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Standardizing $NaOH$ Solutions

APCHEM-KPFM4T

Even when using boiled, distilled water and freshly-opened sodium hydroxide pellets, it is all but impossible to prepare an aqueous solution of $NaOH$ with a precisely known concentration. Instead, one prepares a solution that is as close to the desired molarity as possible, and then tests 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 (aka KHP), $K{ HC }_{ 8 }{ H }_{ 4 }{ O }_{ 4 }(s)$. The structure of KHP is shown below. The acidic hydrogen atom, the one that will react 1:1 with $NaOH$, is underlined.

Created for Albert.io. Copyright 2016. All rights reserved.

A student conducts an experiment in which she will use KHP to standardize a solution of sodium hydroxide.

She rinses the buret twice with distilled water, and then she fills the buret with the $NaOH$ solution that she wishes to standardize, taking care to fill the tip of the buret, 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 flasks, and finally weighs the flask and contents.

She adds distilled water to the flask to a depth of about 2 cm, and she then swirls the flask to dissolve the KHP. Next, she adds three 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?

A

Not rinsing the buret with $NaOH$ solution.

B

Not starting at the 0.00-mL mark for each titration.

C

Not measuring and recording the exact volume of distilled water used to dissolve the KHP.

D

Not adding enough phenolphthalein.