Tag: empirical formula, percentage composition and molecular formula

Questions Related to empirical formula, percentage composition and molecular formula

A mixture contains $2.5$g $CaCO _3$ and $3.0$g $NaCl$. What is the percent by mass of $CaCO _3?$

chemistry the language of chemistry percent composition percentage composition and empirical formula empirical formula, percentage composition and molecular formula water of crystallisation

  1. $45.45$

  2. $75$

  3. $53$

  4. $60$

Show answer
Correct Option: A
Explanation:

Total mass$=2.5+3.0g=5.5g$


$\%$ by mass $CaCO _3=\dfrac{2.5}{5.5}\times 100 = 45.45\%$

Mass of beaker=10g
Mass of mixture+beaker=25g
Mixture contains 30% sand. Find the weight of sand.

chemistry the language of chemistry percent composition percentage composition and empirical formula empirical formula, percentage composition and molecular formula water of crystallisation

  1. 5 g

  2. 4.5 g

  3. 3 g

  4. 3.5 g

Show answer
Correct Option: B
Explanation:

Mass of mixture=$25g-10g=15g$

Percentage of sand=30%=$\frac{Mass  of  sand}{Mass  of  mixture}$
 Mass of sand =$0.3\times15g=4.5 g$

Which of the following compound represents an alkane?

chemistry the language of chemistry percent composition percentage composition and empirical formula empirical formula, percentage composition and molecular formula

  1. ${C} _{5}{H} _{8}$

  2. ${C} _{7}{H} _{16}$

  3. ${C} _{8}{H} _{6}$

  4. ${C} _{9}{H} _{10}$

Show answer
Correct Option: B
Explanation:

Alkanes have general molecular formula is $C _nH _{2n+2}$


$\therefore C _7H _{16}$ is Alkane.

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

The number of $g$ molecules of oxygen in $6.023\times 10^{24} \ CO$ molecules is ________________.

chemistry the language of chemistry percent composition percentage composition and empirical formula empirical formula, percentage composition and molecular formula

  1. $1\ g$ molecule

  2. $0.5\ g$ molecule

  3. $5\ g$ molecules

  4. $10\ g$ molecules

Show answer
Correct Option: C
Explanation:

$1$ mole of $CO=6.023\times 10^{23}$ molecules

$x$ mole of $CO=6.023\times 10^{24}$ molecules
$\therefore x=10$ moles $=10\ g$ molecules
$CO=1$ oxygen atom
$\therefore$ Oxygen molecules $=\dfrac{10}{2}$
$=5\ g$ molecules 
Option $C$ is correct.

It was found from the chemical analysis of a gas that it has two hydrogen atoms for each carbon atoms. At $0^o C$ and $1\ atm$, its density is $1.25\ g$ per litre. The formula of the gas would be ______________.

chemistry the language of chemistry percent composition percentage composition and empirical formula empirical formula, percentage composition and molecular formula

  1. $CH _2$

  2. $C _2H _4$

  3. $C _2H _6$

  4. $C _4H _8$

Show answer
Correct Option: B
Explanation:

$PV=nRT$


$P=\dfrac {dRT}{M}$

$M=\dfrac {1.25\times 0.082\times 273}{1}$

$=27.98$

Molar mass $\equiv 28\ gram/mole$

$\therefore \ $ as empirical formula $=CH _2$

$\therefore \ $ emprical mass $=12+2=14$

$n=\dfrac {28}{14}=2$

$\therefore \ $ Molecular formula $=(CH _2) _2$

$=C _2H _4$

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

When burnt in air, $14.0\ g$ mixture of carbon and sulpher gives a mixture of $CO 2$ and $SO _2$ in the volume ratio of $2:1$, volume being measured at the same conditions of temperature and pressure. Moles of carbon in the mixture is _________.

chemistry the language of chemistry percent composition percentage composition and empirical formula empirical formula, percentage composition and molecular formula

  1. $0.25$

  2. $0.40$

  3. $0.5$

  4. $0.75$

Show answer
Correct Option: C
Explanation:

Consider the mass of $C=x$


mass of $S=14-x$

moles of $C=\dfrac {x}{12};$ moles of $\dfrac {14-x}{32}$

$C+O _2\to CO _2;\quad S+O _2\to SO _2$

moles of $C=$ moles of $CO _2=\dfrac {x}{12}\quad $ moles of $S=$ moles of $SO _2=\dfrac {14-x}{32}$

Given,

$\dfrac {V _c}{V _s}=\dfrac {2}{1}$

$V\alpha $ no. of mole

$\dfrac {\dfrac {x}{12}}{\dfrac {14-x}{32}}=\dfrac {2}{1}$

$\dfrac {x}{12}\times \dfrac {32}{14-x}=\dfrac {2}{1}$

$\dfrac {4x}{42-3x}=1$

$4x=42-3x$

$7x=42$ 

$\boxed {x=6}$

$\therefore \ $ moles of $C$ in mixture $=\dfrac {w}{M}=\dfrac {6}{12}=0.5$

Answer is option $C$

If air is pumped slowly but continuously into a metallic cylinder of strong wall, what would happen to the air inside the cylinder?

chemistry the language of chemistry percent composition percentage composition and empirical formula empirical formula, percentage composition and molecular formula

  1. Temperature of air would increase

  2. Pressure of air would increase

  3. Pressure of air would decrease

  4. Temperature and pressure of air would increase

Show answer
Correct Option: B
Explanation:

Due to continuous pumping number of moles will increase
We know,
$n\propto P$
$n\propto V$
$n\propto \dfrac{1}{T}$
$\therefore$ Pressure will increase.
Option B.

For $10$ min each, at $27^o$C, from two identical holes nitrogen and an unknown gas are leaked into a common vessel of $3$l capacity. The resulting pressure is $4.18$ bar and the mixture contains $0.4$ mole of nitrogen. The molar mass of the unknown gas is?

chemistry the language of chemistry percent composition percentage composition and empirical formula empirical formula, percentage composition and molecular formula

  1. $112$g $mol^{-1}$

  2. $242$g $mol^{-1}$

  3. $224$g $mol^{-1}$

  4. $422$g $mol^{-1}$

Show answer
Correct Option: C

What is the molecular formula of arsenious chloride?

chemistry the language of chemistry percent composition percentage composition and empirical formula empirical formula, percentage composition and molecular formula

  1. $ AsCI _{3}$

  2. $ As _{2}CI _{6}$

  3. $ As _{2}CI _{5}$

  4. $ AsCI _{5}$

Show answer
Correct Option: B

If two compounds have the same empirical formula but different molecular formula, they must have :

chemistry the language of chemistry percent composition percentage composition and empirical formula empirical formula, percentage composition and molecular formula

  1. different percentage composition

  2. different molecular weights

  3. same vapour density

  4. none of these

Show answer
Correct Option: B
Explanation:

If two compounds have the same empirical formula but different molecular formula, they must have different molecular weights.


For example, $CH _2O$ and $C _6H _{12}O _6$ have the same empirical formula but different molecular formula, they have different molecular weights.

Option B is correct.