PrintUnits of Water Chemistry
Elements and molecules have solubilities (the amount that can be dissolved in water before the water is saturated with that element and no more can be held in solution) that depend on their individual properties and styles of bonding to other elements. For example, common table salt (NaCl), when added to water, will dissociate into individual charged ions of Na+ and Cl-. These are held apart from one another "in solution" because they are surrounded and isolated by polar water molecules (Unique Properties of Water section).
Distilled water or "pure" water typically has near-zero concentrations of other components. If concentrations of dissolved elements or compounds are present, they are usually expressed in terms of mass (weight) of the component/unit volume of water, mass element/mass water, or moles element/mass or volume of water.
Typically, the volume of water referenced is a liter (1000 grams--1 kg by mass), and the elemental or component mass is in grams (or milligrams, mg). Milligrams/Liter (mg/L; 0.001g/1000g) or milligrams/kg (mg/kg) is the same as parts per million (ppm) as concentration. You will often see a concentration of a dissolved species in water expressed in either mg/L or ppm.
Molar concentrations, commonly used by chemists, are expressed as a decimal fraction of the mass of Avogadro's Number (a mole) of atoms (6.022 x 1023) of a given element or elements in a compound, equivalent to atomic or molecular mass. For example, a mole of carbon (12C) has a mass of 12 grams, and a mole of carbon dioxide (CO2) has a mass of 44 grams (12C, 16O, 16O). So, if a liter water sample contains 0.044g of carbon dioxide (44 ppm), the carbon dioxide concentration would be 0.01 mole/kg.
Learning Checkpoint
1. The Na concentration in a water sample is 10 ppm. What is the concentration expressed in g/kg?
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2. The mass of a mole of sodium (Na) is about 23 g. A water sample has a dissolved sodium concentration of 0.046 g/kg. What is the Na concentration expressed as moles/L?
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3. Read this article about a contaminated water supply. After mixing in with the 38 million gallons of fresh water already in the reservoir, how many parts per million “contaminant” are there (in other words, what is the proportion of the contaminated water to the total volume)? Assume that the input of “contaminant” is 8 oz.
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