3-quantitive chemistry Flashcards
how to calculate relative formula mass of a compound
work out relative formula masses for all the elements in the compound
add up all the relative atomic masses
how do you find percentage mass of an element in a compound
Ar x number of atoms of that element
————————————— x100
Mr of the compound
a mixture contains 20 % iron ions by mass, what mass of iron chloride would you need to provide the iron ions in 50g of the mixture
1- find mass of iron in the mixture
50 x (20%/100) =10g iron
2-calculate percentage mass of iron in iron chloride
percentage mass of iron=
Ar x number of atoms of that element/ Mr of the compound x100
3- calculate mass of iron chloride that contains 10g of iron
x% 10/ x%
——– = answer
100
what is a mole
name given to an amount of substance
what is avagadros constant
6.02 x 10^23
explain what 1 mole of a substance
one mole of any substance is just an amount of that substance that contains an avagadros number of particles (6.02 x 10^23)
these particles could be atoms, molecules, ions or electrons
why is avagadros constant 6.02 x 10^23
the mass of that number of atoms/molecules of any substance is exactly the same number of grams as the relative atomic/formula mass of the element or compound
1 mole of atom/molecule of any substance will have a mass in grams equal to the relative formula mass for that substance
e.g.
carbon (12) = 1 mole of carbon weighs 12 g
nitrogen gas N2 (2x14)= one mole of N2= 28g
co2 = 44
= 1 mole of co2 is 44g
how do you calculate number of moles
number of moles =
mass in g (of an element/compound)
———————————————-
Mr (of the element / compound )
how is mass conserved in chemical reactions
no atoms are destroyed or created
same number and types of atoms on eachside of a reaction equation
no mass lost or gaied =mass is conserved during a reaction
how can you show mass is conserved in a reaction from an equatiion
add up relative formula masses of the substances on each side of a balanced symbol equation
total mr of reactants equals the total Mr of the products
why might you observe a change of mass of an unsealed reaction vessel
mass increase
if mass increases , one of the reactants is a gas found in air (e.g oxygen) and all products are solids,liquids,gas
before reaction, gas is floating around in the air but not in reaction vessel so you cant account for its mass
when gas reacts to form part of prudct , it becomes contained inside reaction vessel so total mass of stuff inside reaction vessel increases
e.g. when a metal reacts with oxygen in unsealed container, mass of container increass, mass of metal oxide produced= total mass of metal and oxygen reacted from the air
metal + oxygen = metal oxide
why might you observe a change of mass of an unsealed reaction vessel
decrease in mass
if mass decreases , one of the products is a gas and all the reactants is solid liquid or aqeous
before reaction, all reactants are contained in reaction vessel
if vessel isnt enclosed, gas can escape from reaction vessel so total mass inside reaction vessel decreases
e.g. when metal carbonate thermally decomposes to form metal oxide and carbon dioxide, mass of reaction vessel will decrease if not sealed
=(mass of metal oxide & carbon dioxide= mass of metal carbonate decomposed)
metal carbonate=> metal oxide + carbon dioxide
what do the big numbers in front of reactants and products in chemical formulas show you
tells you how many moles of each substance takes part or is formed during the reaction
little number tells you how many atoms of each element there are in each of the substances
how can you work out the balanced symbol equation for a reaction using masses of reactants and products
1-divide mass of each substance by its relative formula mass to find number of moles
2- divide the number of moles of each substance by smallest number of moles in the reaction
3- if any of the numbers arent whole numbers, multiply all numbers by same amount so that they all become whole numbers
4- write balanced symbol equation for reactoin by putting numbers in front of chemical formulas
reactions stop when one reactant is used up
explain how this works
1- reaction stops when all of one of the reactants is used up, any other reactant is in excess , theyre usually added in excess to make sure the other reactant is used up
2- reactant used up is called limiting reactant bc it limits the amount of product formed
3- amount of product formed is directly proportional to the amount of limiting reactant e.g. if u 1/2 amount of limiting reactant, amount of product formed will also 1/2
if you double it doublrs
more reactant=more reactant particles in reaction= more product particles
reactions stop when one reactant is used up
explain how this works
1- reaction stops when all of one of the reactants is used up, any other reactant is in excess , theyre usually added in excess to make sure the other reactant is used up
2- reactant used up is called limiting reactant bc it limits the amount of product formed
3- amount of product formed is directly proportional to the amount of limiting reactant e.g. if u 1/2 amount of limiting reactant, amount of product formed will also 1/2
if you double it doublrs
more reactant=more reactant particles in reaction= more product particles
how can you calculate mass of product formed in reaction by using mass of limiting reactant and balanced reaction equation
1-write out balanced equation
2- work out relative formula mass of reactant and product you want
