Chapter 3 - Amount of Substance Flashcards
what is the quantity amount of substance (n) used for and what is the unit?
- used to count the number of particles in a substance
- measured in the unit mole (mol)
what is one mole?
the amount of a substance that contains 6.02x10^23 particles (avogadros constant)
how do you find the mass of one mole of an element?
it is the relative atomic mass in grams (eg 12g for carbon)
what is the difference between 1 mol of H and 1 mol of H2?
1 mol of H = 1 mole of hydrogen atoms
1 mol of H2 = 1 mole of hydrogen molecules
what is the quantity molar mass (M) used for and what is the unit?
- molar mass is the mass per mole of a substance
- the units of molar mass are gmol^-1
- molar mass is equivilent to Mr
how are the amount of substance (n), mass (m) and molar mass (M) linked?
amount = mass/molar mass
what is molecular formula?
the number of atoms of each element in a molecule
which formula is used in equations?
molecular formula
what is empirical formula?
the simplest whole-number ratio of atoms of each element in a compound
when is empirical formula used?
when talking about giant crystalline structures of atoms/ions that don’t exist as molecules
which elements exist as molecules?
H2, N2, O2, F2, Cl2, Br2, I2, P4 and S8
what is relative molecular mass (Mr)?
- Mr compares the mass of a molecule with the mass of a carbon-12 atom
- it can be calculated by adding up all of the relative atomic masses of the elements in the molecule
what is relative formula mass?
- relative formula mass compares the mass of a formula unit with the mass of an atom of carbon-12
- it can be calculated by adding together the relative atomic masses of the elements in the empirical formula
what is water of crystallisation?
water molecules which are a part of a salt’s crystalline structure
how would you carry out an experiment to find the formula of a hydrated salt?
- weigh an empty crucible
- add the hydrated salt into the crucible and weigh again
- heat the crucible using a pipe-clay triangle on a tripod above a bunsen burner
- weigh the crucible and anhydrous salt
what happens when a hydrated salt is heated?
bonds holding the water within the crystal are broken and the water is driven off, leaving an anhydrous salt
what assumptions are made when you carry out an experiment to find the formula of a hydrated salt?
- all of the water has been lost - some water could be left inside the salt so the salt is still slightly hydrated. this can be improved by heating to a constant mass
- no further decomposition has occurred - many salts decompose further when heated
what is 1 cm3 equivilent to?
1 cm3 = 1 millilitre
what is 1 dm3 equivilent to?
1 dm3 = 1 litre
what is the unit for concentration?
mol dm^-3
what equation links concentration, amount (n) and volume?
amount (n) = concentration x volume
how do you convert from cm3 to dm3?
divide by 1000 (or multiply by 10^-3)
what is a standard solution?
a solution of known concentration, prepared by dissolving an exact mass of the solute in a solvent and making up the solution to an exact volume
what is molar gas volume?
the volume per mole of gas molecules at stated temperature and pressure
what is the volume of 1 mole of gas at RTP?
24dm^3 or 24000cm^3
what is the equation linking amount (n) and volume of a gas?
amount (n) = volume/molar gas volume(24dm^3 or 24000cm^3 at RTP)
what is the ideal gas equation?
pressure x volume = amount (n) x R (constant) x temperature
what is the unit for temperature in the ideal gas equation?
K (Kelvin)
how do you convert from celsius to kelvin?
add 273
what is the unit for pressure in the ideal gas equation?
Pa
how do you convert from kPa to Pa?
x 10^3
what is 1 atm equivilent to?
101kPa (101000Pa)
what is the unit for volume in the ideal gas equation?
m^3
how do you convert from cm^3 and dm^3 to m^3?
cm^3 to m^3 - x 10^-6
dm^3 to m^3 - x 10^-3
what is the room temperature in the ideal gas equation?
19 degrees celsius or 292K
how would you carry out an experiment to find the relative molecular mass of a volatile liquid (boiling point below 100c and liquid at room temp)?
- add a sample of the liquid into a small syringe via a needle and weigh
- inject the sample into a gas syringe and reweigh the small syringe to find the mass of the liquid
- place the gas syringe into a boiling water bath at 100 degrees. the liquid will vaporise producing a gas
not done
what is stoichiometry?
the ratio between the number of moles of each substance in a reaction
how would you carry out an experiment to identify an unknown group 2 metal?
- set up a flask with a gas syringe attached and a bung on the top
- weigh a sample of the metal and add it to the flask
- use a measuring cylinder to add 25cm^3 of HCl to the flask and quickly replace the bung
- measure the maximum volume of gas in the syringe
why would theoretical yield not be achieved in a reaction?
- the reaction may not have gone to completion
- other reactions may have taken place alongside the main reaction
- purification of the product may result in loss of some of the product
how do you calculate percentage yield?
percentage yield = actual yield / theoretical yield x 100
what is the limiting reagent?
the reactant that is not in excess and will be completely used up first
what is atom economy?
a measure of how well atoms have been utilised in a chemical reaction
what do reactions with high atom economies do?
- produce a large proportion of desired products and few unwanted waste products
- are important for sustainability as they make the best use of natural resources
how do you calculate atom economy?
atom economy = sum of molar masses of desired products / sum of molar masses of all products x 100
why is improving atom economy important?
it makes industrial processes more efficient, preserves raw materials and reduces waste
what other factors affect sustainability of a reaction?
- availability of reactants
- percentage yield
