Titration Lab Practice Test

What requirement does a substance have to meet to be a primary standard?

A substance has to be available in a high degree of purity, have a fairly large MoMa, be nonhygroscopic, and be predictable in how it reacts.

The main idea is what makes a substance a good primary standard: it must be extremely pure, stable, and easy to weigh so that the amount of substance in use is known accurately. A substance with high purity ensures you’re measuring the actual compound, not impurities. A fairly large molar mass helps minimize the relative weighing error, since weighing small masses is more sensitive to small mistakes. Being nonhygroscopic means it won’t pull moisture from the air, so the mass you weigh remains the same over time. Finally, it should react with the titrant in a completely known and predictable way, so the stoichiometry is reliable.

Volatile or poorly stable substances would lose or change amount before or during the experiment, which undermines accuracy. A liquid with low density isn’t a criterion for usefulness as a primary standard. A color to indicate endpoint relates to indicators or endpoint detection, not the standard’s suitability.

It must be volatile and poorly stable.

It must be a liquid with low density.

It must be colored to indicate endpoint.