Amount of substance Flashcards
What is the Avagadros constant?
The no. of atoms in 12g of Carbon-12
If we weigh out the RAM of any element how many atoms will there be?
6.022 x 10 ^23 - avagadros constant
What is the RAM?
(2 marks)
- A weighted average mass of an element taking into account its naturally occuring isotopes
- In comparison to 1/12th the mass of a Carbon atom
What is the formula for Ar?
Ar= avg. mass of 1 atom of an element/ 1/12th the mass of a C atom
What is the formula for moles in terms of mass and Mr?
Moles = mass/Mr
What is the formula for moles in terms of concentration and volume?
Moles = conc. x volume
How is a standard solution made?
(6 marks)
Stage 1:
- Weight the sample bottle containing the solid on a 2dp balance
- Transfer to beaker and reweigh sample bottle
- Record difference in mass
Stage 2:
- Add distilled/ deionised water
- Stir with glass rod
- Until all the solid has dissolved
Stage 3:
- Transfer to volumetric flask
- With washings
- Make up to 250cm3 with water
- Shake/ mix
Why should air bubbles be removed from the burette tap?
(1 mark)
Space will fill during titration/ titres are too high
Why should the funnel be removed from the burette during the titration?
(1 mark)
As a droplet from the funnel could enter the burette/ affect volume/ titre
Suggest why rinsing the inside of the conical flask improves the value of the titre
(1 mark)
Ensures all reagents are able to react
Explain why rinsing the conical flask has no effect on the accuracy of the titre
(1 mark)
This does not change the no. of moles of reagents
Suggest how a titration could be improved to reduce the percentage uncertainty in using the burette
(2 marks)
- Larger mass of solid/ more concentrated solution of analyte (conical flask) / less concentrated solution of titrant (burette)
- So a larger titre/ reading will be needed
