C4, Chemical Calculations Flashcards
Formula for number of moles
Number of moles = mass / Ar or Mr
Mass in g
Ar = atomic mass
Mr = relative formula mass
Avogadro’s constant
6.02 x 10^23
What do balanced equations show
They tell you the number of moles of substances are involved in a chemical reaction
What does yield of a chemical reaction mean
How much product is made
Formula for percentage yield
Percentage yield = (actual mass of product produced / max theoretical mass of product possible) x 100%
Factors affecting percentage yield
- product may be lost in handling / left in apparatus
- the reaction may be reversible (as they form they react to reform the reactants)
- some reactants may produce unexpected reactions
What is atom economy
The measure of the extent to which the atoms in the starting materials end up in the desired product
What happens if remaining atoms aren’t used up in other products (atom eco)
They end up as hazardous waste products
Formula for atom economy
% atom economy = (MR of desired product from equation / sum of the MR of the reactants from the equation) x 100%
Definition of concentrations
The amount of a substance dissolved in a given volume of liquid
Formula for concentration
Concentration = amount of solute / volume of solution
Concentration = g/dm^3
Amount of solute = g
Volume of solution = dm^3
What does a more concentrated solution have in comparison to a less
It has more solute in the same volume in comparison to a less concentrated solution
What are titrations used for
To measure accurately what volumes of acid and alkali react together completely
Titrations equation
Number of moles = concentration x volume / 1000
Titration method
1) using a pipette transfer 25cm^3 of an alkali into a conical flask?
2) add a few drops of a suitable indicator to the alkali
3) fill a clean dry burette with acid
4) note starting volume
5) add acid into the conical flask and swirl
6) when indicator starts to change colour add acid drop wise and swirl
7) when colour change lasts 10 secs stop adding acid and note the volume.
8) calculate titre and repeat until 2 concordant results are reached.
