Question

Burning then sample of an organic compound with the weight of 0.00480 g it is afforded: 0.00703 g of the CO2 and 0.00283 g of the H2O, Determine the empirical formula of the compound which consists of hydrogen, carbon and oxygen?

Answer 1

The empirical formula is `"CH"_2"O"`.

Explanation:

We can calculate the masses of `"C"` and `"H"` from the masses of their oxides (`"CO"_2` and `"H"_2"O"`).

`"Mass of C" = 7.03 color(red)(cancel(color(black)("mg CO"_2))) × "12.01 mg C"/(44.01 color(red)(cancel(color(black)("mg CO"_2)))) = "1.918 g C"`

`"Mass of H" = 0.283 color(red)(cancel(color(black)("mg H"_2"O"))) × "2.016 mg H"/(18.02 color(red)(cancel(color(black)("mg H"_2"O")))) = "0.3166 mg H"`

`"Mass of O" = "Mass of compound - mass of C - mass of O" = "4.80 mg - 1.918 mg - 0.3166 mg" = "2.565 mg"`

Now, we must convert these masses to moles and find their ratios.

From here on, I like to summarize the calculations in a table.

`"Element"color(white)(X) "Mass/mg"color(white)(X) "Millimoles"color(white)(m) "Ratio"color(white)(m)"Integers"`
#stackrel(—————————————————-———)(color(white)(ll)"C" color(white)(XXXmm)1.918 color(white)(mmm)0.1597 color(white)(Xlll)1color(white)(Xmmml)1#
`color(white)(ll)"H" color(white)(XXXXm)0.3166 color(white)(mml)0.3145 color(white)(mlll)1.969 color(white)(XXl)2`
`color(white)(ll)"O" color(white)(mmmml)color(white)(ll)2.565color(white)(mmll)0.1603color(white)(mm)1.004color(white)(mml)1`

The empirical formula is `"CH"_2"O"`.