Problem 6 - Chemistry The Central Science

Question

The Earth's oceans have a salinity of \(35 .\) What is the concentration of dissolved salts in seawater when expressed in ppm? What percentage of salts must be removed from sea-water before it can be considered freshwater (dissolved salts \(<500\) ppm \() ?[\operatorname{Section} 18.3]\)

Step-by-Step Solution

Answer

The concentration of dissolved salts in seawater is 350,000 ppm. Approximately 99.86% of salts must be removed from seawater for it to be considered freshwater.

Step 1: Convert salinity to ppm

In the given problem, the Earth's oceans have a salinity of 35. We need to express this value in parts per million (ppm). To do this, we simply multiply the salinity value by 10000. 35 (salinity) x 10000 = 350000 ppm

Step 2: Calculate dissolved salts concentration in seawater

Now that we know the dissolved salts concentration in seawater is 350000 ppm, we can move forward to finding the percentage of salts that must be removed to make it freshwater.

Step 3: Find the percentage of salts to be removed

We are given that to be considered freshwater, dissolved salts should be lesser than 500 ppm. To find the percentage of salts that must be removed to achieve this, we will calculate the difference in ppm values and then divide it by the initial ppm value (350000 ppm). Required change in ppm = Initial seawater ppm - Freshwater ppm Required change in ppm = 350000 ppm - 500 ppm = 349500 ppm Percentage of salts to be removed = (Required change in ppm / Initial seawater ppm) x 100 Percentage of salts to be removed = (349500 ppm / 350000 ppm) x 100 Percentage of salts to be removed ≈ 99.86 % So about 99.86 % of salts must be removed from seawater before it can be considered freshwater.

Key Concepts

Parts Per Million (ppm)

When we talk about 'parts per million' or ppm, we're discussing a way of expressing very dilute concentrations of substances. Imagine taking a million parts of a whole and then specifying how many parts of a specific substance are found within those million total parts. In other words, if you have 1 ppm of a substance in water, it means that out of one million particles of that solution, one particle is the substance of interest.

In our exercise, the salinity of ocean water, which is the amount of salt dissolved in water, is presented as a whole number, but for scientific precision and for certain applications, it's necessary to convert this number into ppm. This aids in understanding the tiny concentrations and comparisons in different water bodies or solutions. The use of ppm is widely accepted in fields like chemistry, environmental science, and engineering, where accurate measurements of concentration are critical.

Dissolved Salts in Seawater

Seawater is notoriously known for its high salinity, which is due to the various salts dissolved in it. These salts come from the erosion of rocks on land, volcanic emissions, and from the ocean's floor. When we measure salinity, we’re essentially measuring the concentration of these dissolved salts.

The primary salts are sodium chloride, magnesium sulfate, and calcium carbonate, among others, which give seawater its distinct saline taste. The concentration of dissolved salts in seawater, as in the exercise, is crucial for marine life, oceanic chemistry, and for various human activities such as desalination. The calculated 350,000 ppm from the exercise outlines the massive difference in salt concentration between seawater and fresh water, and it is this very salinity that supports certain organisms and affects the water's buoyancy and density.

Freshwater Conversion

To make seawater drinkable or usable for most agricultural and many industrial processes, the dissolved salts concentration must be drastically reduced — in essence, converting seawater to freshwater. This process is known as desalination, and it involves several methodologies, including distillation and reverse osmosis.

The aim is to get the salt concentration down to under 500 ppm, which is the general maximum concentration allowed for water to be considered 'freshwater'. According to the exercise, approximately 99.86% of the dissolved salts in seawater need to be removed to reach this level. This indicates just how saline the ocean is and the extent of purification required. Desalination technology has been a cornerstone in providing fresh water in arid regions, and understanding the concentrations involved helps us appreciate the challenges of this critical process.