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VICChemistryQuick questions
Unit 2: How do chemical reactions shape the natural world?
Quick questions on Gravimetric analysis: VCE Chemistry Unit 2
9short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is when gravimetric analysis is used?Show answer
Gravimetric analysis is the right tool when:
What is the principles?Show answer
1. Selective precipitation. Add a reagent that forms an insoluble compound with the analyte but leaves everything else in solution. Use the solubility rules. 2.
What is the standard lab procedure?Show answer
1. Dissolve the sample in a known mass or volume of water (or other suitable solvent). 2. Add the precipitating reagent in slight excess. Add slowly and with stirring.
What is the calculation framework?Show answer
1. Calculate $n(precipitate) = m(precipitate) / M(precipitate)$. 2. Use the stoichiometric mole ratio to find $n(analyte)$.
What is worked example?Show answer
A $1.000\ g$ sample of an alloy is dissolved in nitric acid. All chloride is removed first. Iron is precipitated by adding excess ammonia, giving $Fe(OH)_3$, which is ignited at $800$ deg C to constant mass to give $Fe_2O_3$. The final mass of $Fe_2O_3$ is $0.225\ g$.
What is forgetting to dry to constant mass?Show answer
A wet precipitate weighs more than a dry one. The hallmark of a careful gravimetric analysis is the repeat weigh-and-dry cycle.
What is using the wrong mole ratio?Show answer
Always go through the net ionic equation. $SO_4^{2-}$ to $BaSO_4$ is $1:1$. $PO_4^{3-}$ to $Mg_2P_2O_7$ is $2:1$.
What is adding precipitating reagent in stoichiometric amount instead of excess?Show answer
Equilibrium leaves a small fraction of analyte in solution. The standard practice is to add a moderate excess and check completeness.
What is including paper or crucible mass in the precipitate mass?Show answer
Subtract the pre-recorded empty mass.