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LIQUID SOLUTIONS
Throughout your scientific endeavors you will need to make a number of solutions of solids dissolved in liquids, and of liquids dissolved in liquids. It is obviously necessary to know how to do this reliably. (You'll look like an ignorant klutz in a lab job if you don't know how to do this.) We shall discuss three types of solutions: %(w/v), %(v/v), and molar.
This type of solution is extremely common in biological sciences - including medicine. In order to understand the basic principle, you must realize that the word 'percent' is derived from the Latin words 'per' (for, to, in) and 'centum' (one hundred). Examples:
A 40% aqueous solution of ethanol is made by taking 40 ml of EtOH and adding enough water to make 100 ml. Really simple do-it-in-your-head stuff!
C_{1}V_{1} = C_{2}V_{2}
(concentrations times volumes)
2% x 25 ml = 5% x (Volume of the 5%)
50 = 5 x (Volume of the 5%) = 10 ml of the 5% solution is needed
So, take that 10 of 5%, and add 15 ml of water to give 25 ml of 2%.
*** What is a "mole?" You are familiar with other terms for grouping things - pairs (2), dozens (12), and gross (144) are examples of clusters of a rather small number of items. For larger numbers, we have Astronomical Units (distance between the sun and earth), speed of light, parsecs in astronomy, and in chemistry the "mole" which is 602,000,000,000,000,000,000,000 (six hundred-two sextillion) which is defined as the number of gas molecules in 22,414 mL at standard temperature and pressure (STP) - regardless of the type of gas. What is neat about this 6.02x10^{23}, is that if you take the molecular weight of something, and weigh out that many grams of it, you have 1 mole of it - that number of molecules. Thus if water has a molecular weight of 18, then eighteen grams of water has that many molecules of H_{2}O, which is 1 mole of water. "Molar" is one mole of molecules diluted into a liter by some sort of solvent - usually water. Thus 1 M NaCl is one mole of NaCl (mol.wt. = 57) or 57 grams of NaCl made up to 1 liter of water (not added to a liter, but made up to a total of a liter).
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