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Module 8: Applying Chemical Ideas
Quick questions on Gravimetric analysis and precipitation titrations explained: HSC Chemistry Module 8
10short 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 gravimetric analysis?Show answer
1. Weigh the sample accurately on an analytical balance. 2. Dissolve in a measured volume of water, and acidify with the appropriate dilute acid to remove carbonate and other interferents.
What is the canonical example?Show answer
For a sample of mass $m$ that gives a precipitate of mass $m_{BaSO_4}$:
What is precipitation titration?Show answer
Use when you want speed and do not need part-per-billion precision. The classic Mohr titration measures $Cl^-$:
What is calculation pattern for a precipitation titration?Show answer
For volume of titrant $V_t$ and concentration $c_t$:
What is pH window for the Mohr method?Show answer
The titration must be run at pH 7 to 9.5.
What is forgetting to acidify?Show answer
The most common mark loss is leaving carbonate in solution, which precipitates with $Ba^{2+}$ or $Ag^+$ and inflates the result.
What is confusing moles of precipitate with moles of target ion?Show answer
Always go through the formula. For example, in the magnesium pyrophosphate ignition method, one mole of $Mg_2P_2O_7$ contains two moles of $PO_4^{3-}$-derived phosphorus.
What is running the Mohr titration outside pH 7 to 9.5?Show answer
Memorise the window and the chemical reason for both ends.
What is reading the Mohr end-point too soon?Show answer
Faint pink that disappears on swirling is not the end-point. Wait for a colour that persists for 30 seconds.
What is forgetting to dry to constant mass?Show answer
The first dry weighing is rarely the true mass; reweigh after a second drying cycle to confirm.