Question

An unknown chlorohydrocarbon has $3.55\%$ of chlorine. If each molecule of the hydrocarbon has one chlorine atom only, then chlorine atoms present in 1 g of chlorohydrocarbon are:
$[{\text{Atomic wt}}{\text{. of Cl}} = 35.5{\text{ u}};{\text{Avogadro constant}} = 6.023 \times {10^{23}}{\text{mo}}{{\text{l}}^{ - 1}}]$
A.$6.023 \times {10^{23}}$
B.$6.023 \times {10^{21}}$
C.$6.023 \times {10^9}$
D.$6.023 \times {10^{20}}$

Answer 1

We can calculate the mass of chlorine present from the given percentage composition of chlorine in chlorohydrocarbon. Then we can calculate for 1 g using unitary method and hence molar mass using the molar mass of the chlorine. To calculate the numbers of atoms simply multiply the number of moles with the number of atoms in one mole.
Formula used:
${\text{number of moles}} = \dfrac{{{\text{mass}}}}{{{\text{molar mass}}}}$

Complete step by step answer:
It is given that $3.55\%$ of chlorine is present in the chlorohydrocarbon. This is the percentage composition given to us, which we can define as the amount of solute in grams present in 100 g of solution. So, $3.55{\text{ g}}$ of chlorine is present in 100 g chlorohydrocarbon.
100 g of chlorohydrocarbon contains 3.55 g of chlorine.
1 g of chlorohydrocarbon contains $\dfrac{{3.55}}{{100}}{\text{ g}}$or $0.0355{\text{ g}}$ of chlorine.
Now we will calculate the number of moles of chlorine, the molar mass of chlorine is given as $35.5{\text{ u}}$ .
${\text{number of moles}} = \dfrac{{0.0355}}{{35.5}} = 0.001{\text{ mol}}$
1 mole of a substance contains $6.023 \times {10^{23}}$ atoms of that substance.
Hence $0.001$ moles will contain $0.001 \times 6.023 \times {10^{23}}$ number of atoms.
The number of atoms will be $6.023 \times {10^{20}}$ atoms of chlorine.

Hence, the correct option is D.

Note:
Chlorohydrocarbon compounds are those compounds which contain chlorine, hydrogen and carbon atoms. The change in the number of any one of the atoms, changes the molecular formula and hence the properties also changes. There are various chlorohydrocarbon compounds present that are very important such as chloroform, dichloromethane, trichloroethane and dichloroethene. These are used as solvents for many compounds. The commonly used pesticide DDT is also an organochlorine compound, but the use of DDT has declined due to its toxic nature.. These are used as dry cleaning agents.