Determine the relative molecular mass of a volatile liquid.
initial mass of hypodermic syringe containing ethyl ethanoate=19.198g
final mass of hypodermic syringe containing ethyl ethanoate=18.975g
Atmospheric pressure=101.3kPa
initial volume of graduated gas syringe=l0.0cm3
final volume of graduated gas syringe =85.9cm3
Temperature measured=100oC
a) Calculate the relative molecular mass of ethyl ethanoate.
P (V2 – V1) = n R T = (m/M) R T
(101.3X103)[(85.9-10.0)/106] = [(19.198-18.975)/M] (8.314 )(100+273)
M = 89.9
b)
A student took an initial reading of the graduated gas syringe as 0.0cm3.
Comment whether he is right or wrong. Explain.
Wrong--Empty space is required for injecting ethyl ethanoate liquid
into the graduated gas syringe.
c) List some possible sources of errors in this experiment.
--The vapour produced by the volatile liquid may not behave ideally,
i.e. it may not obey the ideal gas law at the experimental
conditions.
--There is a great error in the measurement of the mass of volatile
liquid injected into the syringe(because the mass is very small).
--There is a possible error in recording the temperature (because the
temperature in the steam jacket may differ from that of the vapour
inside the gas syringe).
--Friction caused by the piston of the large graduated syringe.
d) Suggest THREE precautions in this experiment.
-Rinse the hypodermic syringe with the volatile liquid to ensure
smooth filling.
-Expel all air bubbles from the hypodermic syringe to ensure that it
is completely filled with the volatile liquid.
-Wait for a steady temperature before injecting propanone into the
graduated gas syringe.
-Wait for a steady volume reading, thereby ensuring the complete
vaporization of propanone
e)A student use the same apparatus above to determine the relative molecular mass of
butan-1-ol(boiling point is 118oC). Explain whether he could get an accurate result or
not.
He could not.
The butan-1-ol(boiling point is 118oC) cannot be evaporated
completely at the steam temperature (100oC). Most of the butan-1-ol
liquid remained as liquid(not evaporated).