spano velasco (ds28282) – hw 02 – Fakhreddine – (52395)
1
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001
1.0 points
We have an aqueous solution that contains
26% (by mass) of a hypothetical solute Z. The
formula weight of the solute Z is 175 g/mol.
The density of the solution is observed to be
1
.
4 g/mL. What is the molarity of Z in this
solution?
Correct answer: 2
.
08 mol
/
L.
Explanation:
% of Z = 26%
FW
Z
= 175 g
/
mol
density
soln
= 1
.
4 g
/
mL
The solution is 26% Z by mass.
We can
write this as the ratio
26 g Z
100 g solution
.
The
formula
weight
can
be
written
as
175 g Z
1 mol Z
.
The
density
of
the
solution
is
1
.
4 g/mL, more fully written as
1
.
4 g soln
1 mL soln
.
We want to calculate the molarity of Z,
which has the units of moles Z per liter so
lution.
The density value already has the
volume of solution in the denominator, so we
start with the value and convert from mL to
L:
? g soln
1 L soln
=
1
.
4 g soln
1 mL soln
×
1000 mL soln
1 L soln
=
1400 g soln
1 L soln
We use the percent by mass to convert the
numerator from grams solution to grams Z,
then we use the formula weight of Z to convert
from grams to moles Z:
? mol Z
1 L soln
=
1400 g soln
1 L soln
×
26 g Z
100 g soln
×
1 mol Z
175 g Z
= 2
.
08 mol
/
L
002
1.0 points
What additional information, if any, would
enable you to calculate the molality of a 7.35
molar solution of a nonelectrolyte solid dis
solved in water?
1.
Only
the
density
of
water
would
be
needed.
2.
Only the molecular weight of the solute
would be needed.
3.
None is needed.
4.
Only the density of the solution would be
needed.
5.
Both the density of the solution and
the molecular weight of the solute would be
needed.
correct
Explanation:
molarity =
mol solute
L solution
molality =
mol solute
kg solvent
The density of the solution can be used to
convert volume (1 L) of solution into mass of
solution.
Then the molecular weight of the
solute (given or calculated from the formula)
can be used to convert the number of moles
solute in 1 L solution into mass of solute
in grams.
The mass of the solvent is the
difference between the mass of the solution
and the mass of the solute (both of which
have been calculated). Substitute the values
into the molality formula and calculate.
003
1.0 points
The mole fraction of a certain nonelectrolyte
compound in a solution containing only that
substance and water is 0.100. The molecular
weight of water is 18.0 g/mol.
What addi
tional information is needed to determine the
molality of the solution?
1.
No additional information; the molality
can be calculated from the information given.
correct
2.
The density of the solute.
3.
The density of the solution.
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spano velasco (ds28282) – hw 02 – Fakhreddine – (52395)
2
4.
The molecular weight of the compound.
5.
The mole fraction of water in the solu
tion.
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 Spring '07
 Holcombe
 Chemistry, Mole, Vapor pressure, Molecule, Torr, spano velasco

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