CH 302 Spring 200
8
Worksheet 8 Answer Key
1.
NaH
2
PO4 (conc.
= C
NaH2PO4
) is dissolved in water.
Write the mass balance equation for this system.
Answer:
C
NaH2PO4
= [H
3
PO
4
] + [H
2
PO
4

] + [HPO
4
2
] + [PO
4
3
]
2.
Write the charge balance equation for the solution in question 1.
Answer:
[H
+
] = [OH

] + [H
2
PO
4

] + 2[HPO
4
2
] + 3[PO
4
3
]
3.
Write the charge balance equation for a solution containing all of the following ions: H
+
, OH

, Na
+
, Ba
2+
,
PO
4
3
, Ag
3+
, SO
4
2
, and COOH

.
Answer:
[H
+
] + [Na
+
] + 2[Ba
2+
] + 3[Ag
3+
] = [OH

] + [COOH

] + 2[SO
4
2
] + 3[PO
4
3
]
4.
NaH
2
PO
4
, LiHCO
3
, HCl, NaCl, and LiOH are all dissolved in water.
How many equations are needed
to completely describe this system?
Answer:
In solution, we have: H
+
, OH

, Na
+
, H
3
PO
4
, H
2
PO
4

, HPO
4
2
, PO
4
3
, Li
+
, H
2
CO
3
, HCO
3

, CO
3
2
,
and Cl

.
So we need 12 equations.
5.
Write a charge balance equation for the system described in question 4.
Answer:
[H
+
] + [Na
+
] + [Li
+
] = [OH

] [Cl

] + [H
2
PO
4

] + 2[HPO
4
2
] + 3[PO
4
3
] + [HCO
3

]
+ 2[CO
3
2
]
6.
Write the mass balance equation for H
2
CO
3
(conc. = C
H2CO3
) in water.
Answer:
C
H2CO3
= [H
2
CO
3
] + [HCO
3

] + [CO
3
2
]
7.
Find the pH of 10
8
M HCl like you would have for the last two quizzes.
Then find it using the exact
expression, [H
+
]
2
– [H
+
]C
HCl
– K
w
= 0.
Compare the two answers.
Answer: