Kaplan MCAT General Chemistry Review полностью

Generally, some compounds or elements have different capacities to act in certain ways, not in terms of their characteristics or behaviors, like electronegativity or ionization energy, but rather the ability of certain elements or compounds to act more potently than others in performing certain reactions. For example, one mole of HCl has the ability to donate one mole of hydrogen ions (H⁺) in solution, but one mole of H₂SO₄ has the ability to donate two moles of hydrogen ions, and one mole of H₃PO₄ has the ability to donate three moles of hydrogen ions. Another example to consider is the difference between Na and Mg: One mole of sodium has the ability to donate one mole of electrons, while one mole of magnesium has the ability to donate two moles of electrons. To find one mole of hydrogen ions for a particular acid-base reaction, we could “source” those protons from one mole of HCl, or we could instead use a half-mole of H₂SO₄. If we’re using H₃PO₄, we’d only need one-third of a mole. This is what we mean by “different capacities to act in certain ways.” If we need only one mole of hydrogen ions, it would be a waste to use an entire mole of H₃PO₄, which could donate three times what we need. We are defining the concept of equivalents: How many moles of the “thing we are interested in” (i.e., protons, hydroxide ions, electrons, or ions) will the number of moles of the compound present produce? One mole of hydrogen ions (one equivalent) will be donated by one mole of HCl, but two moles of hydrogen ions (two equivalents) will be donated by one mole of H₂SO₄, and three moles of hydrogen ions (three equivalents) will be donated by one mole of H₃PO₄. Simply put, an equivalent is a mole of charge.

Having discussed the concept of equivalents, we can now introduce a calculation that will be helpful on the MCAT, especially for problems of acid-base chemistry. So far, this discussion has been focused on the mole-to-mole relationship between, say, an acid compound and the hydrogen ions it donates. However, sometimes we need to work in units of mass rather than moles. Just as one mole of HCl will donate one mole (one equivalent) of hydrogen ions, a certain mass amount of HCl will donate one equivalent of hydrogen ions. This amount of compound, measured in grams, that produces one equivalent of the monovalent particle of interest (protons, hydroxide ions, electrons, or ions) is called the gram equivalent weight, and the equation is

Gram equivalent weight = Molar mass/n

where n is the number of protons, hydroxide ions, electrons, or monovalent ions “produced” or “consumed” per molecule of the compound in the reaction. For example, you would need 46 grams of H₂SO₄ (molar mass = 98 g/mol) to produce one equivalent of hydrogen ions, because each molecule of H₂SO₄ can donate two hydrogen ions (n = 2). Simply put, an equivalent weight of a compound is the mass that provides one mole of charge.

If the amount of a compound in a reaction is known and you need to determine how many equivalents are present, use the following equation:

Equivalents = Mass of compound (g)/Gram equivalent weight (g)

Finally, we can now introduce the measurement of normality without too much fear of causing the almost-always-fatal head explosion. Normality, as the term suggests, is a measure of concentration. The units for normality are equivalents/liter. A 1 N solution of acid contains a concentration of hydrogen ions equal to 1 mole/liter; a 2 N solution of acid contains a concentration of hydrogen ions equal to 2 moles/liter. The actual concentration of the acidic compound may be the same or different from the normality, because different compounds have different capacities to donate hydrogen ions. In a 1 N acid solution consisting of dissolved HCl, the molarity of HCl is 1 M, because HCl is a monoprotic acid, but if the dissolved acid is H₂SO₄, then the molarity of H₂SO₄ in a 1 N acid solution is 0.5 M, because H₂SO₄ is a diprotic acid. The conversion from normality of acid solution to molarity of acidic compound is

Molarity = Normality/n

where n is the number of protons, hydroxide ions, electrons, or monovalent ions “produced” or “consumed” per molecule of the compound in the reaction.