Question

Our professor was replaced 1/2 way through the semester and we are left lost. We never went over this. This was given as one question but has 3 parts. If they need to be broken down into different questions please let me know, part B & C seem more like they are together than A, but they are all clumped into one question worth 20 points.

a) What volume of toluene do you need to add to 1 mL of ethyl acetate to make an equi-molar mixture? The densidy of ethyl acetate (C₄H₈O₂) is 0.898 g/ml and the density of toluene (C₇H₈) is 0.867 g/mL.

b) List the following compounds, acetic acid, diethyl ether, ethanol, hexane, octane, pyridine) in order of elution from a gc column that separates by boiling point only.

	diethyl ether	ethyl acetate	ethanol	pyridine	acetic acid	octane
boiling point °C	34.6	77.1	78.37	115.6	118.1	125
molecular weight	46	76	46	79	60	114
order of elution

c) Explain the relative boiling points in terms of the structure and intermolecular forces in the molecules for: Diethyl ether, ethyl acetate, ethanol, pyridine, acetic acid.

Answer 1

a)

equimolar mixture means the two components are having same concentrations or same number of moles.

mass of ethyl acetate = density x volume = 0.898 g/ml x 1 ml = 0.898 g

moles of ethyl acetate = mass / molar mass = 0.898 / 88 = 0.01020 mol

As the mixture to be equimolar,

moles of ethyl acetate = moles of toulene

=> moles of toulene = 0.01020

=> mass of toulene = moles x molar mass = 0.01020 x 92 = 0.9384 g

=> volume of toulene = mass / density = 0.9384 / 0.867 = 1.08 mL

b) The chemical with lower bp will stay in the column shorter and elutes out earlier,(i.e. lower retention time).

The chemical with higher bp will stay in the column longer and have higher retention time.

Therefore the order of elution is,

diethyl ether, ethyl acetate, ethanol, pyridine, acetic acid, octane

c) Boiling point increases with increase in the magnitude of intermolecular forces.