The properties of a good primary standard solution include the following except?

Analyze the question: We're asked which option is NOT a property of a good primary standard. This is an "except" question, so we look for the false property.

Concept: Primary standard A primary standard is a substance used to prepare a solution of exactly known concentration by direct weighing. Examples include anhydrous sodium carbonate, oxalic acid dihydrate, and potassium hydrogen phthalate.

A good primary standard should:

• Have a high degree of purity (so the mass weighed is truly the substance).
• Be stable, not hygroscopic, deliquescent, or efflorescent, and not react with air (CO₂, O₂).
• Have a high molar mass, so that weighing errors are small relative to the amount weighed.
• Be readily available and reasonably cheap.
• Be soluble in water.
• React completely and in a known way (stoichiometrically).

Go through the options:

• A. High molecular mass: a desirable property (reduces relative weighing error). ✔
• B. Low molecular mass: not desirable. A small molar mass means a small mass is needed for a given number of moles, so weighing errors become significant. ✘
• C. High degree of purity: essential. ✔
• D. Readily available: desirable. ✔

Answer: The property that is NOT a feature of a good primary standard is low molecular mass, which is option B.

Exam tip: Remember the word PHASS: Purity, High molar mass, Available, Stable, Soluble. In an "except" question, look for the option that contradicts one of these, and "low molecular mass" does.