Chemistry 12 Worksheet 4-7 Indicators Answer Key

Chemistry 12 Worksheet 4-7 Indicators Answer Key

Chemistry 12 Worksheet 4-7 focuses on indicators, detailing the behavior of various indicators in different pH environments. This answer key provides solutions to questions about the color changes of indicators like HInd, Orange IV, and methyl orange, as well as calculations for pKa and Ka values. It is designed for high school chemistry students preparing for exams and understanding acid-base chemistry. The worksheet includes equilibrium equations and analysis of weak acids, making it a valuable resource for mastering indicator concepts.

Key Points

  • Includes answers for Chemistry 12 Worksheet 4-7 on indicators.
  • Covers pH ranges and color changes for various indicators.
  • Provides calculations for pKa and Ka values of weak acids.
  • Explains the equilibrium equations for indicators like HInd.
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Chemistry 12 Worksheet 4-7- Indicators
Chemistry 12—Worksheet 4-7- Indicators Page 1 of 3 Pages
Chemistry 12
Worksheet 4-7—Indicators
Name _________________________________
Due Date ______________________________
Correct & Hand In by ___________________________
This worksheet covers material from class notes and Student Workbook pages 159-163
1. An indicator HInd is yellow in 0.1M NaOH and blue in 0.1M HCl. The pH range
in which the colour change occurs in this indicator is 3.6 - 5.2.
a) Write the equilibrium equation describing this indicator.(1)
______________________________________________________
b) What is the colour of HInd? _______ What is the colour of Ind
-
? _______ (2)
c) What is the pH at the transition point of this indicator? ____________ (1)
d) What is the value of pKa for this indicator? ______________ (1)
e) What is the Ka of this indicator? _____________________ (1)
f) At pH = 2.2, this indicator is the colour ______ and [HInd]
(>,<,=)
___ [Ind
-
].(2)
g) At pH = 7.0, this indicator is the colour ______ and [HInd]
(>,<,=)
___ [Ind
-
].(2)
h) At pH = 11.3, this indicator is the colour ______ and [HInd]
(>,<,=)
___ [Ind
-
].(2)
i) At pH = 4.4, this indicator is the colour ______ and [HInd]
(>,<,=)
___ [Ind
-
].(2)
j) At pH = 4.3, this indicator is the colour ______ and [HInd]
(>,<,=)
___ [Ind
-
].(2)
k) In 0.001M HNO
3
, this indicator is the colour ____ and [HInd]
(>,<,=)
___ [Ind
-
].(2)
l) In 0.001M KOH, this indicator is the colour _____ and [HInd]
(>,<,=)
___ [Ind
-
].(2)
m) At the transition point, is [H
3
O
+
] = Ka (indicator.)? _______(1)
21
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Chemistry 12 Worksheet 4-7- Indicators
Chemistry 12—Worksheet 4-7- Indicators Page 2 of 3 Pages
2. A solution turns yellow when Orange IV is added and red when methyl orange is added.
Give the approximate pH range of the solution. (1)
Answer pH = ______- ______
3. A solution turns yellow when chlorophenol red is added and also yellow when methyl
orange is added. Give the approximate pH range of the solution. (1)
Answer pH = ______- ______
4. A solution turns magenta when phenolphthalein is added and yellow when alizarin
yellow is added. Determine the approximate [H
3
O
+
]. (1)
Answer [H
3
O
+
] = ____________
5. A 0.10 M solution of a weak acid HX turns red in both chlorophenol red and in
neutral red indicator.
a) Determine the approximate pH of this solution of HX. ____________ (1)
b) Determine the Ka of the weak acid HX (Not the Ka (indicator)!)
(Hint: Use an ICE table!) (2)
Answer Ka = ____________
6. An indicator “George Green” (HGg) turns yellow when [H
3
O
+
] drops below
1.2 x 10
-4
M and turns blue when [H
3
O
+
] rises above 1.8 x 10
-3
M. (Notice 2 SD’s)
a) Find the pH range over which the indicator changes colour.(2SD’s) (2)
pH Range _______- _______
b) Determine the pKa of the indicator “GeorgeGreen”. (1) _________________
c) What colour would 0.00019 M HCl be in this indicator? (1) ______________
d) What colour would 0.010 M NaOH be in this indicator? (1) _______________
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Chemistry 12 Worksheet 4-7- Indicators
Chemistry 12—Worksheet 4-7- Indicators Page 3 of 3 Pages
e) What colour would 0.10 M CH
3
COOH be in this indicator?
(
Show how you got [H
3
O
+
]
) (2)
Answer _______________
7. An indicator HInd turns yellow in 0.10 M HCl and blue in 0.10 M NaOH.
a) Write the equation describing the equilibrium in HInd. (1)
________________________________________________________
b) What colour is HInd? (1) _______ What colour is Ind
-
? (1) _________
c) HInd is green in the range pH = 5.4 to pH = 6.2. Determine the Ka of HInd.(1)
Ka =__________________________
d) When a few drops of HInd are added to a weak acid HA
1
, the colour is
yellow. Which is the stronger acid, HInd or HA
1
? (1)
Answer _______is the stronger acid.
e) When a few drops of HInd are added to a weak acid HA
2
, the colour is
blue. Which is the stronger acid, HInd or HA
2
? (1)
Answer _______is the stronger acid.
f) Which acid is stronger, HA
1
, or HA
2
? (1) _____________________
g) List the acids HInd, HA
1
and HA
2
in order of strength from strongest to weakest.(1)
____________> ___________ > ___________
h) List the bases Ind
-
, A
1
-
, and A
2
-
, in order of strength from strongest to weakest.(1)
____________> ___________ > ___________
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FAQs of Chemistry 12 Worksheet 4-7 Indicators Answer Key

What color does the indicator HInd turn in different pH levels?
The indicator HInd is yellow in acidic conditions (pH < 3.6) and blue in basic conditions (pH > 5.2). At the transition point around pH 4.4, HInd appears green, indicating a mix of both forms. This behavior illustrates how indicators can signal the acidity or basicity of a solution.
How do you determine the pKa of an indicator?
The pKa of an indicator can be determined using the formula pKa = -log(Ka), where Ka is the acid dissociation constant. In the case of HInd, the pKa is calculated based on the pH at which the color change occurs, typically around the midpoint of the transition range. This value helps in understanding the strength of the acid in comparison to other acids.
What is the significance of using indicators in acid-base titrations?
Indicators are crucial in acid-base titrations as they provide a visual cue for the endpoint of the titration. They change color at specific pH levels, allowing chemists to determine when the reaction between the acid and base is complete. This visual change helps in accurately measuring the concentration of unknown solutions.
What is the role of weak acids in the context of indicators?
Weak acids play a significant role in the behavior of indicators, as their dissociation in solution affects the pH and, consequently, the color of the indicator. For example, in the presence of a weak acid like HX, the pH can influence whether the indicator appears in its acidic or basic form. Understanding this relationship is key for students studying acid-base chemistry.
How does the concentration of an acid affect the color of an indicator?
The concentration of an acid directly influences the pH of the solution, which in turn affects the color of the indicator. A higher concentration of acid typically results in a lower pH, causing the indicator to shift to its acidic color. Conversely, diluting the acid raises the pH, leading to the basic color of the indicator.

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