Experimental Investigations ❮ 303❯ Answers and Explanations
- A—Separating materials in solution normally
involves a physical change such as removing the
solvent through evaporation. B and C will work
on a heterogeneous mixture but not a solution
(homogeneous mixture). D is a chemical change. - A—Molarity is moles per liter. The moles are
readily determined from the grams of NaOH.
Then, simply measuring the volume (and con-
verting to liters) is all that is needed to determine
the molarity. B may seem like a good option
because titration is a common method for deter-
mining the molarity of a solution; however, one
base (KOH) cannot be used to titrate another
base (NaOH), as an acid is needed. C could be
used to determine the molality of the solution;
however, it is easier to determine the molality
from the mass of NaOH and the mass of water;
to convert to molarity, another measurement
would be required. The vapor pressure would
help determine the molarity, but it would not be
easy to get to the molarity from the vapor pres-
sure. - C—If the solute contains an electrolyte, the
solution will conduct electricity and the van’t
Hoff factor, i, will be greater than 1. The choices
do not include any conductivity measurements;
therefore, it is necessary to determine the van’t
Hoff factor. It is possible to determine the van’t
Hoff by measuring the osmotic pressure, the
boiling-point elevation, or the freezing-point
depression. The freezing-point depression may
be found by measuring the freezing point of the
solution and comparing the measured freezing-
point depression to that expected for a nonelec-
trolyte. - A—To calculate the molarity, the moles of urea
and the volume of the solution are necessary. It
is possible to calculate the volume of the solution
from the density of the solution and the mass of
the solution (110 g). The mass of urea and the
molar mass of urea give the moles of urea.- A—The units on each side must match. The rate
has the units of M/s (or M s-^1 or mol L-^1 s-^1 ).
Substituting units for symbols changes the rate
law from Rate = k [Br-] [ BrO 3 - ] [H+]^2 to
M/s = k [M ][M ] [M^2 ] or M/s = k[M^4 ]. From
this, k must have the units 1/(s M^3 ) or s-^1 M -^3. - A—At G, all the C 2 O 42 - has been converted to
HC 2 O 4 - , and the moles of HC 2 O 4 - will equal
the moles of C 2 O 42 - originally present. It will
require an equal volume of acid to titrate an
equal number of moles of HC 2 O 4 - , as required
for the C 2 O 42 -. - B—The reaction with HCl converts all the
C 2 O 42 - to HC 2 O 4 -. The Na+ has not reacted, so
it is still present. The Cl- is from the HCl and
remains in solution because it has not reacted.
The H+ from the acid reacted with the C 2 O 42 - to
form HC 2 O 4 - and is no longer present (in a sig-
nificant amount). Other than water, all species are
strong electrolytes and exist as ions in solution. - B—A source of cations and anions is necessary
for the operation of a cell to keep the charges
in each compartment neutral. If there is no salt
bridge, there is no ion source and the cell cannot
operate (zero voltage). - A—The size of the electrode is irrelevant to the
cell voltage. - B—The radioactive decay process follows first-
order kinetics; only B will give a linear graph
for a first-order process. A applies to zero-order
kinetics. C applies to second-order kinetics. D is
not a kinetics graph.