Rank these chemical species by increasing pH of an 0.1 M aqueous solution of each. That is, imagine making an 0.1 M solution of each species. Select 1 next to the species that makes the solution with the lowest pH. Select 2 next to the species that makes the solution with the next higher pH, and so on.
Notice that some of the rankings have been filled in for you already. Also notice that water is on the list. For that particular case, just compare the pH of pure water to the pH of the other solutions.
Note for advanced students: for all charged species, you may assume the necessary counterions act as neither acids nor bases.
| species | relative pH of 0.1 M aqueous solution |
| :-------------- | :-------------------------------------- |
| [tex]$H _3 O ^{+}$[/tex] | 1 (lowest) |
| [tex]$C _6 H _5 COOH$[/tex] | -3 |
| [tex]$C _6 H _5 COO ^{-}$[/tex] | (Choose one) |
| [tex]$HONH _2$[/tex] | 8 (highest) |
| [tex]$HONH _3{ }^{+}$[/tex] | (Choose one) |
| HCOOH | 2 |
| [tex]$HCOO ^{-}$[/tex] | (Choose one) |
| [tex]$H _2 O$[/tex] | (Choose one) |
Answer (2)
Rank the chemical species by increasing pH.
Identify acids and bases and their relative strengths.
Use tool calculations to determine pH values: H ON H 3 + (3.52), H 2 O (7.0), H CO O − (8.37), C 6 H 5 CO O − (8.60).
Final ranking: H ON H 3 + (4), H 2 O (5), H CO O − (6), C 6 H 5 CO O − (7).
Explanation
Problem Analysis We are asked to rank the pH of 0.1 M aqueous solutions of the given chemical species from lowest to highest. We are given the following species: H 3 O + , C 6 H 5 COO H , C 6 H 5 CO O − , H ON H 2 , H ON H 3 + , H COO H , H CO O − , and H 2 O . Some of the rankings are already provided: H 3 O + is 1 (lowest pH), H COO H is 2, C 6 H 5 COO H is 3, and H ON H 2 is 8 (highest pH). We need to determine the relative pH ranking of C 6 H 5 CO O − , H ON H 3 + , H CO O − , and H 2 O .
Identifying Acids and Bases First, let's identify the acids and bases in the list. The acids are H 3 O + , C 6 H 5 COO H , H COO H , and H ON H 3 + . The bases are C 6 H 5 CO O − , H CO O − , H ON H 2 , and H 2 O . We know that strong acids have the lowest pH, and strong bases have the highest pH.
Ranking Acids We are given that H 3 O + has the lowest pH (1), H COO H is 2, and C 6 H 5 COO H is 3. This indicates the order of acidity among these species. H ON H 3 + is the conjugate acid of the weak base H ON H 2 . We can calculate the pH of each of these solutions to determine their relative ranking.
Calculating pH Values To rank the remaining species, we need to consider their relative acidity or basicity. We can use the K a values for the weak acids and K b values for the weak bases to compare their strengths. However, since we are not given these values, we will use the given rankings and general chemical knowledge along with the tool calculations.
From the tool calculations, we have the following pH values:
H COO H : pH = 2.37
C 6 H 5 COO H : pH = 2.60
H ON H 3 + : pH = 3.52
H 2 O : pH = 7.0
H CO O − : pH = 8.37
C 6 H 5 CO O − : pH = 8.60
Final Ranking Based on these pH values, we can rank the species as follows:
H 3 O + (given)
H COO H (given)
C 6 H 5 COO H (given)
H ON H 3 + (pH = 3.52)
H 2 O (pH = 7.0)
H CO O − (pH = 8.37)
C 6 H 5 CO O − (pH = 8.60)
H ON H 2 (given)
Conclusion Therefore, the final ranking is: H 3 O + (1), H COO H (2), C 6 H 5 COO H (3), H ON H 3 + (4), H 2 O (5), H CO O − (6), C 6 H 5 CO O − (7), H ON H 2 (8).
Examples
Understanding the pH of different chemical solutions is crucial in various real-world applications. For instance, in agriculture, knowing the pH of the soil helps farmers select the right crops and optimize their growth. In medicine, maintaining the correct pH balance in the body is essential for proper bodily functions. In environmental science, monitoring the pH of water sources helps assess pollution levels and protect aquatic life. By understanding the relative acidity or basicity of different substances, we can make informed decisions and solve practical problems in many fields.
To rank the chemical species by increasing pH in a 0.1 M solution, we identify acids and bases based on their strengths. The order from lowest pH to highest is: H 3 O + , H COO H , C 6 H 5 COO H , H ON H 3 + , H 2 O , H CO O − , C 6 H 5 CO O − , and H ON H 2 at the highest pH. Therefore, the final ranking is according to their respective pH values.
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