Praktikum Kimia Dasar Modul 2. Stoikiometri

Main ideas and lessons conveyed

Overview of Stoichiometry Practicum (Modul 2)

The video explains that there are three practicum activities within stoichiometry:

1. Determining an empirical formula (via combustion and composition analysis).
2. Determining the limiting reactant and salt precipitation (using a salt mixture, filtration/centrifugation, and mass measurement).
3. Volumetric (titration) analysis to find the molar concentration of an unknown strong acid.

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Practicum 1: Empirical Formula (Combustion/Decomposition of a Compound)

Goal / objectives

• Determine the empirical formula of two compounds using a reaction/combination approach.
• Determine the mole ratio of magnesium to oxygen (Mg:O) and use it to find the empirical formula.

Main procedures described

1) Prepare crucibles

• Wash the crucibles.
• Heat a crucible using a Bunsen burner setup connected to gas.
• Use two crucibles:
  • one with a lid
  • one without a lid

2) Weighing steps

• Weigh the crucible (especially after heating) to obtain an initial/empty mass.
• Clean the sample area using sandpaper to remove impurities (mentioned).
• Weigh again to obtain the mass of the sample by subtraction:
  • mass(sample) = mass(after) − mass(empty)

3) Combustion/heating

• Turn on the Bunsen burner.
• Heat the sample and wait until it burns completely.
• Every ~30 seconds, check flame behavior:
  • If flames are still present, continue heating.
  • Close/cover as needed and keep monitoring.

4) Cooling and final weighing

• Stop heating when burning ceases (no more flame/emission).
• Allow the crucible to cool (partly mentioned: cool to room temperature).
• Place in a desiccator to prevent moisture absorption.
• Weigh again to get the final mass after reaction.

5) Determining the result

• Use the measured masses to determine the mole ratio (Mg:O).
• From that ratio, determine the empirical formula.

Example numeric values mentioned (subtitle artifacts)

• Crucible/sample mass discussed with values like 3.83…94 and later 38.5 g (exact accuracy unclear due to subtitle errors).
• For decomposition using CaCO₃, mass values mentioned include about 25 g and 26.7…87 g (as transcribed).

Additional concept mentioned

• Decomposition reaction of a pure compound, specifically CaCO₃, referenced to find a ratio and/or empirical formula from decomposition products.

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Practicum 2: Limiting Reaction and Salt Precipitation

Purpose / objectives

1. Determine the limiting reactant in a salt mixture.
2. Determine the percent composition of each substance in the salt mixture.

Materials / chemical setup described

• A salt mixture composed of:
  • CaCl₂ (calcium chloride)
  • K₂C₂O₄ (potassium oxalate)
• Filter paper
• Beaker and Erlenmeyer flask
• Centrifuge tubes (about two)
• Distilled water
• An oven for drying the precipitate (mentioned)
• A watch glass (or “project glass” due to subtitle error)

Step-by-step procedure described

• Weigh salt mixture

  • Weigh 1 gram of the salt mixture into a container (watch glass/project glass).

• Prepare and weigh filter paper

  • Weigh the filter paper first to obtain its mass (around 0.40…7 g, as transcribed).

• Dissolve the salt mixture

  • Transfer the salt to a beaker.
  • Add distilled water up to about 150 mL.

• Filter/collect precipitate

  • Fold filter paper and place in the filtration setup.
  • Transfer filtrate using centrifugation:
    • Use two centrifuge tubes
    • Wait until solids settle
    • Take the clear liquid (avoid the settled solids)

• Combine or transfer

  • Pour the clear liquid into an Erlenmeyer flask.

• Centrifugation/clarity check

  • The subtitles note the solution becomes clearer after centrifugation.

• Test for excess reactant

  • A solution is given as K₂C₂O₄ 0.5 M and CaCl₂ 0.5 M.
  • A mixing test in a tube shows a change to infer whether one reagent is in excess.

• Recover and dry precipitate

  • Place filter paper with precipitate on a watch glass.
  • Dry in an oven until dry.

• Weigh to find anhydrous calcium oxalate

  • Determine the mass of anhydrous calcium oxalate formed from 1 gram of the salt mixture.
  • A value mentioned is 0.6 g (accuracy uncertain due to subtitle errors).

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Practicum 3: Volumetric Analysis (Acid–Base Titration)

Purpose / objective

• Determine the molar concentration of an unknown strong acid.

Materials / chemicals / tools listed

• 0.1 M NaOH (in a burette)
• Indicator (pp indicator mentioned; likely phenolphthalein)
• HCl with unknown concentration
• Erlenmeyer flask
• Measuring pipette
• Burette
• Possibly a bulb/syringe for transferring solutions (mentioned via transcription)

Step-by-step titration procedure described

• Prepare NaOH

  • Put 0.1 M NaOH into the burette.

• Prepare HCl sample

  • Take 25 mL of HCl (unknown concentration) using a pipette.
  • Transfer into an Erlenmeyer flask.

• Add indicator

  • Add pp indicator (subtitle references “only 12”; exact meaning unclear).

• Titrate

  • Add NaOH from the burette into the Erlenmeyer while shaking.

• Determine endpoint

  • Record the burette final volume at the endpoint.
  • Subtitle indicates 27.5 mL NaOH was used.

• Calculation (conceptual)

  • Use acid–base stoichiometry and titration volume to calculate the molarity of HCl.
  • The computed concentration is not clearly shown in the subtitles (noted as “so we don’t calculate…”, likely due to transcription or completion).

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Speaker(s) / sources featured

• Presenter / instructor (unnamed): Conducts and explains the three chemistry practicums.
• No other named speakers, institutions, or sources are clearly identifiable from the subtitles.