Ok, 3 questions from the topic of Atoms, molecules and
stoichiometry.
1. 2 volumes of hydrogen are mixed with 5 volumes of dry air at
100 deg cel and atmospheric pressure. The mixture is sparked so
that the combustion of hydrogen goes to completion. The air
contains oxygen and nitrogen in the molar ratio of 1:4, what is the
mole fraction of water vapour in the resulting mixture at 100 deg
cel?
Can someone guide me in the necessary steps? Anyway, when we do
the question, do we have to take into account that the oxygen is
being combusted too? (because they just said the mixture is
sparked... so perhaps O2 is undergoing combustion, not sure abt
this one)
2. A 0.0353g sample of a volatile organic compound when
vaporized completely at 100kPa and 300K occupied a vol. of
20cm3. When this vol. of vapour was completely burnt in
excess O2, 40cm3 of CO2 and 40cm3 of water vapour were formed. All
gaseous vol. were measured under identical conditions.
What could be the molecular formula of the organic compound?
C3H8
C2H2O
C2H4O
C2H3N
Dont have to help me solve this one. Just wanna ask, why cant we
use just the first part of the question to deduce the molecular
formula?
eg. no. of moles of the gaseous compound = (20/1000) / 24 =
x
0.0353g / x = Mr of the organic compound
because if i use this method, i realized the answer derived does
not tally with that given by the answer booklet. so this method
cannot be used, but why cant it be?
3. Most elements contain more than 1 isotope, atoms that have
the same chemical properties but diff masses.
Which expression correctly defintes the terms relative atomic
mass of the element? (which we know was "weight average mass of an
atom of the element realtive to 1/12 the mass of a 12-C atom")
a. the mass of 1 atom of an element relative to the mass of 1
atom of 12-C
b. the mass of 1 mol of atoms of an element divided by
6.02X10^23
c. the mass of 1 mol of atoms of an element relative to the mass
of 1 atom of 12-C
d. the mass of 1 mol of atoms of an element relative to the mass
of 1 mol of 12-C atom
I wrote the answer as A but B is the answer. After looking at
it, I realized A is wrong because its taking the mass of 1 atom of
12-C not 1/12 the mass. But I dont get why B is wrong. Can anyoen
subsequently explain why the rest are wrong too?
Thanks.
Q1. Let the moles of H2, O2 and N2 be 2, 1 and 4 respectively.
H2 + 0.5 O2 --> H2O
Hence 2 moles of H2 require 1 mole of O2 for complete
combustion, and will produce 2 moles of H2O.
So after the combustion is complete, there will be 2 moles of
HO2(g) and 4 moles of N2(g), with no H2(g) or O2(g) present.
Hence mole fraction of H2O(g) = 2 / (2+4) = 1/3
Q2. You can indeed use the 1st part of the question to calculate
the molar mass. P V = n R T. Since you know P, V, R and T, you can
calculate number of moles. Since you know sample mass and number of
moles, you can calculate molar mass. (In your working, you tried to
use 24dm3 as molar volume, which does not hold true under these
conditions of pressure and temperature.)
However, notice that two of the four options have identical
molar mass. Hence, you will require the next part of the question's
data to further narrow down the identity of the compound. Since
20cm3 of the compound when completely burnt in
excess O2, produced 40cm3 of CO2 and 40cm3 of water vapour, we
deduce that one molecule of the compound contains 2 atoms of C, and
that the ratio of C to H in the compound must be 2:4.
Q3.
b. "the mass of 1 mol of atoms of an element divided by
6.02X10^23"
Since "mole" = "6.02X10^23", mass of 1 mole of atoms
divided by 1 mole = mass of atom. (this is still an incomplete
definition, as the units of mass are not stated, which should be
amu or "atomic mass unit")
c. "the mass of 1 mol of atoms of an element relative to the
mass of 1 atom of 12-C"
Should be "the average mass of 1 atom of an element
relative to 1/12 the mass of 1 atom of 12-C"
d. "the mass of 1 mol of atoms of an element relative to the
mass of 1 mol of 12-C atom"
Should be "the average mass of 1 mol of atoms of an element
relative to 1/12 the mass of 1 mol of 12-C atom"