Question

From 320 mg. of $O_2$, $6.023\times {10}^{20}$ molecules are removed, the no. of moles remained are

• A. 9 x 10^-3 moles
• B. 9 X 10-2 moles
• C. Zero
• D. 3 X 10-3 moles

Answer 1

Answer: (A)

The explanation for the correct option

Option(D) 9 x 10^-3 moles

The number of moles of a given element can be calculated by using the equation:

$No.ofmoles$ = $\frac{Wt}{M.Wt}$ or $\frac{No.ofmolecules}{N_A}$

Hence, the number of moles of oxygen in 320 gm is $No.ofmoles=\frac{givenmass}{molarmass}=\frac{320\times {10}^{-3}}{32}=10\times {10}^{-3}moles$

Hence, the number of moles in $6.022\times {10}^{20}$ molecules is $No.ofmoles=\frac{No.ofmolecules}{N_A}=\frac{6.022\times {10}^{20}}{6.022\times {10}^{23}}=1\times {10}^{-3}$

Hence, the no. of moles remained are $10\times {10}^{-3}-1\times {10}^{-3}=9\times {10}^{-3}moles$

Hence Option(D) 9 x 10-3 moles is the remaining moles.