The empirical formula is the simplest whole number molar ratio of the elements in the compound.
We must convert the masses of "C"C and "H"H to moles and then find the ratio.
Step 1. Calculate the mass of "H"H
"Mass of H" = "mass of hydrocarbon - mass of C" = "2.00 g - 1.68 g = 0.32 g"Mass of H=mass of hydrocarbon - mass of C=2.00 g - 1.68 g = 0.32 g
Step 2. Calculate the moles of "C"C and "H"H
"Moles of C" = 1.68 color(red)(cancel(color(black)("g C"))) × "1 mol C"/(12.01 color(red)(cancel(color(black)("g C")))) = "0.1399 mol C"
"Moles of H" = 0.32 color(red)(cancel(color(black)("g H"))) × "1 mol H"/(1.008 color(red)(cancel(color(black)("g H")))) = "0.317 mol H"
From this point on, I like to summarize the calculations in a table.
bbul("Element"color(white)(m) "Mass/g"color(white)(m) "Moles"color(white)(m) "Ratio"color(white)(m)×4color(white)(m)"Integers")
color(white)(mm)"C" color(white)(XXXm)1.68 color(white)(Xmll)0.1399 color(white)(Xm)1color(white)(mmml)4color(white)(mmmml)4
color(white)(mm)"H" color(white)(XXXm)0.32 color(white)(mmll)0.317 color(white)(Xml)2.27color(white)(mm)9.08 color(white)(mmm)9
The molar ratio is "C:H = 4:9".
The empirical formula is "C"_4"H"_9.
Here is a video that illustrates how to determine an empirical formula.
