Answer please both part with explanation and step by step solution...

<p><span class="eqneditor-formula" data-value="(a)\ 6.51\times10^{19}\ molecules"><span class="katex"><span class="katex-mathml"></span><span class="katex-html"><span class="base"><span class="strut" style="height:1em;vertical-align:-.25em;"></span><span class="mopen">s</span><span class="mord mathnormal">s</span><span class="mclose">s</span><span class="mspace"> </span><span class="mord">s</span><span class="mord">s</span><span class="mord">s</span><span class="mord">s</span><span class="mspace" style="margin-right:.2222222222222222em;"></span><span class="mbin">s</span><span class="mspace" style="margin-right:.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:.8141079999999999em;vertical-align:0em;"></span><span class="mord">s</span><span class="mord"><span class="mord">s</span><span class="msupsub"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:.8141079999999999em;"><span style="top:-3.063em;margin-right:.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mtight">s</span><span class="mord mtight">s</span></span></span></span></span></span></span></span></span><span class="mspace"> </span><span class="mord mathnormal">s</span><span class="mord mathnormal">s</span><span class="mord mathnormal" style="margin-right:.01968em;">s</span><span class="mord mathnormal">s</span><span class="mord mathnormal">s</span><span class="mord mathnormal">s</span><span class="mord mathnormal" style="margin-right:.01968em;">s</span><span class="mord mathnormal">s</span><span class="mord mathnormal">s</span></span></span></span></span></p><p><span class="eqneditor-formula" data-value="(b)\ 1.74\times10^{4}\ molecules"><span class="katex"><span class="katex-mathml"></span><span class="katex-html"><span class="base"><span class="strut" style="height:1em;vertical-align:-.25em;"></span><span class="mopen">s</span><span class="mord mathnormal">s</span><span class="mclose">s</span><span class="mspace"> </span><span class="mord">s</span><span class="mord">s</span><span class="mord">s</span><span class="mord">s</span><span class="mspace" style="margin-right:.2222222222222222em;"></span><span class="mbin">s</span><span class="mspace" style="margin-right:.2222222222222222em;"></span></span><span class="base"><span class="strut" style="height:.8141079999999999em;vertical-align:0em;"></span><span class="mord">s</span><span class="mord"><span class="mord">s</span><span class="msupsub"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:.8141079999999999em;"><span style="top:-3.063em;margin-right:.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mtight">s</span></span></span></span></span></span></span></span></span><span class="mspace"> </span><span class="mord mathnormal">s</span><span class="mord mathnormal">s</span><span class="mord mathnormal" style="margin-right:.01968em;">s</span><span class="mord mathnormal">s</span><span class="mord mathnormal">s</span><span class="mord mathnormal">s</span><span class="mord mathnormal" style="margin-right:.01968em;">s</span><span class="mord mathnormal">s</span><span class="mord mathnormal">s</span></span></span></span></span></p><p> </p>