Topic 5 - Amount of Substance Flashcards
Avogadro’s constant
6.02 × 10^23 mol^-1
Molar mass
The mass per mole of the substance in g mol^-1
Empirical formula
the simplest whole number ratio of the elements present in one molecule or formula unit of the compound
Molecular formula
the actual number of atoms of each element in a molecule of the compound
Accuracy
How close a measured quantity is to the true value.
The true value is often unknown in chemical analysis, so estimations of the measurement uncertainty is needed.
Acid-base indicator
substances used to show changes in pH of solutions and detect end-points in titrations (e.g. phenolphthalein, methyl orange, bromothymol blue)
Atom economy
a measure of how efficiently a chemical reaction converts the atoms in its reactants to atoms in the product
Atom economy equation
(Mr of atoms in product/sum of Mr of all products) × 100%
Bias
the systematic deviation of laboratory test results from the actual value
arises from systematic errors, which affect all measurements in the same way and do not average out
Concentration
how much solute is dissolved in a certain volume of solution (g/dm^3 or mol/dm^3)
Concentration/g dm^-3 equation
mass of solute (g)/volume of solution (dm^3)
Concentration/mol dm^-3 equation
amount of solute (mol)/volume of solution (dm^3)
Dilution
the process of adding more solvent to a solution to lower the concentration
End point
the point at which a colour change shows that enough of the solution in the burette has been added to react with amount of chemical in the flask
Equivalence point
reached when the amount of reactant added from a burette is just enough to react exactly with all the measured amount of chemical in the flask as shown by the balanced equation
Measurement uncertainty
unavoidable differences between measured values and true values arising from random and systematic errors of measurement
Ideal gas equation
pV = nRT
p = N/m^2 or Pa
V = m^3
T = K
n = moles
R = gas constant (J K^-1 mol^-1)
Limiting reactant
a substance which is present in an amount which limits the theoretical yield
Molar volume
the volume of one mole of the gas molecules under specified conditions
Percentage composition
the percentage by mass of each of the elements in a pure sample of a compound
Precise
when repeat measurements have values which are close to each other. Precise measurements have a small random error
Pressure
force per unit area in Pa
1 Pa = 1 N/m^2
Primary standard
a chemical which can be weighed out accurately to make up a standard solution
Relative atomic mass (Ar)
the mean mass of the atoms of an element relative to 1/12 of the mass of an atom of the isotope carbon-12
Relative formula mass (Mr)
the sum of the relative atomic masses of all the atoms in a substance’s formula
Relative molecular mass
the sum of the relative atomic masses of all the atoms in a substance’s molecular formula (formula mass but for molecules)
Standard solution
a solution with an accurately known concentration
Strong acids
acids that are fully ionised when they dissolve in water
Titration
a volumetric analysis technique for finding the concentrations of solutions and to investigate the amounts of chemicals involved in reactions
Volume
the amount of space taken up by a sample (m^3)
Weak acid
acids that are only slightly ionised when they dissolve in water
organic acids e.g. ethanoic acid, citric acid
inorganic acids nitrous acid, chloric (I) acid
% yield equation
actual yield/theoretical yield × 100
Writing ionic equations
- write out the full equation for the reaction
- replace the formulae of ionic compounds by their separate ions
- remove spectator ions
Writing ionic half equations
- make the electrons balance
- combine the equations
- cancel electrons on each side out
Acid + metal
-> salt + hydrogen
Acid + base
-> salt + water
Acid + alkali
-> salt + water
Acid + metal oxide
-> salt + water
Acid + metal hydroxide
-> salt + water
Acid + metal carbonate
-> salt + water + carbon dioxide
Acid + metal hydrogencarbonate
-> salt + water + carbon dioxide
Amount of substance
the means of counting atoms
symbol: n
unit: mol
Mole
the amount of substance that contains the same number of particles as there are atoms in 12.0 g of carbon-12
Ideal gases
gases which obey the gas laws perfectly
Avogadro’s law
equal volumes of gases at the same temperature and pressure contain equal numbers of molecules
mol, vol, 24 equation
mol = vol (dm^3)/24
or
mol = vol (cm^3)/24,000
Concentration equations
mol. = conc. × vol. (dm^3)
mol. = (conc. × vol. (cm^3))/1000
conc. (mol/dm^3) = amount of solute (mol)/vol. of solution (dm^3)
conc. (g/dm^3) = mass of solute (g)/vol. of solution (dm^3)
Preparing a standard solution
- dissolve a weighted sample of a primary standard in water
- mark the solution up to a definite volume in a graduated flask
reasons for carrying out dilutions
- to make a solution with the concentration needed for a particular experiment from a standard solution
- to dilute an unknown sample for analysis
- to give a concentration suitable for titration
diluting a solution (method)
- take a measured volume of the more concentrated solution with a pipette and run it into a graduated flask
- fill the flask to the mark with purified water
concentration volume equation
CaVa = CbVb
Phenolphthalein colour change
- bright pink in alkaline solutions
- colourless in acidic/near-neutral solutions
Methyl orange colour change
- yellow in alkaline solutions
- orange in neutral solutions
- red in acidic solutions
