a
(a) \(200\,mg\) of \(C{O_2}\) = \(200 \times {10^{ - 3}} = 0.2\,gm\)

\(44\,gm\) of \(C{O_2}\) = \(6 \times {10^{23}}\) molecules

\(0.2\,gm\) of  \(C{O_2}\) = \(\frac{{6 \times {{10}^{23}}}}{{44}} \times 0.2 = 0.0272 \times {10^{23}}\) \( = 2.72 \times {10^{21}}\) molecule

Now \({10^{21}}\) molecule are removed.

So remaining molecules \( = 2.72 \times {10^{21}} - {10^{21}}\) \( = {10^{21}}(2.72 - 1)\) =\(1.72 \times {10^{21}}\) molecules

Now, \(6.023 \times {10^{23}}\)molecules = \(1\) \(mole\)

\(1.72 \times {10^{21}}\) molecules \( = \frac{{1 \times 1.72 \times {{10}^{21}}}}{{6.023 \times {{10}^{23}}}} = 0.285 \times {10^{ - 2}}\)

\( = 2.85 \times {10^{ - 3}}\).