# Question 4bd23

##### 2 Answers
Jan 4, 2015

The empirical formula is $C H$. The molecular formula is ${C}_{6} {H}_{6}$.

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:

${A}_{r} C = 12$
${A}_{r} H = 1$

Ratio C : H by moles:

$\frac{92.26}{12} : \frac{7.74}{1}$

= $7.69 : 7.74$

Dividing through by 7.69 we get:

1 : 1.006 which we can round down to:

1 : 1

So the empirical formula is $C H$

To find the molecular formula we find how many ${M}_{r}$ units of $C H$ will fit into the ${M}_{r}$ of benzene.

${M}_{r} C H = 12 + 1 = 13$

$\frac{78.1}{13} = 6$

So the molecular formula is ${C}_{6} {H}_{6}$.

Jan 4, 2015

The empirical formula is ${\left(C H\right)}_{n}$ and the molecular formula is ${C}_{6} {H}_{6}$.

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:

$\text{For C} : \frac{7.69}{7.69} = 1$

$\text{For H} : \frac{7.74}{7.69} \cong 1$

This is your empirical formula: ${\left(C H\right)}_{n}$ - 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 $n$:

$78.10 = \left(1 \cdot 12.0 + 1 \cdot 1.00\right) \cdot n = 13.0 \cdot n$

$n = \frac{78.10}{13.0} = 6$

Therefore, your molecular formula is ${C}_{6} {H}_{6}$.