422

THINK ABOUT IT

Selling bottled water is a big business. Some companies claim to get their water from mountain streams. Some say that their water is quite pure. Still others claim their water contains beneficial minerals. How could you determine how pure a water sample is?

Can molarity calculations be used to identify a toxic substance?

To answer this question, you will explore

EXPLORING THE TOPIC

Accounting for the Mass of a Solution

Suppose that you want to know how pure a sample of water is. Examine the illustration and consider how you might use mass to make a determination of purity.

What masses do you expect if the samples are all pure water? Recall that mass per volume is equal to density. The density of pure water is 1 g/mL. If a 100 mL sample is pure water, then you expect a mass of 100 g.

The sample in the first beaker may be pure water. The other two samples must contain solutes that add more mass to the same volume, making them denser (and therefore heavier) than pure water.

THE DENSITY OF A SOLUTION

Suppose you have three samples of salt water with the concentrations shown in the illustration at the top of the next page. Take a moment to consider how the densities of the three solutions compare. Why does the ball float higher in the solution on the right?

423

As the density of the solution increases, an object floats higher. That is why when you swim in very salty water, you float with more of your body out of the water. As the concentration increases, there are more units of NaCl per volume of solution. The higher the salt concentration, the denser the solution.

SAME MASS, DIFFERENT SOLUTES

Imagine that you add 10 g of three different solids to three different beakers and then add enough water to make 100 mL of solution. The solutions all have approximately the same mass—the mass of about 100 mL of water plus 10 g of solute. But do they have the same concentration?

Copper (II) sulfate, CuSO₄ has the largest molar mass. That means there are fewer formula units of CuSO₄ in 10 grams compared with NaOH. While you’ve added the same mass to each sample, you have not added the same number of moles. Dissolving 10 grams of the compound with the higher molar mass will result in a lower concentration.

424

Drinking Water Standards

Environmental Protection Agency standards protect public health by limiting contaminants in drinking water. The maximum contaminant levels for fluorine, iron, and barium in drinking water are given in mg/L in the table.

Based on the numbers in the table, you might conclude that iron, Fe, is the most toxic because the allowable level is the smallest by mass. However, these contaminants interact with your body atom by atom. So you need to compare the contaminants by number of atoms.

Find the concentration of the potential toxins in moles per liter. To do so, convert the maximum contaminant level milligrams to grams, and then divide by the molar mass.

In moles per liter, the maximum contaminant level for barium, Ba, is the smallest numerical value. This means less of it is tolerated in our drinking water, making barium the most toxic of the three elements.

LESSON SUMMARY

425

Can molarity calculations be used to identify a toxic substance?

When a substance is dissolved in water, it adds to the mass of the solution. One mole of one substance may have a very different molar mass than one mole of another substance. So different solutions of the same concentration can be differentiated by weighing them. Comparing contaminants by number of atoms may give a different picture of toxicity than comparing them by mass.