Post Lab Questions and Explanations

Post Lab Questions - Experiment 1

Question 4: Effect of Partial Submersion on Density Measurement
  • Scenario: Solid is not completely submerged in water.
  • Effect: Reported density will be higher than the true value.
  • Explanation: The volume measured will be less than the actual volume as it’s not fully submerged. Since density is calculated as mass divided by volume (Density = Mass / Volume), a smaller volume leads to a higher calculated density.
Question 6: Effect of Evaporation on Density Measurement
  • Scenario: Volatile liquid evaporates before measurement.
  • Effect: Reported density will be lower than the true value.
  • Explanation: The loss of mass due to evaporation decreases the recorded mass. Since density is also mass divided by volume, a lower mass results in a lower density measurement.
Question 4.A.2: Effect of Contamination on Percent Water in Hydrated Salt
  • Scenario: Sample contaminated with anhydrous salt.
  • Effect: Reported percent water will be too low.
  • Explanation: The contamination increases the total mass of the sample without adding any water, thus the calculated percent water (mass of water/total mass) is lowered.
Question 6.B.1: Effect of Overheating on Percent Water Measurement
  • Scenario: Hydrated salt overheated causing thermal decomposition.
  • Effect: Reported percent water will be too high.
  • Explanation: The decomposition releases gas which makes it appear more water has been lost than actually was. Leads to an overestimation of the percent water.
Identification of Reagents for Distinction

  1. Distinguishing CaCl2 from CaCO3:

    • Reagent: HCl (hydrochloric acid)
    • Observation: CaCO3 bubbles (CO2) while CaCl2 dissolves without bubbles.

  2. Aqueous Mixtures Analysis:

    • a. Potassium Carbonate + Hydrochloric Acid → Fizzing (CO2 gas forms)
    • b. Zinc Chloride + Silver Nitrate → White precipitate (AgCl forms)
    • c. Magnesium Chloride + Sodium Hydroxide → White precipitate (Mg(OH)2 forms)
    • d. Ammonium Nitrate + Sodium Hydroxide → Strong ammonia smell (NH3 gas forms)
Cation Tests

  1. Ag+ vs. Mg2+ Distinction:

    • Reagent: HCl
    • Observation: Ag+ forms white precipitate (AgCl), while Mg2+ remains clear.

  2. HCl vs. H2SO4 Distinction:

    • Reagent: BaCl2
    • Observation: H2SO4 forms white precipitate (BaSO4), while HCl stays clear.

  3. Ba2+ vs. Cu2+ Distinction:

    • Reagent: Na2SO4
    • Observation: Ba2+ forms white precipitate (BaSO4), Cu2+ remains clear.

  4. Cu2+ vs. Mg2+ Distinction:

    • Reagent: NH3
    • Observation: Cu2+ forms blue precipitate, deep blue in excess NH3; Mg2+ forms white precipitate.
Copper Experiment Observations

  1. Evolution of Gas:

    • Gas Formula: NO2
    • Color: Brown

  2. Reaction with NaOH:

    • Note: Excess HNO3 reacts with NaOH first, preventing immediate Cu(OH)2 precipitation.

  3. Impact of No Centrifuging:

    • Explanation: Loss of solid copper when pouring off liquid causes decreased percent recovery.

  4. Incomplete CuO Reaction with H2SO4:

    • Effect: Reported percent recovery will be too low.
    • Reason: Remaining CuO does not convert to copper metal, limiting recovery.
Vinegar Analysis**

  1. Importance of Mass Measurement:

    • Reason: Mass is more accurate and less affected by temperature/density changes.

  2. Volume Measurement Implications:

    • Requirement: Density of vinegar to convert volume to mass.

  3. Effect of Immediate Burette Reading:

    • Effect: Reported percent acetic acid would be too high; inaccurate reading may lead to using more NaOH than needed.

  4. Endpoint Overshoot in Titration:

    • Effect: Reported percent acetic acid will be too high; additional NaOH after endpoint leads to calculation of excess acetic acid.
Limiting Reactant Analysis

  1. Effect of Filter Paper Porosity:

    • Impact: Reported percent of limiting reactant will be too low due to loss of precipitate.

  2. Incomplete Air Drying of Precipitate:

    • Impact: Reported mass will be too high since excess water adds weight.

  3. Temperature Impact in Drying Oven:

    • Effect: Reported percent by mass will be too high; lower temperature prevents full drying, leading to excess moisture left in the precipitate ball.