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S1: SOLUTIONS & DILUTIONS INTRODUCTION The chemistry of life depends upon specific molecules interacting with each other. Successful interaction requires that the molecules be dissolved in water. One or more molecular species dissolved in water constitutes as solution. The molecules dissolved in water are called solutes, while the water is referred to as solvent. At this point you should review the discussion of solutions in your text book. A key property of solutions is the concentration of solute. To a nonscientist, concentration is most often thought of in terms of percent. A “10% glucose solution” is something most of us can easily relate to and visualize (for every 100 parts of solution, 10 parts are glucose). The “parts” in a percent solutions usually refer to weight. Thus, the 10% glucose solution would consist of 10 grams of glucose for every 100 grams of solution (10 g glucose in 90 g H 2 O). 10 grams Glucose 10 grams Sucrose 90 ml H2O 10% Glucose 10% Sucrose Figure 1. Comparison of the composition of solutions by weight Noting the concentration of solutions in percent by weight can lead to problems because it doesn’t give any information about the concentration of molecules. For example, in comparing a 10% solution of glucose and a 10% solution of sucrose (Fig. 1), you see that each has the same weight of sugar (10g per 90g H 2 O), but because a glucose molecule is about half the size of a sucrose molecule (Fig. 2), the glucose solution has about twice the concentration of dissolved solute molecules. The weight of sugar in each solution is the same, but there are twice as many sugar molecules in the glucose solution. CH OH 2 O OH OH OH Glucose : Molecular weigth = 180 OH CH OH 2 O OH OH O CH OH 2 HOCH 2 OH OH O Sucrose: Molecular weight = 342 Figure 2. Comparison of molecular sizes of glucose and sucrose To get around this problem, solutions are often described by their solute concentration using the Molar designation. A Mole of a substance is equal to its molecular weight in grams. Thus for any type of substance, the number of molecules in a Mole is always the same; namely 6.022 x 10 23 (this is an extremely large number).
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Molar solutions are simply the number of moles of solute in one liter of solution. Figure 3 compares how 1 Molar glucose and sucrose solutions would be made. The concentration of solute molecules in both solutions is the same: 6.022 X10 23 molecules per liter of solution. 1 Mole of Glucose (180 g) 1 Mole of Sucrose (342 g) 1.0 Liter Add enough water to make 1 liter of solution. 1 Molar Solution (1M Glucose) 1 Molar Solution (1M Sucrose) Add enough water to make 1 liter of solution. Figure 3. Comparison of 1 Molar solutions of glucose and sucrose Since a great deal of experimental lab work in biology involves solutions, the ability to work with and manipulate them is very important. In this lab you will investigate several techniques for working with solutions. In biology lab work, concentrations are often given in micro-moles or μM. A μM is one
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