3- find out how many moles there are of substance yk mass of
4- use balanced equation to work out how many moles there wll be of other substance
(how many molesofproduct will be made of this many molesof reactant)
use this number of moles to calculate the mass
also look at the ratio of moles
what is the mass of the product known as
yield of a reaction
masses you calculate from mass of product formed in reaction by using mass of limiting reactant and balanced reaction equation
isknown as theoretical yield of a reaction
but amount of product you get will be less than you calculated
one mole of gas occupies,…
24 dm^3 at 20 degrees celisuis
at the same temperature and pressure, equal number of moles of any gas will occupy the same volume
what is the formula to find the volume of a known mass of any gas at r.t.p
volume of gas= mass of gas
—————- x 24
Mr of gas
r.t.p is 20 at 1 atm
1 mole of any gas occupies 24dm^3
for reactions between gases, how can you use the volume of 1 gas o find the volume of another
e.g. how much co2 is formed when 30 dm^3 of oxygen reacts with carbon monoxide
2CO(g)+ O2 => 2CO2(g)
1 mole of O2=> 2 moles of CO2
1 volume of O2=> 2 volumes of CO2
so 30dm^3 of O2 => (2x30dm^3)=60dm^3 of CO2
what is concentration
reactions that take place between substances that are dissolved in a solution
amount of a substance (mass or number of moles) in a certain volume of a solution is concentration
the more solute (the substance thats dissolved) there is a given volume, the more concentrated the solution
how can you measure concentration
volume of solvent(dm^3)
or number of moles of solute(mol)
————————————-
volume of
how can you measure concentration
volume of solvent(dm^3)
or number of moles of solute(mol)
————————————-
volume of solvent (dm^3)
you can convert to dm^3 by dividing by 1000
what do titrations help you find
lets you find the volume needed for 2 solutions to react together completely
if you know concentration of one of the solutions, you can use volume from titration experiment with reaction equation to find concentration of other solution
how can you find concentration of a substance in mol/dm^3
no.of moles= conc x volume
how do you convert mol/dm^3 to g/dm^3
find concentration in mol/dm^3
mass =moles x Mr
1-work out relative formula mass
2- convert concentration in moles into concentration in grams
mass in grams= moles x relative formula mass
what does atom economy tell you
atom economy or utilisation of a reaction tells you how much of the mass of the reactants is wasted when manufacturing a chemical and how much ends up as useful product
how do you calculate atom ecomony
relative formula mass of
desired products
———————————— x100
relative formula mass of
all reactants
1-identify desired product
2-work out Mr of all reactants
3- work out total Mr of desired product
4-use formula to calculate atom economy
what does 100% atom economy mean
all atoms in reactants have been turned into useful desired products
the higher the atom economy, the greener the prcess
why is higher atom economy better for profits and the environment
1-low atom economy uses up resources very quickly, make lots of waste materials that need to be disposed, makes reactions unsustainable ,raw materials will run out and have to go somewhere
2- low atom economy reactions arent profitable, raw materials are expensive to buy, products=expensive to remove and dispose of responsibly
so u need to find a use for the waste products , come up with a reaction what gives useful by products
reactions with highest atom economy only have 1 product , atom economy of 100%
more products=lower atom economy
what are factors to consider when choosing which reaction to use to make a certain product
atom economy
yield
rate of reaction
position of equilibrium for reversible reactions
a reaction with a low atom economy that produces useful by products might be used
what is percentage yield
it compares actual yield to theoretical yield
the amount of product you get is known as yield
the more reactants you start with, the higher the actual yield will be
100% yield means you got all product you expected to get
0% means no reactants were converted into product
why should industrial processes have as high a percentage yield as possible
to reduce waste and costs
how do you calculate percentage yield
mass of product made(g)
——————————- x 100
maximum theoretical mass
of product (g)
in real life why will you never be able to get a 100% yield
some product or reactant always gets lost along the way and that goes for big industrial processes as well as skl lab experiments
how this happens depends on what sort of reactio it is and what apparatus is used
1) not all reactants react to make a product, in reversible reactions products can turn back to reactants so yield will never be 100%
e.g. in haber process, reverse reaction is also happening so reaction never goes to completion
2) might be side reactions- reactants might react differently than expected, may react with gases in air or impurities in reaction mixture might form extra products
3- when you filter liquid to get solid particles, you lose bit of liqui/solid
if you want to keep liquid, youll lose bit that remains with solid and filter paper (stays a bit wet) if you want to keep solid , some will be left behind when you scrape it off filter paper
youll lose a bit of material when transferring from 1 container to another even if you manage not to spill it , some gets left behind inside surface of old container
