Question

A sample of a compound contains 32.0 g C and 8.0 g H. Its molar mass is 30.0 g/mol. What is the compound’s molecular formula?

Answer 1

Answer:  C2H6

Explanation:

g g/mol AVG   0.375         g/mol  

C 32   12        2.667 1 12    2   24

H   8     1             8         3       3    6      6

                     15           30

Obtain the weighted average of C and H by multiplying the grams of substance times its repective molar mass.  The AVG column is this result.  The number for C is 2.667, but we know we need a whole number for both C and H, so divide both averages by (1/2.667) 0.375 to convert C to a whole number.  H also converts to a whole number with this ratio, which is further evidence we now have the correct ratio of C and H:  they are 1:3 C:H, so the empirical formula is CH3.  The actual formula could be a multiple of this, such as C2H6, C3H9, C4H12, etc.  But we are given the molar mass if this compound, 30 g/mole,  ! mole of CH3 would only be 15 g/mole.  One mole of C2H6, however, is exactly 30 g/mole.  That matches the value we were given, so the formula is C2H6.