A primary standard must NOT be hygroscopic, as absorption of atmospheric moisture prevents accurate weighing.

• High purity: Essential criterion; a primary standard substance must possess a certified purity exceeding 99.9%.
• Stability in air: Essential; the substance must not oxidize, absorb carbon dioxide, or degrade upon exposure to ambient air.
• Hygroscopic nature: Undesirable defect; absorbing moisture introduces uncontrolled mass errors during gravimetric preparation, disqualifying a chemical as a primary standard.
• High equivalent weight: Desirable property because a higher molecular/equivalent weight minimizes relative weighing errors.

The mass of NaOH required is calculated as Molarity x Volume x Molecular Weight = 0.5 mol/L x 2 L x 40 g/mol = 40 g.

• 60 g: Calculation error.
• 20 g: The mass present in only 1 litre of a 0.5 M solution (0.5 x 40 = 20 g).
• 80 g: The mass present in 2 litres of a 1.0 M solution (2 x 40 = 80 g).
• 40 g: Exact stoichiometric calculation: 0.5 M = 0.5 moles per litre; in 2 litres there is 1.0 mole of NaOH, which weighs exactly 40.0 g.

• Concentrated analytical-grade sulfuric acid (~98% H2SO4) has a standard specific gravity of 1.84 g/mL (density of 1.84 g/cm3) and normality of approximately 36 N.
• 70 g/mL corresponds to concentrated orthophosphoric acid (85% H3PO4).
• 19 g/mL is the specific gravity of concentrated hydrochloric acid (37% HCl, ~12 N).
• 06 g/mL is typical of dilute aqueous solutions such as 15% acetic acid.

• 3 is the inverse ratio (HNO3:HCl).
• Aqua regia (royal water) is prepared by mixing concentrated hydrochloric acid (HCl) and concentrated nitric acid (HNO3) in a 3:1 volumetric ratio, capable of dissolving noble metals like gold and platinum.
• 9:4 is incorrect.
• 9:5 is incorrect.