A compound was found to have 85.7% carbon and 14.3% hydrogen. Its molecular mass is 84. What is its empirical formula? What is its molecular formula?

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Answer 1 · 2016-12-13T08:44:51

$"empirical formula"$ $=$ $CH_2$ .

$"molecular formula"$ $=$ $C_6H_12$

AS with all these problems, we assume (for simplicity) $100*g$ of unknown compound.

And thus there are $(85.7%xx100*g)/(12.011*g*mol^-1)$ with respect to carbon, i.e. $7.14*mol*C$

And $(14.3%xx100*g)/(1.00794*g*mol^-1)$ with respect to hydrogen, i.e. $14.1*mol*H$ .

And thus the $"empirical formula"$ , the simplest whole number ratio that defines constituent elements in a species is $CH_2$ .

Now the $"molecular formula"$ is alway a whole number multiple of the $"empirical formula"$ :

$"empirical formula"xxn$ $=$ $"molecular formula"$

So $(12.011*g*mol^-1+2xx1.00794*g*mol^-1)xxn=84*g*mol^-1.$

So $n=6$ , and $"molecular formula"$ $=$ $6xxCH_2$ $=$ $C_6H_12$