Question #4bd23

2 Answers
Jan 4, 2015

The empirical formula is #CH#. The molecular formula is #C_6H_6#.

Write down the ratio of the elements by mass.

%C = 100 - 7.74 = 92.26.

Ratio C : H by mass:

C : H = 92.26 : 7.74

To get the ratio by moles we divide by the #A_r# values:

#A_rC=12#
#A_rH=1#

Ratio C : H by moles:

#(92.26)/(12):(7.74)/(1)#

= #7.69:7.74#

Dividing through by 7.69 we get:

1 : 1.006 which we can round down to:

1 : 1

So the empirical formula is #CH#

To find the molecular formula we find how many #M_r# units of #CH# will fit into the #M_r# of benzene.

#M_rCH=12+1=13#

#(78.1)/(13)=6#

So the molecular formula is #C_6H_6#.

Jan 4, 2015

The empirical formula is #(CH)_n# and the molecular formula is #C_6H_6#.

Since we know that a hydrocarbon only contains carbon and hydrogen, we can use the percent of hydrogen given to determine how much carbon the compound contains

#%"carbon" = 100%-7.74% = 92.26%#

The next step in determining the empirical formula is to divide each element's percentage by its atomic mass

#"For C": (92.26%)/12.0 = 7.69#

#"For H": (7.74%)/(1.00) = 7.74#

You then divide each of these two numbers by the smallest one to get the ratios of the two elements in the molecule:

#"For C": 7.69/7.69 = 1#

#"For H": 7.74/7.69 ~= 1#

This is your empirical formula: #(CH)_n# - the ratio between carbon and hydrogen atoms is #1:1#. We now have to determine how many of each the molecule contains.

This is done by using benzene' molar mass to determine the value of #n#:

#78.10 = (1 * 12.0 + 1 * 1.00) * n = 13.0 * n#

#n = 78.10/13.0 = 6#

Therefore, your molecular formula is #C_6H_6#.