I am using Lanthanum(III) nitrate hydrate (Product No. 238554, Sigma-Aldrich) for the synthesis of LaCrO₃. The Certificate of Analysis (CoA) mentions the formula as La(NO₃)₃·xH₂O, with a molecular weight of 324.92 g/mol (anhydrous basis), but it does not specify the exact value of x.
I have the following doubts:
1. For stoichiometric calculations, should I use the given anhydrous molecular weight (324.92 g/mol), or do I need to assume a value for x?
2. If x is not known, can this lead to significant error in mole calculations and affect the final stoichiometry of my compound?
3. Does this compound absorb more moisture over time (i.e., become more hydrated), and would that change the effective molecular weight?
If you only have the mass of the hydrated compound (La(NO₃)₃·xH₂O):
You cannot accurately calculate moles without knowing the exact value of x, because the molar mass changes depending on how many water molecules are present.
You could dry your reagent to constant weight, either by heating or in a desiccator, and treat the residue as the anhydrous salt when calculating molar quantity. This way, variable hydration won’t introduce stoichiometric errors. Additionally, you could standardize lanthanum in your solution by EDTA titration or spectrometrically.
You can measure water content by Karl-Fisher titration and calculate.
- Badges
- Science topic
- Similar topics
- Geoscience
- Atmospheric Sciences
- Climate Science
- Climate Change