles\times M\\ \qquad \qquad \qquad \qquad \qquad \quad \quad =0.25\times 98\\ \qquad \qquad \qquad \qquad \qquad \quad =24.5g\quad of\quad { H } _{ 2 }{ SO } _{ 4 }$

$4 g$ atom of $Ag$ contains: 

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

  1. $108 g$

  2. $4 g$

  3. $432g$

  4. none of these

Show answer
Correct Option: C
Explanation:

$1g$ atom of $Ag$ implies $1$ mole of it

Hence, $4g$ atom is $4$ moles of $Ag$
$1$ moles of $Ag= 108g$
$\therefore 4g$ atom of $Ag$ contains $4 \times 108=432g$

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

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

  1. $640$ grams

  2. $160$ grams

  3. $80$ grams

  4. $320$ grams

Show answer
Correct Option: D
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.

In the following charge,  $3Fe + 4{H _2}O \to F{e _2}{O _4}$ If the atomic weight of iron is $56$, then its equivalent weight will be

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

  1. 42

  2. 21

  3. 63

  4. 84

Show answer
Correct Option: D

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

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

  1. 75%

  2. 25%

  3. 60%

  4. 80%

Show answer
Correct Option: A
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 $Cu _2O$.