Answer
2.91 million
Work Step by Step
Calculate the number of moles of methane using Avogadro's number:
$n=\dfrac{N}{N_A}$
$n=\dfrac{8.00\cdot 10^6}{6.022\cdot10^{23}}=1.33\cdot10^{-17}\ mol$
Calculate the molar mass of methane:
Atomic weights: C - 12.011, H - 1.008
$M=12.011+4\cdot1.008=16.043\ g/mol$
Calculate the mass in this amount of methane:
$n=\frac mM$
$m=1.33\cdot10^{-17}\ mol\cdot 16.043\ g/mol$
$m=2.13\cdot10^{-16}\ g$
This is the same as the mass of the propane sample. Calculate the molar mass of propane:
$M=3\cdot 12.011+8\cdot 1.008=44.097\ g/mol$
Calculate the number of moles of propane:
$n=\frac mM$
$n=\dfrac{2.13\cdot10^{-16}\ g}{44.097\ g/mol}=4.83\cdot10^{-18}\ mol$
Calculate the number of molecules of propane:
$n=\frac N{N_A}$
$N=4.83\cdot10^{-18}\ mol\cdot 6.022\cdot 10^{23}=2.91\cdot 10^6\ molecules$
The sample has 2.91 million propane molecules.
