Tag: some basic concepts of chemistry

Questions Related to some basic concepts of chemistry

There are two isotopes of an element with atomic mass $z$. Heavier one has atomic mass $z+2$ and lighter one has $z-1$, then an abundance of lighter one is:

determination of atomic and isotopic mass some basic concepts of chemistry chemistry

  1. $66.6\%$

  2. $96.7\%$

  3. $6.67\%$

  4. $33.3\%$

Show answer
Correct Option: A
Explanation:

Abundance of an isotope can be calculated as :


Let abundance of isotope with atomic mass $Z+2$ is x
Then abundance of isotope $Z-1$ is $1-x$

$(Z+2)x+(Z-1)(1-x)\quad =Z$

On solving this, we get
$x=\dfrac { 1 }{ 3 } $
Therefore,% abundance of isotope with atomic no.$Z+2$ is =$=\dfrac { 1 }{ 3 } \times 100=33.33$%
Therefore,% abundance isotope with atomic no. $Z-1$ is =$100-33.33=67.66%$%
Therefore isotope with atomic $Z-1$ is higher in % abundance

$ _{ 17 }^{ 35 }{ Cl }$ and $ _{ 17 }^{ 37 }{ Cl }$ are two isotopes of chlorine. If average atomic mass is $35.5$ then ratio of these two isotopes is:

determination of atomic and isotopic mass some basic concepts of chemistry chemistry

  1. $35 : 37$

  2. $1 : 3$

  3. $3 : 1$

  4. $2 : 1$

Show answer
Correct Option: C
Explanation:

Solution:- (C) $3 : 1$

Let $x$ and $y$ be the fraction of ${ _{17}^{35}{Cl}}$ and ${ _{17}^{37}{Cl}}$ in ${ _{17}^{35.5}{Cl}}$.
Therefore,
Average at. mass $= \cfrac{x \times 35 + y \times 37}{x + y}$
$35.5 = \cfrac{35x + 37y}{x+y}$
$\Rightarrow 35.5 x + 35.5 y = 35x + 37y$
$\Rightarrow 35.5 x - 35x = 37 y - 35.5 y$
$\Rightarrow 0.5 x = 1.5 y$
$\Rightarrow \cfrac{x}{y} = \cfrac{1.5}{0.5} = \cfrac{3}{1}$
Hence the ratio of ${ _{17}^{35}{Cl}}$ and ${ _{17}^{37}{Cl}}$ in ${ _{17}^{35.5}{Cl}}$ is $3 : 1$.

If isotopic distribution of $ C-12 $ and $ C-14 $ is  98 %  and  2 %  respectively, what would be the number of $ C-14 $ isotope in $ 12 gm $  carbon sample?

determination of atomic and isotopic mass some basic concepts of chemistry chemistry

  1. $ 1.032 \times 10^{22} $

  2. $ 3.01 \times 10^{23} $

  3. $ 5.88 \times 10^{23} $

  4. $ 6.02 \times 10^{23} $

Show answer
Correct Option: A
Explanation:

1 mole of carbon contains $N _A$ molecules.


$14g$ of $C-14$ contains $6.023 \times 10^{23}$ atoms

$1g$ of $C-14= \dfrac{6.023 \times 10^{23}}{14}$


$12g$ of $C-14$ atom contains $=\cfrac {6.023 \times 106{23}\times 12}{14}$

                                                  $=5.16 \times 10^{23}$ atoms

Now $2$% of $5.16 \times 10^{23}=\cfrac {5.16 \times 10^{23}\times 2}{100}$

                                         $=1.032 \times 10^{22}$ atoms .

Chlorine has two naturally occurring isotopes, $^{35}Cl$ and $^{37}Cl$. If the atomic mass of Cl is 35.5 the ratio of natural abundance of $^{35}Cl$ and $^{37}Cl$ is closest to :

determination of atomic and isotopic mass some basic concepts of chemistry chemistry

  1. $ 3 : 5 $

  2. $3 : 1$

  3. $2 : 5 $

  4. $4 : 1$

Show answer
Correct Option: B

A sample of perfect gas that initially occupies $15.0L$ at $300K$ and $1.0$ bar is compressed isothermally. To what volume must the gas be compressed to reduce its entropy by $5.0J/K$? $\left[ \ln { 0.36 } =-1.0,\ln { 2.7 } =1.0 \right] $

determination of atomic and isotopic mass some basic concepts of chemistry chemistry

  1. $5.4L$

  2. $8.22L$

  3. $40.5L$

  4. $5.56L$

Show answer
Correct Option: A

If an element $z$ having atomic weight $x$ exists in two isotopes of mass number $(x-1)$ and $(x+2)$ then, the percentage abundance of heavier isotopes is:    

determination of atomic and isotopic mass some basic concepts of chemistry chemistry

  1. $25\%$

  2. $66.6\%$

  3. $33.3\%$

  4. $20\%$

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Correct Option: B

A system undergoes a process in which the entropy change is $+5.51J{K}^{-1}$. During the process, $1.50kJ$ of heat is added to the system at $300K$. The correct information regarding the process is

determination of atomic and isotopic mass some basic concepts of chemistry chemistry

  1. the process is thermodynamically reversible

  2. the process is thermodynamically irreversible

  3. the process may or may not be thermodynamically reversible

  4. the process must be isobaric

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Correct Option: B

The table shows the numbers of particles present in the nuclei of four atoms or ions.

protons neutrons electronic structure
$1$ $18$ $22$ $2, 8, 8$
$2$ $19$ $20$ $2, 8, 8$
$3$ $19$ $21$ $2, 8, 8, 1$
$4$ $20$ $20$ $2, 8, 8, 2$

Which two particles belong to the same element?

determination of atomic and isotopic mass some basic concepts of chemistry chemistry

  1. $1$ and $2$

  2. $1$ and $4$

  3. $2$ and $3$

  4. $2$ and $4$

Show answer
Correct Option: C
Explanation:

$\text{An element always consist same number of proton and electrons.}$

$\text{Number of neutrons can be changed in an element based on its isotrops.}$
$\text{Option C is correct.}$

The average atomic mass of copper is $63.546$ amu .Natural copper consists of two iostopes: $^{63} Cu$ and $^{65} Cu$.Their natural abundances are $69.09\%$ and $30.91\%$ respectively. If the mass of $^{63} Cu$ isotope is $62.9298$ amu ,What is the mass of $^{65} Cu$ isotope?

determination of atomic and isotopic mass some basic concepts of chemistry chemistry

  1. 64.9000

  2. 65.1233

  3. 64.9233

  4. 65.1933

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Correct Option: C

Europium consists of $47.8$% Eu-$151$ and $52.2$% Eu-$153$. Calculate average atomic mass?

determination of atomic and isotopic mass some basic concepts of chemistry chemistry

  1. $152.2$

  2. $152.1$

  3. $151.9$

  4. $152$

Show answer
Correct Option: D
Explanation:

Average atomic mass is given by:


$M _{av}=\dfrac{151\times0.478+153\times0.522}{0.478+0.522}=152.044\ gm/mol$