Tag: masses of atoms and molecules

Questions Related to masses of atoms and molecules

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

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

  1. $0.1N _{A}$

  2. $0.2N _{A}$

  3. $0.3N _{A}$

  4. $N _{A}$

Show answer
Correct Option: D
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 $D$

In an ionic compound, mole ratio of cation to anion is $1:2$. If atomic masses of metal and non-metal respectively are 138 and 19, then correct statement is :

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

  1. molecular mass of compound is 176

  2. formula mass of compound is 176

  3. formula mass of compound is 157

  4. molecular mass of compound is 157

Show answer
Correct Option: B
Explanation:

1 mole of ionic compound contains 1 mole of cations and 2 moles of anions.
The formula mass of the compound is $138+2(19)=176 \ g/mol$.

Which of the following statements are true?

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

  1. $1$ mole $H$ atoms $=6.02\times 10^{23}H$ atoms.

  2. $6.02\times 10^{23}H$ atoms have a mass of $1.008$ g.

  3. The formula mass of $O _2=32.00$ amu.

  4. All of the above are true.

Show answer
Correct Option: D
Explanation:

(A) $ 1 $ mole of any substance contains Avogadro's number of atoms or molecules. Thus, $1$ mole of H atoms corresponds to $=6.02\times 10^{23}H$ atoms.
Hence, the option A is true.
(B) $6.02\times 10^{23}H$ atoms have mass equal to molar mass of H. it is equal to $1.008$ g.
 Hence, the option B is true.
(C) The formula mass of $O _2=32.00 amu$. Thus, $1$ oxygen molecule weighs $32$ amu and $1$ mole of oxygen molecules weighs $32$ g.
Hence, the option C is true.
As we know, 1 mole is the collection of $6.02\times 10^{23}$ entities. Here entities may represent atoms, ions, molecules.
And mass of one mole of H is $1.008$ g so $6.02\times 10^{23}H$ atoms have a mass of $1.008$ g.
The formula mass of $O _2= 2(8p+8n) = 2\times16 = 32.0$ amu.

The relative atomic mass of $N$ and relative molecular mass of $N _2$ are:

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

  1. $14, 28$

  2. $7, 14$

  3. $14, 14$

  4. $7, 28$

Show answer
Correct Option: A
Explanation:
Relative atomic mass is the ratio of the average mass of one atom of an element to one-twelfth of the mass of an atom of carbon-12.

The relative atomic mass of N is 14  and relative molecular mass of $N _2$ is 28.

Hence, option A is correct.

Find the relative molecular mass of methyl alcohol $(CH _3OH)$, if $160 gm$ of the alcohol on vaporization has a volume of $112$ litres at STP.

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

  1. $16 gm$

  2. $32 gm$

  3. $160 gm$

  4. $80 gm$

Show answer
Correct Option: B
Explanation:

As we know, one mole of a gaseous substance occupies $22.4$ litres volume.


Mole $= \dfrac{w}{M}$


$\dfrac{160}{M} = \dfrac{112}{22.4}$

$M = 32 gm$

Also, molar mass $=2\times vapour  density$
Therefore, vapour density $= 16 gm$

The element chlorine consists of mixture of 75.53% $ _{17}$Cl$^{35}$ and 24.47% $ _{17}$Cl$^{37}$ having a mass of 34.97 amu and 36.95 amu, respectively. The atomic weight of chloride is:

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

  1. 35.25

  2. 35.45

  3. 36.25

  4. 36.45

Show answer
Correct Option: B
Explanation:

Given - $ _{17} Cl^{35} - 75.53$% and mass of $Cl$ 34.97 amu
$ _{17} C^{37} - 24.47$% and mass of $Cl$ 36.95 amu
$\therefore $ Atomic weight of chlorine
$= \displaystyle \frac{(75.53 \times 34.97)+(24.47 \times 36.95)}{100} = \frac{3545.45}{100}$
$=35.45$

The formula of oxide of a metal is $MO$ and equivalent weight of metal is $20$. The atomic weight of metal is

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

  1. $40$

  2. $20$

  3. $30$

  4. $10$

Show answer
Correct Option: A
Explanation:

Valency of $M$ in $MO=2$
Equivalent wt. of $M = 20$
Atomic wt. of $\text{M = Equivalent wt. Valency}$
$\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;=20\times2=40$

Two flasks A and B of equal volumes are kept under similar conditions of temperature and pressure. If flask A holds 16.2 g of gas X while flask B holds 1.012 g of hydrogen, calculate the relative molecular mass of gas X:

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

  1. 20 g

  2. 32 g

  3. 28 g

  4. 44 g

Show answer
Correct Option: B
Explanation:

A mole is equal to the relative molecular mass of a gas. As given, the number of moles of gas X and hydrogen are same in the two flasks.
Therefore no. of moles in 2g of hydrogen gas = 1
No. of moles in 1.01g of hydrogen gas = 1/2
Now the weight of gas X that contains 1/2 moles = 16.2g
Weight of gas X that contains 1mole = $\frac{16.2}{1/2}$ = 32g

The amount in parts by weight of sulphur present in one sulphuric acid molecule:

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

  1. $16$

  2. $32$

  3. $64$

  4. $48$

Show answer
Correct Option: B
Explanation:
$H _2SO _4$ is sulphuric acid. 
Molecular mass of $H _2SO _4= 2+32 + (4 \times 16)=38 \ g$
Weight of sulphur in it $= 32 \ g$
So, amount in parts by weight of sulphur in one sulphuric acid molecule is $32 \ g$

The molecular mass of a salt of oxy acid of chlorine of a divalent metal which contains more number of oxygen atoms than its corresponding '-ic' acid is $239\ g/mole$.
What is the atomic mass of the metal?

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

  1. 24

  2. 39

  3. 40

  4. 30

Show answer
Correct Option: C
Explanation:

The formula of the salt with molecular weight 239 is $Ca(ClO _{4}) _{2}$ and thus the divalent metal is calcium with atomic weight 40.