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This is the question in my text book:

From $392\:\mathrm{mg}$ of $\ce{H2SO4}$, $1.204 \times 10^{21}$ molecules are removed. How many moles of $\ce{H2SO4}$ are left?

What I tried doing:

First I tried calculating the number of moles in $392\:\mathrm{mg}$ of $\ce{H2SO4}$, which came to $\frac{0.392\:\mathrm{g}}{98\:\mathrm{g\:mol^{-1}}} = 0.004\:\mathrm{mol}$, and then multiplying the of moles by Avagadro's number to get the number of molecules in $392\:\mathrm{mg}$. I then tried subtracting "$1.204 \times 10^{21}$ molecules," but even after doing all this, and trying a few more methods, my answer was not correct.

The correct answer is: $2.0 \times 10^{-3}$ molecules

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1) correct, $4 \cdot 10^{-3}$ moles of $\ce{H2SO4}$

2) $1.204 \cdot 10^{21}$ molecules divided by $6.023 \cdot 10^{23}$ molecules/mole gives $2 \cdot 10^{-3}$ moles

3) subtracting the moles found in step 2 from the moles found in step 1 yields $2 \cdot 10^{-3}$ moles, the correct answer.

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    $\begingroup$ So, we have to convert everything first into moles, and then subtract them. :-D thanks for clearing the doubt. :-) $\endgroup$ – Anoneemus May 18 '15 at 17:29

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