Question

Question #4bd23

Answer 1

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`.

Answer 2

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`.