Lab 2 Flashcards

What is gravimetric analysis?

Quantitative analysis based on mass measurements.

What is the analyte in Experiment 2?

Al³⁺.

What is the goal of gravimetric analysis?

Convert the analyte into a solid, collect it, dry it, weigh it, and calculate the amount present.

Major advantage of gravimetric analysis?

High precision.

Major disadvantage of gravimetric analysis?

Slow and inefficient.

What is a coordination compound?

A metal ion bound to ligands.

What is a ligand?

A molecule or ion that donates an electron pair to a metal.

Is a ligand a Lewis acid or Lewis base?

Lewis base.

Is a metal ion a Lewis acid or Lewis base?

Lewis acid.

What is a monodentate ligand?

A ligand that binds through one atom.

What is a bidentate ligand?

A ligand that binds through two atoms.

What ligand is used in Experiment 2?

8-hydroxyquinoline (hq).

Why is hq bidentate?

It binds Al³⁺ through one nitrogen and one oxygen atom.

What is a chelator?

A multidentate ligand.

What precipitate forms in Experiment 2?

Al(hq)₃.

Is Al(hq)₃ soluble or insoluble?

Insoluble.

Why is Al(hq)₃ useful for gravimetric analysis?

It precipitates and has a large molar mass.

What is the formula of the precipitate?

Al(hq)₃.

What is the molar ratio of hq to Al³⁺ in Al(hq)₃?

3:1.

What is the coordination number of Al³⁺ in Al(hq)₃?

6.

Why is the coordination number 6?

Three bidentate ligands donate six electron pairs.

Why is pH important in Experiment 2?

pH determines which form of hq is present and whether it can bind Al³⁺.

What happens below pH 5?

hq is protonated and cannot effectively bind Al³⁺.

What happens above pH 10?

hq binds well, but Al³⁺ forms unwanted side products.

What side products form at high pH?

Al(OH)₃, Al(OH)₄⁻, and Al₂O₃.

What pH is used in the experiment?

About pH 4.

What reagent adjusts the pH?

Ammonium acetate buffer.

Which reagent should be limiting?

Al³⁺.

Which reagent is used in excess?

hq.

Why is hq added in excess?

To drive precipitation of Al(hq)₃.

What happens if too much hq is used?

hq contaminates the precipitate and causes an artificially high Al³⁺ concentration.

What is Ksp?

Solubility product constant.

What does a small Ksp mean?

Low solubility.

How can precipitation be increased?

Add excess hq to shift equilibrium toward solid Al(hq)₃.

Why heat the solution initially?

Increase solubility and speed the reaction.

Why allow precipitation overnight?

Produces larger, purer crystals.

Why use vacuum filtration?

To separate solid precipitate from liquid.

What is a desiccator used for?

Keeping samples dry.

Why cool crucibles in a desiccator?

Prevent moisture absorption before weighing.

Why should crucibles not be handled with bare hands?

Oils and moisture change the measured mass.

Why dry precipitates to constant mass?

Ensures all water is removed and mass is accurate.

What volume of unknown was used in each trial?

10.00 mL.

What measurement is ultimately used to calculate [Al³⁺]?

Mass of Al(hq)₃ precipitate.

If precipitate mass increases, calculated [Al³⁺] increases or decreases?

Increases.

What statistics were reported?

Average, standard deviation, and %RSD.