How many molecules are in 35.5 g of #"Cl"_2# ?

Your strategy here will be to

• use the molar mass of chlorine gas, #"Cl"_2#, to calculate how many moles you have in that sample
• use Avogadro's number to convert the number of moles to number of molecules

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Chlorine gas has a molar mass of #"70.906 g mol"^(-1)#, which tells you that one mole of chlorine gas has a mass of #"70.906 g"#. Use the molar mass as a conversion factor to help you go from grams of chlorine gas to moles

#35.5 color(red)(cancel(color(black)("g"))) * "1 mole Cl"_2/(70.906color(red)(cancel(color(black)("g")))) = "0.5007 moles Cl"_2#

Now, a mole of a molecular compound is defined as #6.022 * 10^(23)# molecules of said compound #-># this is known as Avogadro's number.

In your case, the sample of chlorine gas will contain

#0.5007 color(red)(cancel(color(black)("moles Cl"_2))) * (6.022 * 10^(23)"molec. Cl"_2)/(1color(red)(cancel(color(black)("mole Cl"_2)))) = color(green)(|bar(ul(color(white)(a/a)color(black)(3.02 * 10^(23)"molec Cl"_2)color(white)(a/a)|)))#

The answer is rounded to three sig figs.