Question

If 10${}^{21}$ molecules are removed from 200 mg of CO${}_2$, the number of moles of CO${}_2$ left is :

• A. $2.88\times {10}^{-3}$
• B. $28.8\times {10}^{-3}$
• C. $0.288\times {10}^{-3}$
• D. $1.66\times {10}^{-3}$

Answer 1

The correct option is A $2.88\times {10}^{-3}$

Answer:-

Molar mass of $C{O}_2$ = 44g

Given mass = 200 mg = 0.2g

Number of moles = $\frac{0.2}{44}=\frac{1}{220}$

$\text{Number of moles}=\frac{\text{number of molecules}}{\text{Avogadro's number}}$

$\Rightarrow \text{Number of molecules}=\text{Avogadro's number}\times \text{number of moles}$

$\Rightarrow \text{No. of molecules}=6.022\times {10}^{23}\times \frac{1}{220}=2.73\times {10}^{21}\text{Molecules in}\frac{1}{220}moles$

As ${10}^{21}$ molecules are removed,

$\therefore$ No. of molecules left = $2.73\times {10}^{21}-{10}^{21}=1.73\times {10}^{21}$

$\therefore \text{No. of moles}=\frac{\text{No. of molecules}}{\text{Avogadro's number}}=\frac{1.73\times {10}^{21}}{6.23\times {10}^{23}}$

$=2.88\times {10}^{-3}$

Hence, option $A$ is correct.