Question

From $392 \,mg$ of $H _{2} SO _{4}, 1.204 \times 10^{21}$ molecules are removed. How many moles of $H _{2} SO _{4}$ are left?

• A. $2.0 \times 10^{-3}$
• B. $1.2 \times 10^{-3}$
• C. $4.0 \times 10^{-3}$
• D. $1.5 \times 10^{-3}$

Answer 1

Answer: (A)

$392 \,mg =392 / 1000\, g$

No. of moles of $H _{2} SO _{4}=\frac{0.392}{98}=0.004$

As $N=n N_{A} $

$\therefore n=\frac{N}{N_{A}}=\frac{1.204 \times 10^{21} \text { molecules }}{6.023 \times 10^{23} \text { molecules }}=0.002$ moles

No. of moles of $H _{2} SO _{4}$ left $=0.004-0.002$

$=0.002=2.0 \times 10^{-3}$