The properties of a good primary standard include the following except

Analyze the question: You are asked which option is NOT a property of a good primary standard (a substance used to prepare a standard solution of exactly known concentration, e.g. anhydrous Na₂CO₃, oxalic acid, or potassium hydrogen phthalate).

Concept: Properties of a good primary standard

1. High purity (C): the mass weighed must truly be the substance, with no impurities.
2. High molar mass (A): a larger mass is weighed for a given number of moles, so the percentage error from weighing is smaller.
3. Stable and not hygroscopic or deliquescent: it must not absorb water or CO₂ from the air, or change on standing or drying.
4. Readily available and affordable (D): it should be easy to obtain.
5. Soluble in water, and it reacts completely and in a known way (definite stoichiometry).

Checking each option:

• A. High molecular mass: this is a desirable property, so it is not the answer.
• B. Low molecular mass: this is not desirable. A small molar mass means a small mass is weighed, so weighing errors become significant. This is the exception.
• C. High degree of purity: this is essential, so it is not the answer.
• D. Readily available: this is desirable, so it is not the answer.

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

Exam tip: For "except" questions, find the option that is the opposite of a known property. Remember "PHASE": Purity, High molar mass, Available, Stable, Easily soluble. "Low molar mass" contradicts the "H".