Amount of Substance Flashcards
Define relative atomic mass (Ar)
Average mass of one atom of an element/ 1/12th the mass of a C12 atom
Define relative molecular mass (Mr)
Average mass of one molecule/ 1/12th the mass of a C12 atom
Define the Avogadro constant
The Avogadro Constant is the number of atoms in 12g of C12
What is the formula involving moles, mass and Mr or Ar?
Moles = mass/Ar or Mr
What is the formula for percentage yield?
% yield = amount of specific product formed/ theoretical maximum amount of product that could be formed x 100
What is the formula for concentration?
concentration = moles/volume
What are the steps in a titration?
- Fill a burette with the acid of know concentration.
- Accurately measure out 25cm3 of the alkali standard solution using a pipette and transfer to a conical flask.
- Add a few drops of a suitable indicator to the alkali.
- Slowly add the acid from the burette into the conical flask, swirling constantly. As soon as the indicator changes colour the solution has turned neutral so stop adding the acid. Record the volume of acid which has been added.
- Repeat the process until you get two results which are within 0.1cm3 of each other. The mean of these two results are then used to calculate an average titre.
- Once an average titre has been determined it can be used to calculate the unknown concentration.
What are concordant results?
Concordant results are any results that are within +/- 0.1 of one another
How do you calculate percentage error?
Error of the equipment/amount measured x 100
What are the steps to make up a standard solution?
Stage 1: transfers known mass of solid
a) Weigh the sample bottle containing the solid on a (2 dp) balance
b) Transfer to beaker* and reweigh sample bottle
c) Record the difference in mass
Stage 2: Dissolves in water
a) Add distilled / deionised water
b) Stir (with a glass rod) or swirl
c) Until all solid has dissolved
Stage 3: Transfer, washing and agitation
a) Transfer to volumetric / graduated flask using a funnel
b) With washings
c) Make up to 250cm3 / mark with water
d) Shakes/inverts/mixes
What is the ideal gas equation?
pV = nRT
What are the components of the ideal gas equation and their standard units?
p = Pressure measured in Pa
V = Volume measure in m3
n = number of moles
R = the ‘molar gas constant’ which is always 8.31 JK-1mol-1
T = temperature measured in K
What is the definition of the empirical formula?
The empirical formula of a compound is the simplest whole number ratio of atoms of each element
What is the definition of the molecular formula?
The molecular formula of a compound is the actual number of atoms of each element
What is combustion analysis?
It is a method of determining the empirical formula where a sample of an unknown compound is burned in an excess of oxygen and the products of combustion analysed.
