How to Determine Percent of Alkaline With Titration
Titration is a standard laboratory procedure for the chemical analysis of materials and solutions. Sometimes called volumetric analysis, titration involves determining the volume of a solution, called a titrant, that is chemically equivalent to another substance reacting with it. Titrant is added carefully to a solution of the other substance from a measuring instrument called a burette until the "end-point" of the titration is reached. Often the end-point is indicated by a color change of the solution due to the presence of a chemical indicator. A common type of titration involves neutralization of an alkaline compound (base) with an acid or an acid with a base.
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Instructions
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Suppose a chemistry problem asks for the percent of sodium hydroxide, NaOH, in a water solution. Assume 25.00 milliliters of the solution is titrated with a standard hydrochloric acid solution of concentration 0.129 molar HCL to an end-point indicated at 32.72 mL.
Note the molar concentration, or molarity (M), is the number of moles of a substance contained in a volume of 1 liter.
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Write the balanced equation of the chemical reaction: NaOH + HCL = NaCL + H2O.
Note the molar ratio of the reaction is 1 mole NaOH / 1 mole HCL.
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Determine the number of moles of HCL required to neutralize the NaOH, as follows:
32.72 ml HCL x (1 L / 1000 mL) x 0.129 moles HCL / L = 0.00442 moles HCL.
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Calculate the molar concentration of NaOH, as follows: 0.00422 moles HCL x (1 mole NaOH / 1 mole HCL) / 25 mL x 1000 mL / 1 L = 0.169 M
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Calculate the weight of NaOH per liter of solution by multiplying the molar concentration of NaOH by its molar mass, or formula weight, as follows: 0.169 moles NaOH / L x 40.00 grams NaOH / mole NaOH = 6.76 grams NaOH / L.
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Assume the density of the solution is 1.00 since the solution is dilute. Therefore 1 liter of solution weighs 1,000 grams. Determine the percent of alkaline as follows: 6.76 g NaOH / 1000 g x 100 = 0.676 percent by weight.
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Tips & Warnings
The density of concentrated alkaline solutions may depart significantly from that of water and must therefore be accurately known. Density is easily measured by weighing a known volume of solution and dividing the weight by the volume.
A pH meter is often used to indicate the end point of an acid-base titration. A pH of 7.0 indicates a neutral solution containing no acid or base.
