PQs Flashcards
(61 cards)
Why do you rinse the burette with the solution it will contain (not water)?
To ensure any water droplets left inside don’t dilute the solution, which would affect the concentration and give inaccurate volume readings.
Why do you rinse the pipette with the solution it will contain?
To ensure the correct concentration is delivered into the conical flask — rinsing with water would dilute the solution and affect the number of moles added.
Why do you not rinse the conical flask with the solution?
Rinsing the flask with solution would introduce an unknown amount of extra moles, affecting the result. Only rinse with distilled water (which doesn’t affect moles).
Why is it important to remove the funnel from the burette during titration?
To prevent extra drops of solution from entering the burette, which would change the volume recorded and make your titre inaccurate.
Why is a white tile used under the conical flask?
To make the colour change at the endpoint easier to see.
What is meant by “concordant titres”?
Titres that are within ±0.10 cm³ of each other — this increases reliability and reduces random error.
How could you make your titration results more accurate?
• Use a white tile
• Remove funnel after filling burette
• Use concordant results only
• Add dropwise near endpoint
• Repeat titration for consistency
Why are rough titrations done first?
To get a quick estimate of the endpoint — helps plan more accurate repeat titrations by avoiding overshooting.
Why is it better to use a pipette instead of a measuring cylinder?
Pipettes are more accurate and have a lower uncertainty than measuring cylinders.
Why is phenolphthalein used in a titration between a strong acid and strong base?
Actually, phenolphthalein is best for strong base + weak acid, because it has a sharp colour change at the equivalence point.
For strong acid + strong base, methyl orange or phenolphthalein could both be used depending on what’s clearer.
Describe how to make a standard solution of sodium carbonate (Na₂CO₃)
- Accurately weigh the required mass of Na₂CO₃ using a balance and a weighing boat.
- Transfer the solid into a beaker and dissolve it in a small volume of distilled water.
- Rinse the weighing boat into the beaker with distilled water to ensure all solid is transferred.
- Pour the solution into a 250 cm³ volumetric flask using a funnel.
- Rinse the beaker and funnel into the flask to ensure all solution is transferred.
- Add distilled water to the flask until the bottom of the meniscus sits on the mark.
- Stopper the flask and invert several times to mix thoroughly.
Why do you rinse the beaker, funnel, and stirring rod into the volumetric flask?
To ensure all of the solute is transferred, so the concentration is accurate — you don’t want to leave any of the substance behind.
Why do you invert the volumetric flask several times after making up to the mark?
To ensure the solution is fully mixed and has a uniform concentration throughout.
Why should you not blow air into the pipette filler or pipette tip?
Blowing introduces extra volume or contaminants, making the volume delivered inaccurate.
What error would be caused if the solution was made up above the calibration mark?
The solution would be more dilute, so the concentration would be lower than intended.
Why is a volumetric flask better than a measuring cylinder for making up a standard solution?
Volumetric flasks are more precise, with a much lower uncertainty in volume measurement
How does using a wet beaker (not dried after rinsing) affect the concentration?
It would dilute the solution slightly, giving a lower concentration than intended.
What does “weighing by difference” mean?
Weigh the full container (e.g. weighing boat with solid), then reweigh after tipping out the solid. Subtract the second value from the first to get accurate mass transferred.
What type of balance should be used for making a standard solution?
A 2 decimal place (0.01 g) or 4 decimal place (0.0001 g) balance for higher precision.
What is the meniscus and how do you read it correctly?
The meniscus is the curved surface of the liquid. Always read the volume at the bottom of the meniscus at eye level to avoid parallax error.
Why is a polystyrene cup used in calorimetry?
Because it’s a good insulator, which helps reduce heat loss to the surroundings and improves accuracy
What are common sources of error in a simple calorimetry experiment?
• Heat loss to the surroundings
• Incomplete combustion (if using a fuel)
• Evaporation of fuel or liquid
• Assuming solution has same density/specific heat capacity as water
• Not stirring evenly → uneven temperature
What effect would heat loss to the surroundings have on the calculated ΔH?
It would cause a lower temperature change, so the calculated enthalpy change (ΔH) would be less exothermic than it should be.
How can you reduce errors in a calorimetry experiment?
• Use a lid on the calorimeter
• Use a draught shield or insulation
• Use a digital thermometer or temperature probe
• Stir continuously for even heating
