Compounds that should not be used as primary standards in titrimetric analysis are: A. mathrmNa_2mathrmCr_2mathrmO_7$\mathrm{Na}_{2}\mathrm{Cr}_{2}\mathrm{O}_{7}$ B. Oxalic acid C. mathrmNaOH$\mathrm{NaOH}$ D. mathrmFeSO_4 cdot 6mathrmH_2mathrmO$\mathrm{FeSO}_{4} \cdot 6\mathrm{H}_{2}\mathrm{O}$ E. Sodium tetraborate Choose the most appropriate answer from the options given below:

### Related Formula A primary standard is a highly pure, stable chemical substance with a known, definite composition that can be directly weighed. It must not be hygroscopic, deliquescent, or easily oxidized by air. ### Core Logic Evaluate each substance: - mathrmNa_2mathrmCr_2mathrmO_7$\mathrm{Na}_2\mathrm{Cr}_2\mathrm{O}_7$: Highly hygroscopic (unlike mathrmK_2mathrmCr_2mathrmO_7$\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7$), absorbs water from air, composition varies rightarrow$\rightarrow$ *Cannot be used*. - Oxalic acid: Highly stable, non-hygroscopic, definite composition rightarrow$\rightarrow$ *Can be used*. - mathrmNaOH$\mathrm{NaOH}$: Extremely hygroscopic, absorbs moisture and mathrmCO_2$\mathrm{CO}_2$ from air rightarrow$\rightarrow$ *Cannot be used*. - mathrmFeSO_4 cdot 6mathrmH_2mathrmO$\mathrm{FeSO}_4 \cdot 6\mathrm{H}_2\mathrm{O}$: Easily oxidized by air, unstable composition rightarrow$\rightarrow$ *Cannot be used*. - Sodium tetraborate (Borax): Highly stable, clear hydration state rightarrow$\rightarrow$ *Can be used*. ### Step 1: Select incorrect standards The compounds that should *not* be used are A, C, and D. This corresponds to Option (4). ### Pattern Recognition Sodium salts of di-chromate (mathrmNa_2mathrmCr_2mathrmO_7$\mathrm{Na}_2\mathrm{Cr}_2\mathrm{O}_7$) are highly hygroscopic, whereas potassium di-chromate (mathrmK_2mathrmCr_2mathrmO_7$\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7$) is non-hygroscopic and works as an excellent primary standard. Deliquescent bases like mathrmNaOH$\mathrm{NaOH}$ and easily oxidized transition metal salts like Mohr's salt counterparts can never serve as primary standards. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Chemistry: Practical Chemistry