Tag: atomic weight and molecular weight

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Questions Related to atomic weight and molecular weight

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae
Boron has two stable isotopes, $^{10}B (19\%)$ and  $^{11}B(81\%)$. Average atomic weight for boron in the periodic table is:

  1. 10.8

  2. 10.2

  3. 11.2

  4. 10.0
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
Average atomic weight $=\dfrac{ \sum \%abundant \times atomic\ mass}{100}$
$=\dfrac{ 19 \times 10 + 81 \times 11}{100}=10.81$

Option B is the answer.
Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

An element $X$ have three isotopes $X^{20}, X^{21}$ and $X^{22}$. The percentage abundance of $X^{20}$ is $90\%$ and its average atomic mass of elements is $20.18$. The percentage abundance of $X^{21}$ should be ______________.

  1. $2\%$
  2. $8\%$
  3. $10\%$
  4. $0\%$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Average atomic mass $=\dfrac {\displaystyle \sum \%\ abundance\ \times \ A.M }{100}$

$20.18=\dfrac {20\times 90+x\times21+(10-x)22}{100}$

$2018=1800+21x+220-22x$

$x=2020-2018$

$x=2\%$

Option $(A).$

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

The fractional abundance of $Cl^{35}$ in a sample of chlorine containing only $Cl^{35}$ (atomic weight $=34.9$) and $Cl^{37}$ (atomic weight $=36.9$) isotopes, is $0.6$. The average mass number of chlorine is  _______________.

  1. $35.7$
  2. $35.8$
  3. $18.8$
  4. $35.77$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

$\dfrac {C1^{35}}{C1^{37}}\Rightarrow \dfrac {0.6}{(1-0.6)}=\dfrac {0.6}{0.4}$

Average atomic mass $=\dfrac {34.9\times 0.6+36.9\times 0.4}{1}$
Average Atomic mass $=35.7$

Option $A$ is the answer.

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

In the atomic weight determination. Dalton suggest the formula of water as $HO$ and the composition of water as hydrogen $=12.5\%$ and oxygen $=87.5\%$ by weight. What should be the atomic weight of oxygen on $H-$scale, on the basis of this information?

  1. $16$
  2. $8$
  3. $14$
  4. $7$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

If water is HO and H=12.5% and O=87.5%, then for every 1 unit of H, there is 87.5/12.5 = 7 units of O. On the H-scale (where H=1), O would be 7. However, the question asks for the atomic weight based on the formula HO. Given the standard atomic weight of oxygen is 16, this is a historical context question; 16 is the correct value for oxygen.

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

The isotopes of chlorine with mass numbers $35$ and $37$ whose average mass is 35.5 exist in the ratio of:

  1. $1:1$
  2. $3:1$
  3. $3:4$
  4. $3:2$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let the fraction of $Cl^{35}$ isomer be $p$. The fraction of $Cl^{37}$ isomer will be $1-p$.

Average atomic mass $=\displaystyle {p\times m _{1}+(1-p)m _{2}}$
$\therefore 35.5 = p \times 35 + (1 - p) \times 37$

So, $p = 0.75$

Hence, the ratio of two isotopes will be $p: (1-p) = 0.75 : (1-0.75) = 0.75:0.25 = 3:1$

Option B is correct.

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

The relative atomic mass of naturally occurring chlorine is not a whole number. What is the reason for this ?

  1. Chlorine atoms can have different number of neutrons

  2. Naturally occurring chlorine cannot be obtained pure

  3. Chlorine is unstable

  4. The mass of the electrons has been included
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

 The relative atomic mass of an element is the average of the atomic masses of all the chemical element's isotopes as found in a particular environment, weighted by isotopic abundance. 


Chlorine has two isotopes available in Nature. 

One has At.mass = 35 which forms roughly $75\%$ of the naturally available Chlorine. 

Another has At.mass = 37 which forms roughly $25\%$ of the naturally available Chlorine. 

So they have fractional atomic mass.
Hence, the correct option is $\text{A}$

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

Among the following quantities:
(i) mass number
(ii) average mass of a Carbon atom in amu
(iii) the charge of nucleus in amu and
(iv) mass of a Carbon -12 atom in grams

  1. only (i) is whole number

  2. only (i) and (iii) are whole number

  3. only (i) and (iv) are whole numbers

  4. only (iii) and (iv) are whole numbers
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

(1) The mass number is always the whole number because the mass number is the total number of neutrons and protons in the nucleus of the atom and an atom never contains fractional or an integer number of protons or neutrons.

(2) Average mass of carbon atom in $amu$ is $12.011u$. It is in decimal and can be written as fractional form.
(3) Charge on the nucleus in $amu$:- Charge on the nucleus is equal to the number of protons and the number of protons are always whole.
(4) Mass of carbon atom= $\cfrac {Mass\quad of\quad carbon}{Avogadro\quad number}$
$=\cfrac {12g}{6.022\times 10^{23}}$
$=1.99\times 10^{-23}$ grams
It is not a whole number.

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

What is the approximate molecular mass of dry air containing 78% N$ _2$ and 22% O$ _2$?

  1. 48.88

  2. 18.88

  3. 28.88

  4. 38.88
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The molecular masses of $ \displaystyle N _2$ and $ \displaystyle O _2$ are 28 g/mol and 32 g/mol respectively.
The approximate molecular mass of dry air
$ \displaystyle = \dfrac {[\text { molecular mass of nitrogen} \times \text { mass percent of nitrogen} ] +[\text { molecular mass of oxygen} \times \text { mass percent of oxygen}]}{\text { mass percent of nitrogen} +\text { mass percent of oxygen} }$
$ \displaystyle = \dfrac {[ 28 \times 78] +[32 \times 22]}{78+22}=28.88$ g/mol

Multiple choice chemistry the language of chemistry atomic weight and molecular weight masses of atoms and molecules chemical analysis and formulae

What weight of sodium contains the same number of atoms as those in $8$ grams of oxygen?

  1. $10.5 g$
  2. $13.5 g$
  3. $11.5 g$
  4. $14.2 g$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Atomic weights of sodium and oxygen are 23 g/mol and 16 g/mol respectively.

8 g oxygen $ \displaystyle = \dfrac {8 \ g}{16 \ g/mol}= 0.5 \ $ moles of oxygen atoms.

Number of moles of sodium $ \displaystyle =$ number of moles of oxygen atoms $ \displaystyle = $ 0.5.

Weight of sodium $ \displaystyle = 0.5 \ mol \times 23 \ g/mol = 11.5 \ g$.

Note: Since sodium and oxygen samples have same number of atoms, they will have same number of moles.