How to determine the molecular formula of this organic compound?
An organic compound containing only carbon, hydrogen and oxygen was analysed gravimetrically. When completely oxidized in air, #0.900 g# of the compound produced #1.80 g# of carbon dioxide and #0.736 g# of water. A separate #2.279 g# sample, when vaporized in a #1.00 dm3# vessel at #100°C# , had a pressure of #84 kPa# . Determine the molecular formula of the compound.
An organic compound containing only carbon, hydrogen and oxygen was analysed gravimetrically. When completely oxidized in air,
1 Answer
WARNING! Long answer! The molecular formula is
Explanation:
Calculate the empirical formula.
We can calculate the masses of
Now, we must convert these masses to moles and find their ratios.
From here on, I like to summarize the calculations in a table.
The empirical formula is
Use the Ideal Gas Law to calculate the molar mass
#color(blue)(bar(ul(|color(white)(a/a)PV = nRTcolor(white)(a/a)|)))" "#
Since
we can write the Ideal Gas Law as
#color(blue)(bar(ul(|color(white)(a/a) PV = m/MRTcolor(white)(a/a)|)))" "#
We can rearrange this to get
#M = (m/V)(RT)/P#
or
#color(blue)(bar(ul(|color(white)(a/a)M = (mRT)/(PV)color(white)(a/a)|)))" "#
∴
Calculate the molecular formula
The empirical formula mass of
The molecular mass is 84.2 u.
The molecular mass must be an integral multiple of the empirical formula mass.
The molecular formula must be twice the empirical formula.