Amount of substance Flashcards
Describe how we can carry out a titration. [7]
- Unknown [ ] Solution into conical flask using a pipette
- Known [ ] solution in burette
- Add 2 drops of indicator
- Run known solution into flask (slowly near endpoint)
until indicator just changes colour - Read volume from burette
- Repeat until concordant
How do you calculate % uncertainty?
(uncertainty / measurement made) x 100
What are the steps in working out what reactant is in excess
Step 1 - calculate amount of each reactant in mols
Step 2- use balanced equation to work out which is in excess
Step 3 - use the limiting reactant for calculations
How can we calculate volumes of gases reacting?
equal volumes of under gases under same conditions will have equal moles
therefore, ratio of volumes = ratio of moles
What are some errors involved in using a gas syringe?
gas escapes before bung inserted
syringe sticks
some gases like CO2 are soluble in water so true amount isn’t measured
Why does the crucible need to be dry?
if the lid were wet, mass loss would be inaccurate as water would be lost when heating frm the crucible
Why do we use a lid? Water of crystallisation
lid prevents the loss of solid from the crucible
Why do we not used large amounts of hydrated calcium sulfate?
large amounts may mean decomposition is likely to be incomplete
Why don’t we use small amounts?
percentage uncertainty is too high
Devise a method to remove the water crystals from hydrated calcium sulfate crystals (6)
Weigh an empty clean dry crucible and lid .
•Add 2g of hydrated calcium sulfate to the crucible and weigh again
•Heat strongly with a Bunsen for a couple of minutes
•Allow to cool
•Weigh the crucible and contents again
•Heat crucible again and reweigh until you reach a constant mass ( do this to ensure reaction is complete).
Why is a higher atom economy more beneficial? (4)
- High atom economy is desirable for industrial processes
- They are better for the environment as they produce less waste
- More efficient use of raw material means high atom economy issustainable
- Sometimes waste is expensive to dispose of safely so less waste can becheaper /less money has to be spent separating useful product form waste products
What does the percentage yield tell you?
Tells you how wasteful a process is, as it determines how much of the product is lost during the process
Why is a white tile used during titrations?
You can more easily see the colour changes and end points
How do we reduce uncertainties in titration?
- Replacing measuring cylinders with pipettes / burettes which have lower apparatus uncertainty
- Increasing the volume and concentration of the substance in the conical flask to increase the titre volume
When should we use phenolphthalein over methyl orange? Vice versa?
Use Phenolphthalein if NaOH is used
Use Methyl Orange if HCl is used
Why can distilled water be used during a titration to wash the sides of the flask?
Water does not affect the titration as it doesn’t change the number of moles of each reactant
Why do we need to make sure the jet space in the burette is filled?
If it isn’t filled, it will lead to errors if it fills during the titration, leading to a larger than expected titre reading.
What does concordant mean?
within 0.10 cm^3 of each other
means the results are accurate and reproducible
Give two ways to reduce uncertainties in measuring mass
- Using a more accurate balance or a larger mass
- Weighing by difference to calculate the actual mass of solid transferred
Describe the steps of creating a Volumetric Solution
- Weigh the sample bottle containing the solid on a (2 dp) balance.
- Transfer solid to beaker and reweigh sample bottle.
- Record the difference in mass.
- Add distilled water and stir with a glass rod until all the solid has dissolved.
- Transfer to a volumetric flask with washings.
- Make up to the 250cm3 mark with distilled water.
- Invert flask several time
