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From 392mg of $H_2SO_4,1.204\times 10^{21}$ molecules are removed.How many moles of $H_2SO_4$ are left?

$\begin{array}{1 1}(a)\;2.0\times 10^{-3}& (b)\;1.2\times 10^{-3} \\(c)\;4.0\times 10^{-3}& (d)\;1.5\times 10^{-3}\end{array}$

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392mg=$\large\frac{392}{1000}$g
No of moles of $H_2SO_4=\large\frac{0.392}{98}$$=0.004$
As $N=nN_A$
$\therefore n=\large\frac{N}{N_A}$
$\Rightarrow \large\frac{1.204\times 10^{21}\text{molecules}}{6.023\times 10^{23}\text{molecules}}$
$\Rightarrow 0.002$moles
No of moles of $ H_2SO_4$ left =$0.004-0.002$
$\Rightarrow 0.02$
$\Rightarrow 2.0\times 10^{-3}$
Hence (a) is the correct option.
answered Mar 21, 2014 by sreemathi.v
 

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