Problem 68 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

A certain sample of rainwater gives a yellow colour with methyl red [pH range \(4.2(\) red \()-6.2(\) yellow \()\) ]and a yellow colour with phenol red [pH range \(6.4\) (yellow) \(-8.0\) (red)]. What is the approximate \(\mathrm{pH}\) of the water? Is the rainwater acidic, neutral, or basic? (a) \(6.3\), acidic (b) \(6.1\), acidic (c) \(6.5\), acidic (d) \(6.3\), basic

Step-by-Step Solution

Answer

The approximate pH of the water is 6.3, and the rainwater is acidic. Hence, the correct answer is (a) 6.3, acidic.

Step 1 - Determine the possible pH range based on methyl red

We are told that the rainwater gives a yellow colour with methyl red, which changes colour from red to yellow between pH 4.2 and 6.2. Therefore, the pH of the rainwater must be greater than 6.2, since at pH 6.2 it starts turning yellow.

Step 2 - Determine the possible pH range based on phenol red

We are also told that the rainwater gives a yellow colour with phenol red, which changes colour from yellow to red between pH 6.4 and 8.0. Therefore, the pH of the rainwater must be less than 6.4, since at pH 6.4 it is yellow and not yet turning red.

Step 3 - Find the pH interval where both indicators agree

Both indicators suggest that the water is in the yellow range; it is yellow with methyl red at a pH above 6.2 and yellow with phenol red at a pH below 6.4. The pH value where both indicators will show yellow lies between 6.2 and 6.4.

Step 4 - Approximate the pH of the rainwater

The only approximate pH value given in the options that lies within the intersection of the pH ranges from both indicators is 6.3.

Step 5 - Determine the nature of the rainwater

The pH value of 6.3 is less than the neutral pH value of 7, which means the water is slightly acidic.

Step 6 - Select the correct answer

The approximate pH of the water is 6.3, and it is acidic. Thus, option (d) 6.3, basic is incorrect, while option (a) 6.3, acidic is the correct answer.

Key Concepts

Acid-base indicators

Understanding acid-base indicators is crucial for interpreting pH changes in solutions. These substances change color at specific pH levels, visually indicating whether a solution is acidic, basic (alkaline), or neutral. Methyl red and phenol red, as mentioned in the exercise, are typical examples. Methyl red shifts from red to yellow over a pH range of 4.2 to 6.2, while phenol red changes from yellow to red between pH 6.4 and 8.0.

It's important to note that indicators are carefully selected based on their transition range to match the expected pH range of the solution being evaluated. Therefore, by using a combination of indicators, one can pinpoint the approximate pH of a solution more accurately. In this exercise, the yellow coloration with both indicators suggested a pH range where they overlap, leading to the correct interpretation that the pH was around 6.3.

Determining pH of solutions

Determining the pH of a solution involves finding its exact acidic or basic nature. The pH scale ranges from 0 to 14, with 7 being neutral. Values below 7 indicate acidity, and those above point to alkalinity. To determine the pH of a solution, indicators like litmus, phenolphthalein, or electronic pH meters can be utilized.

When using indicators, the color change provides a qualitative measure of pH. For a more precise determination, a pH meter, which measures the hydrogen ion concentration electronically, would serve better. In classroom or instructional settings, the use of discrete pH indicators is often a convenient and accessible method, as was effectively demonstrated in the step-by-step breakdown of the exercise provided.

Chemical properties of rainwater

Rainwater's chemical properties can reveal much about local and global environmental conditions. Typically, pure rainwater has a pH close to that of neutral water, which is 7. However, due to dissolved carbon dioxide and pollutants, rainwater often exhibits slightly acidic characteristics, usually with a pH ranging from 5.5 to 6.5.

When determining rainwater's acidity as in our exercise, it's crucial to understand this natural acidity due to carbon dioxide, which forms carbonic acid upon contact with water. In comparison, any pH level significantly below this range might indicate the presence of stronger acids, such as sulfuric or nitric acid, which can result from industrial emissions and lead to acid rain—a serious environmental issue.