Quantitive Chemistry Flashcards
what is the relative formula mass?
sum of all the relativeatomic masses of the atoms in the molecular formula
how do you work out the percentage mass of an element in a compound?
Alittle r x number of atoms of that element / Mlittle r of compound x 100
what is Alittle r?
RAM
what is Mlittle r?
RFM
what is a balanced equation?
a symbol equation with the same number of atoms of each element on both sides
give the balanced equation of magnesium + oxygen.
2Mg + O2 > 2MgO
what is 1 mole equal to?
6.02x10^23 particles of a substance
what is avogadro’s constant?
6.02x10^23
how do you work out the number of moles?
mass in grams / RFM (Mlittle r)
what is the conservation of mass?
no atoms are created or destroyed in a chemical reaction, so the total masses of reactants and products are also the same.
why would you find a decrease in mass in an unsealed reaction vessel?
a gas is made during the reaction and escapes the vessel, so its mass is no longer accounted for
why would you find an increase in mass in an unsealed reaction vessel?
a gas from the air is a reactant, so its mass is added to the mass in the vessel
how can a balanced equation tell you the moles?
the number in the front is the number of moles.
Mg + 2HCl > MgCl + H
so theres 1 mole of Mg reacting with 2 moles of HCl
how do you balance an equation using masses?
divide mass by RFM to find the number of moles of each substance > divide each number of moles by the smallest number of moles > if results aren’t whole numbers, mulitply them by the same number so that they are whole > put these numbers in front of the chemical formulae.
what is a limiting reactant?
a reactant that gets completely used up in a reaction, so limits the amount of product formed.
all the other reactants are in excess
if you know the mass of the limiting reactant, how can you work out the mass of a product?
write the balanced equation for the reaction > divide the mass of the limiting reactant by the RFM to find the number of moles > use the balanced equation to find the number of moles of the product > multiply this number of moles by the RFM of the product to work out its mass.
tell me about gases.
at the same temperature and pressure, one mole of gas will occupy the same volume.
at room temperature and pressure, one mole of any gas occupies 24 dm3
what is room temperature and pressure?
20C and 1atm
how do you work out the volume of gas?
volume of gas = mass / RFM x 24
what is concentration?
amount of substance dissolved in a certain volume of solution
what happens if you increase the amount of solute
concentration increases
what happens if you increase the volume of solvent?
concentration decreases
what are teh two ways to measure concentration?
1: mass of solute / volume of solvent = g/dm3
2: number of moles of solute / volume of solvent = mol/dm3
how do you find an unknown concentration from the result of a titration?
multiply the konwn concentration by measured volume to work out the number of moles of reactant 1 > use reaction equation to work out how many moles of reactant 2 have reacted > divide number of moles of reactant 2 by its volume to get the unknown concentration
what is reactant 1 of a titration?
measured volume, known concentration
what is reactant 2 of a titration?
known volume, unknown concentration (+ indicator)
what is atom economy?
also called atom utilisation
the percentage of the mass of reactants that ends up as useful products
how do you calculate the atom economy?
total RFM of desired products / total RFM of all reactants x 100
what are 3 advantages of using reactions with higher atom economies?
1: use up resources at a slower rate
2: don’t produce a lot of waste
3: more profitable
what is yield?
the amount of product made in a reaction
what is percentage yield?
a comparison of the amount of product you actually get (yield) with the maximum you could theoretically get
how do you calculate percentage yield?
mass of products actually made / maximum theoretical mass of product x 100
what does higher % yield mean?
less waste and lower costs
what are 3 common reasons for less than 100% yield?
1: reaction is not complete - eg, its reversible
2: side reeactions use up some of the reactants or product
3: some product is lost when it’s separated from the reaction mixture
