The average salinity of normal seawater, expressed as the percentage of dissolved salts by mass, is approximately equal to which of the following values?

Introduction / Context:
Salinity is a basic concept in geography and oceanography, describing how much dissolved salt is present in seawater. It is usually expressed either in parts per thousand or as a percentage. For general knowledge and competitive exams, students are expected to know the approximate average salinity value of the world's oceans, which remains fairly stable globally, except in special regions such as enclosed seas or areas with large freshwater input.

Given Data / Assumptions:

• The question asks for the average salinity of seawater as a percentage.
• Options are 35 percent, 3.5 percent, 3 percent, and 2.5 percent.
• Average ocean salinity is commonly cited as around 35 parts per thousand.
• This value needs to be converted properly into a percentage to match one of the options.

Concept / Approach:
Average salinity of seawater is about 35 parts per thousand, which means 35 grams of salts per 1000 grams of seawater. To convert parts per thousand to a percentage, divide by ten. Thus, 35 parts per thousand is equal to 3.5 percent by mass. Therefore, among the options given, 3.5 percent is the correct approximate representation of average seawater salinity.

Step-by-Step Solution:
Step 1: Recall that typical open ocean salinity is about 35 parts per thousand. Step 2: Understand that parts per thousand means grams of salt per 1000 grams of water. Step 3: Convert 35 parts per thousand into percentage by using the relation: percentage = (parts per thousand) / 10. Step 4: Perform the conversion: 35 / 10 = 3.5, so salinity is approximately 3.5 percent. Step 5: Choose 3.5 percent from the list of options as the value that matches this calculation.

Verification / Alternative check:
An alternative check is to think of realistic percentages. If the ocean contained 35 percent salt by mass, it would be almost one third salt, which is unrealistically high. Everyday table salt solutions used in kitchens and food preservation are usually much less concentrated than that. On the other hand, a value around a few percent aligns with everyday understanding that seawater is salty yet still mostly water. This consistency confirms that 3.5 percent is the appropriate figure.

Why Other Options Are Wrong:
The 35 percent option misinterprets the 35 parts per thousand figure by forgetting the division by ten, making the value ten times too large. The 3 percent and 2.5 percent options are lower than the widely accepted average and would underestimate ocean salinity. While local seas or diluted coastal waters can have different values, the global open ocean average is closer to 3.5 percent, not these smaller numbers.

Common Pitfalls:
Many students remember the 35 and quickly choose 35 percent without performing the unit conversion, which is a typical trap. Others may think that a smaller percentage such as 2.5 or 3 is safer because it sounds less extreme, but this goes against standard textbook values. Carefully distinguishing between parts per thousand and percent is crucial for avoiding these errors in environmental and physical geography questions.

Final Answer:
The average salinity of seawater is approximately 3.5% by mass.