1 • Chapter 0 = The Analytical Process You should read this chapter. • Chapter 1= Chemical Measurements Section 1-1: SI Units and Prefixes You should know this material. Section 1-2: Conversion Between Units You should know this material. Section 1-3: Chemical Concentrations Solution = solute + solvent Solute = minor species in solution. Solvent = major species in solution. Most common solvent in CHM 3120: water. Concentration = amount of solute contained in a given volume or mass of solvent.
2 • Molarity (M) • M is the number of moles of solute per liter of solution M = moles of solute / liters of solution moles of solute = weight of solute (g)/formula weight of solute (g) or moles of solute = mass of solute (g)/formula mass of solute (g) or moles of solute = mass of solute (g)/molecular mass of solute (g) • Example: How many grams of Boric Acid [B(OH)3, FM 61.83] should be used to make 2.00 L of 0.0500M solution? moles of solute = 2.00 L x 0.0500M = 0.1 mol mass of solute = moles of solute x formula mass mass of solute = 0.1 mol x 61.83 g = 6.183 g • Electrolyte An electrolyte dissociates into ions in aqueous solution. A strong electrolyte dissociates almost completely. Example: MgCl2→ Mg+ + MgCl+ After dissociation, 89% exists in the form of Mg2+ and 11% in the form of MgCl+ The molarity of a strong electrolyte is referred to as Formal Concentration (F) to indicate that the substance is really converted into other species in solution.
3 A weak electrolyte is partially split into ions in solution. Example: Acetic Acid, CH3CO2H Note: If you dissolve 0.01000 mol of acetic acid in 1.000L, you will have: a 0.01000 F solution or a 0.00959M solution Why? because 4.1% is dissociated into CH3CO2- (acetate ion) and 95.9% remains as CH3CO2H. Nonetheless, we usually say that the solution is 0.01M and understand that some of the acid is dissociated
4 • Molality (m) • m is the number of moles of solute per kilogram of solvent (not solution!) • The advantage of molality over molarity is that molality does not change with temperature. • The molarity of a solution changes with temperature because the volume of solution changes with temperature. • Percent Composition • It is usually expressed as weight percent (wt%) • wt% = [mass of solute / mass of total solution or mixture] x 100 • Converting weight percent into molarity • Example: Find the molarity of HCl in a reagent labeled “37.0 wt% HCl, density = 1.188g/mL”. 37.0 wt% = There are 37.0 g of HCl in 100 g of reagent density = 1.188g/mL = allows us to calculate the volume of 100 g of reagent density (g/mL) = mass (g) / volume (mL) volume (mL) = mass (g) / density (g/mL) volume = 100 g / 1.188 g/mL = 84.1 mL
5 We know that: M = moles of solute / volume of solution in liters Moles of solute = mass of solute (g) / formula mass of solute (g) Formula mass of HCl = 36.46 g/mol Mass of solute = 37 g Volume of solution = 84.1 mL = 84.18 x 10-3 L Substituting these values
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