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Given the following indicators on a pH stick, what is the PH of a solution that yields x: red, Y: blue, and Z: red?
Indicator X: pka = 4.96; when deprotonated it goes from yellow to red
Indicator Y: pka = 6.83; when deprotonated it goes from yellow to blue
Indicator Z: pka = 9.08; when deprotonated it goes from red to blue
A. 5
B. 6
C. 7
D. 8
Solution: Because Indicator Y is blue, the pH must be at least one unit greater than 6.83 (the pka of Indicator Y). Because Indicator Z is red, the pH must be at least one unit less than 9.08 (the pka of Indicator Z). The only value greater than 7.83 and less than 8.08 is 8, choice D.
Now, I knew the answer was either 7 or 8, but how do you know the pH must be "at least one unit less than 9.08" and "greater than 7.83"? Why do you need to add/subtract "1"?
Thank you in advance!
Indicator X: pka = 4.96; when deprotonated it goes from yellow to red
Indicator Y: pka = 6.83; when deprotonated it goes from yellow to blue
Indicator Z: pka = 9.08; when deprotonated it goes from red to blue
A. 5
B. 6
C. 7
D. 8
Solution: Because Indicator Y is blue, the pH must be at least one unit greater than 6.83 (the pka of Indicator Y). Because Indicator Z is red, the pH must be at least one unit less than 9.08 (the pka of Indicator Z). The only value greater than 7.83 and less than 8.08 is 8, choice D.
Now, I knew the answer was either 7 or 8, but how do you know the pH must be "at least one unit less than 9.08" and "greater than 7.83"? Why do you need to add/subtract "1"?
Thank you in advance!
