Question

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

• A. $2.89\times {10}^{-3}$
• B. $28.8\times {10}^{-3}$
• C. $0.289\times {10}^{-3}$
• D. $1.68\times {10}^{-2}$

Answer 1

The correct option is A $2.89\times {10}^{-3}$

$\begin{array}{c}\text{Number of}{c}_2\text{molecules in}200mg=\frac{200\times {10}^{-3}}{44}\times 6.022\times {10}^{23}\\ =2.7373\times {10}^{21}.\\ \text{Now,}{10}^{21}\text{molecules are removed.}\\ \text{so, left number}=\begin{array}{c}(2.7373-1)\times {10}^{21}\\ =1.7373\times {10}^{21}\end{array}\end{array}$

$\begin{array}{c}\text{Hence, no. of moles left}=\frac{1.7373\times {10}^{21}}{6.022\times {10}^{23}}=2.89\times {10}^{-3}moles\\ \text{so option (A) is correct.}\end{array}$