Standardization of a Hydrochloric Acid Solution

1 Page 1 Spring 2011 Standardization of a Hydrochloric acid Solution The purpose of this experiment is to test your lab technique and examine potential sources of error in your measurements. You will do this by standardizing Hydrochloric acid that will be used in future experiments by titration with sodium hydroxide Solution . BUT, the sodium hydroxide Solution has not been standardized. Before you can use the NaOH(aq) to standardize your HCl(aq), you will have to standardize the NaOH(aq) using the primary solid acid standard, potassium hydrogen phthalate. Standardizing NaOH(aq) Potassium hydrogen phthalate (KHP, KC8H5O4) is a solid, monoprotic acid . Weight out of KHP (record the mass to the nearest milligram) and place it in a 125mL Ehrlenmeyer flask. Add ~30mL of water and 2-3 drops of phenolphthalein indicator. (It s OK if all of the KHP does not initially dissolve, it will dissolve during the titration.)

2 Titrate this Solution with NaOH(aq) to a persistent endpoint. (If you swirl the flask and the color fades, you are not at the endpoint.) Calculate the concentration of the NaOH(aq) stock. Repeat this titration until your average result has an error that you consider acceptable. Standardizing HCl(aq) Pipette of HCl(aq) into a 125mL Ehrlenmeyer flask and add two drops of phenolphthalein indicator. Titrate this Solution with NaOH(aq) to the endpoint. Calculate the concentration of the HCl(aq) stock. Repeat this titration until your average result has an error that you consider acceptable. NOTES: To standardize a Solution means to determine its actual concentration. When an acid or base Solution is prepared from commercially available starting materials, we are fairly confident in its concentration to 1 or perhaps 2 significant figures, but if we need to be more certain of its concentration we usually need to titrate it to determine its actual concentration.

3 Titrations are stoichiometry problems. Treat them that way. If you are not using a balanced chemical equation and if you don t include a mol-to-mol conversion in your calculation, you are not doing things correctly. Even if the mol-to-mol conversion is mathematically trivial because the stoichiometry is 1:1, you have to include that term in your calculation. KHP and KHPH are abbreviations/acronyms for potassium hydrogen phthalate, they are not the chemical formula of potassium hydrogen phthalate. Make sure you use the correct chemical formula when you are determining the molar mass. Burettes are used by taking the difference in your initial and final readings. When you are doing a titration, record your initial reading and then observe the Solution you are titrating to determine when to stop. You shouldn t even look at the burette until you reach an endpoint, then take a final reading and subtract to determine the total volume added to the titrations.

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