Topic 5 - Formulae, Equations and Amounts of Substances Flashcards
What is a mole?
the number of atoms found in exactly 12g of carbon-12
-unit for amount of substance
What is Avogadro’s constant?
the number of particles in one mole
6.02 x10^23
What is molar mass (M)?
the mass per mole of something
this is the same as relative molecular mass Mr
mass =
Mr x no.moles
no.moles =
concentration x volume
concentration (in g dm-3) =
mass/volume
no. particles =
mols x 6.023x10^23 (Avogadro’s constant)
What is the emperical formula?
the smallest whole number ratio of atoms of each element in a compound
What is the molecular formula?
the formula that shows the actual number of atoms of each type in a molecule
How do you calculate emperical formula?
- calc mols of each element (mass/Mr)
- divide by lowest no. mols (to get simplest ratio)
percentage of an element in a compound =
Ar of element x no.atoms in formula
____________________________ x100
Mr
What actions should be taken to ensure the volume of acid at the end point of a titration is accurate?
- add acid drop by drop
- swirl conical flask
- rinse sides of flask with distilled water between additions
- use white tile
- compare colour of end point with previous titrations
- rinse end of jet of burette with distilled water
Why should the acid be added drop by drop at the end of the titration?
to avoid overshooting the end point (ensure not too much acid was added)
Why should the conical flask be swirled during a titration?
to ensure all the acid and alkali react (create a homogeneous mixture)
Why should the sides of the conical flask be rinsed with distilled water between additions in a titration?
to rinse all the reactants into the solution so they can all react
Why should a white tile be used in a titration?
so the end-point can clearly be seen
Why should the colour of end point be compared with previous titrations?
to ensure consistency of the end point
Why should the jet of the burette be rinsed with distilled water in a titration?
to ensure all the acid is in the conical flask
percentage yield (%) =
obtained mass
_____________ x100
theoretical mass
Why is 100% yield not obtained?
- incomplete rxn
- loss in transfer
- impurities formed
- reversible rxn
- incorrect measurements
What is the ideal gas equation?
pV = nRT
What are the units for pressure in the ideal gas equation?
Pa
What are the units for volume in the ideal gas equation?
m3
How do you convert from cm3 to m3?
÷1000000
What are the units for temperature in the ideal gas equation?
K
How do you convert from °C to K?
+273
How do you make a standard solution (with known conc)?
- weigh out mass of container and solute combined
- dissolve solute in distilled water
- reweigh container (to calc the exact mass that was added to calc exact conc of standard soln)
- tip soln into a volumetric flask
- rinse beaker with distilled water and add washings to volumetric flask (to make sure none is left behind)
- top flask up to correct volume with more distilled water
- place stopper on volumetric flask and turn upside down a few times (to thoroughly mix)
What are concordant results?
results within ±0.1cm3 of eachother
percentage uncertainty =
uncertainty x no. readings
_____________________ x100
reading
How can you minimise percentage error/uncertainty?
- use more precise equipment
- using larger volumes
What is a hydrated salt?
a salt produced when enough water is left for crystallisation so that water molecules surround the metal ion in the salt
How is a hydrated salt produced?
- small portions of base/metal are added to acid until no more reacts (can be done over heat to increase rate of rxn/how much can be dissolved)
- filter to remove residue
- heat evaporating basin containing saturated solution of salt (heat reduces vol and crystals begin to form)
- transfer solution to crystallising dish (to allow water to evaporate so that hydrated salt is produced)
