Chemistry

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Concentration

The amount of dissolved material in a solution is called the concentration and can be expressed in units, such as grams per liter or ounces per gallon. Chemists sometimes use percent to indicate concentration, and by convention indicate whether the percent is by weight or volume. A 10 percent solution of alcohol in water would normally be thought of as 10 volumes of alcohol in 90 volumes of water. A 10 percent solution of sodium chloride is thought of as 10 weight units of salt in 90 weight units of water. Scientists often use parts per million (or billion) when the amount of solute is very small. For chemical purposes, expressing concentration in terms of the number of molecules (or ions) in solution is often preferable.

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Mole

The mole is one of the seven fundamental units in the International System of Units (SI). The mole is the unit used for measuring the amount of a substance and is defined as the amount of a substance containing the same number of atoms, molecules, or ions as the number of atoms in 12 grams of ¹²C. Because there are 6.022 × 10²³ atoms of carbon in 12 grams of ¹²C, this number (6.022 × 10²³), known as Avogadro's number, is the amount of matter containing 6.022 × 10²³ atoms, molecules, or ions. (see Avogadro’s Law).

The mole concept provides a means of calculating how many atoms, ions, or molecules are in a sample by weighing the substance. From the definition of atomic weight, the amount of any element that has a mass (in grams) equal to its atomic weight (available on the periodic table) will contain 6.022 × 10²³ atoms. Thus, 4.0026 grams of helium, 32.0064 grams of sulfur, and 200.59 grams of mercury each contain 6.022 x 10²³ atoms.

Similarly, a mole of a molecular substance (6.022 × 10²³ molecules) is the amount of the substance whose mass (in grams) is equal to its molecular weight. Molecular weight is derived by summing the atomic weights of the atoms composing a molecule. For example, 70.906 grams (2 × 35.453) of Cl₂ contains 6.022 × 10²³ molecules (one mole) of Cl₂.

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Chemists use the same principle to measure the number of ions in a compound. For example, one mole of sodium ions (Na⁺) has a mass of 22.9898 grams (atomic weight of Na is 22.9898). One mole of NaCl has a mass of 58.443 grams (22.9898 + 35.453).

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Molarity

Molarity is the concentration of a substance in solution and is expressed as moles of solute per liter of the solution. Thus, a 0.1 molar (abbreviated 0.1 M) solution of sodium chloride contains 5.8443 (58.443 × 0.1) grams of NaCl per liter of solution.

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Molality

Molality, a term less frequently used than molarity, is the number of moles of solute in 1000 grams of solvent. Thus, a 0.1 molal solution of sodium chloride in water has 5.8443 grams of NaCl in 1000 grams of H₂O.

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Normality

Normality is the number of equivalents per liter of solution. For acid-base-salt systems, an equivalent is the amount of the substance that will gain or lose one mole of H⁺ ions. For instance, one mole of sulfuric acid (H₂SO₄), which has a mass of 98.0795 grams, produces two moles of H⁺, or two equivalents. Therefore, a one molar solution of sulfuric acid is a two normal (2 N) solution. A 0.1 N solution (containing 0.1 moles of H⁺) of sulfuric acid contains 4.90397 grams of H₂SO₄ per liter of solution ([98.0795/2] × 0.1).

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