192 ❯ STEP 4. Review the Knowledge You Need to Score High
- C—The reaction is
Sr^2 +(aq) + SO 42 - (aq) → SrSO 4 (s)The strontium nitrate solution contains
()
+
70.0 mL0.20 mol Sr(NO )
1, 000 mL1 mol Sr
1 mol Sr(NO)32 2
32= 0.014 mole Sr^2 +
The sodium sulfate solution contains
()
−
30.0 mL
0.10 mol NaSO
1, 000 mL1 mol SO
1 mol Na SO24 4
2
24= 0.0030 mole SO 42 -
The strontium and sulfate ions react in a 1:1 ratio,
so 0.0030 mole of sulfate ion will combine with
0.0030 mole of strontium ion, leaving 0.011 mole
of strontium in a total volume of 100.0 mL. The
final strontium ion concentration is
0.011 mol Sr+
100.0 mL1 mL
0.001 L2
= 0.11 M- B—A (nitrous acid) and D (acetic acid) are weak
acids. Weak acids and bases are weak electrolytes.
C (ethanol) is a nonelectrolyte. Potassium nitrate
(B) is a water-soluble ionic compound; such
compounds are normally strong electrolytes. - B—The number of moles of chloride ion
needed is
(400 mL)1.0 mol Cl
1,000 mL
−
= 0.40 mole Cl-The initial number of moles of chloride ion in
the solution is
()
−
400 mL0.30 mol MgCl
1,000 mL2 mol Cl
1 mol MgCl2
2= 0.24 mole Cl-
The number of moles needed
= (^) []()−
−
0.40 0.24 mol Cl −
1 mol CaCl
2 mol Cl(^2)
= 0.080 mole CaCl 2
- D—Both acids are strong acids and yield 1 mole
of H+ each. Calculate the number of moles of H+
produced by each acid. Divide the total number
of moles by the final volume. The calculation is
() ()
+
++
30.0 mL
0.50 mol H
1, 000 mL 70.0 mL1.00 mol H
1, 000 mL=
0.085 mol H+
100.0 mL1 mL
0.001 L^
= 0.085 mole H+- C—To produce a molar solution of any type, the
final volume must be the desired volume. This
eliminates answer D. B involves mass of water
instead of volume. A calculation of the required
mass will allow a decision between A and C:
()
3.0 L
0.20 mol K PO
L212 g KPO
1 mol K PO34 34
34
= 130 g K 3 PO 4- D—The calculation is
() 5.0 mol MgSO
1,000 mL 100.0 mL120.4 g MgSO
1 mol MgSO44
4
= 60.0 g MgSO 4- D—This is a dilution problem: Vbefore =
MV
M()()
()afterafter
before()
()()
6.0 M HNO 0.500 L
16.0 M HNO1,000 mL
1 L3
3
= 190 mL- A—To calculate the molarity, the moles of urea
and the volume of the solution are necessary. The
density of the solution and the mass of the solu-
tion give the volume of the solution (it may be
necessary to convert to liters). The mass of urea
and the molecular weight of urea give the moles
of urea. - C—The strong electrolyte with the greatest con-
centration of ions is the best conductor. D is a
weak electrolyte, not a strong electrolyte. The
number of ions for the strong electrolytes may be
found by simply counting the ions: A, 2; B, 2; C, 4.
The best conductor has the greatest value when
the molarity is multiplied by the number of ions.