chem titration help

Primary standard is a standard solution (a solution with known concentration), which is made by dissolving a known mass of solute to form a known volume of solution. The concentration is calculated by dividing the number of moles of solute (mass over molar mass) by the volume of the solution.

Sodium hydroxide is not suitable to be used as a primary standard. This is because sodium hydroxide absorbs moisture and carbon dioxide in air, and thus the sample of sodium hydroxide contains pure sodium hydroxide and an unknown amount of impurities. Therefore, the exact mass of sodium hydroxide in the sample weighed out is unknown.

Hydrated sodium carbonate is not suitable to be used as a primary standard. This is because the number of water of crystallization in each mole of hydrated sodium carbonate (Na2CO3•nH2O) varies from 1 to 10, depending on the humidity of air. Therefore, the molar mass of hydrated sodium carbonate is unknown.