# How do you convert 15.0 g to C_2H_6?

I think you mean how do you convert a mass of $15 \cdot g$ of ethane to moles of ethane.
$\text{Moles of ethane}$ $=$ (15.0*cancelg)/(30.07*cancelg*mol^-1)=??mol.
And thus a mass of $15 \cdot g$ of ethane represents $\frac{1}{2}$ a mole of ethane. How many ethane molecules are there in such a molar quantity?