Even using boiled, distilled water and freshly-opened sodium hydroxide pellets, it is all but impossible to prepare an aqueous solution of
with a precisely known concentration. Instead, one prepares a solution that is as close to the desired molarity as possible, then test it against a primary standard, a compound that can be obtained in high purity.
The most common primary standard for analyzing aqueous sodium hydroxide is potassium hydrogen phthalate (
KHP),
KHC8H4O4(s). The structure of
KHP is shown in the figure. The acidic hydrogen atom, the one that will react
1:1 with
NaOH, is underlined.
A student conducts an experiment in which she will use KHP to standardize a solution of sodium hydroxide. She rinses the burette twice with distilled water, then fills the burette with the NaOH solution that she wishes to standardize, taking care to fill the tip of the burette, as well as the barrel.
She records the initial volume of NaOH titrant as 0.35 mL. She weighs an empty 125−mL Erlenmeyer flask, then adds a sample of KHP to the flask, and weighs the flask and contents.
She adds distilled water to the flask to a depth of about 2 cm, then swirls the flask to dissolve the KHP. Next, she adds 3 drops of phenolphthalein indicator and titrates her first sample.
Which of the following experimental errors will result in a value of the concentration of NaOH that is lower than it should be?
