QUANTITATIVE CHEM Flashcards
how to work our relative formula mass of compound/molecule
how do you work out % mass of an element in a compound
add up relative atomic masses of all atoms
relative atomic mass of element (Ar)/ relative formula mass of compound(Mr) x100
Calculate percentage by mass of H in CH₄
H Ar=1 CH₄ Mr= 16
1x4 / 16 x 100 = 25%
how many g in a tonne
1000000
what is avagadros constant
What is a mole of any substance
Symbol for moles
6.02x10²³
an amount of substance that contains 6.02x10²³ particles- atoms, molecules,ions, electrons
mol
Why is Avagadro Constant used
one mole of a substance will have mass EQUAL to Ar / Mr of that substance
eg
N₂ has Mr of 28. One mole of N₂ is 28g
C Ar= 12, One mole of C is 12g
CO₂ Mr= 44 One mole of CO₂ is 44g
12 g of Carbon, 28g of N₂, 44g of CO₂ all contain one mole (6.02x10²³) atoms/molecules
formula triangle for number of moles in a given mass
——mass——
Mr——–mol
state law of conservation of mass
no atoms lost/made during chemical reaction.
so mass of products= mass of reactants (mass is concerved)
why would there be a change in mass in an unsealed vessel
An increase-
3 points
A decrease-
4 points
reactant/product is gas
-A REACTANT is a gas.
-BEFORE reaction, gas is floating, not contained in vessel, mass NOT taken into account
-Gas reacts to form product, its contained inside vessel, TOTAL mass inside vessel INCREASES
-A PRODUCT is gas,
-REACTANTS are s/ l / aq.
-BEFORE reaction, all reactants contained in vessel.
-Gas escapes as its formed. Gas X in vessel-Mass of gas X taken into account.- TOTAL mass inside vessel DECREASES
eg of when
mass seems to increase
mass seems to decrease
(due to gasses)
Equations for these with State symbols
- Metal + Oxygen react —-> MetalOxide produced w/ greater mass than metal. (o₂ floating in air, mass not accounted)
-mass of MetalOxide = TOTAL mass of reactants
Metal(s) + Oxygen (g) = Metal oxide(s)
-Metal carbonate thermally decomposes to form Metal Oxide + CO₂
-CO₂ escapes into atmosphere
-metal oxide is the only solid product
Metal carbonate(s) = Metal oxide(s) + Carbon Dioxide (g)
why does mass in a unsealed container increase/ decrease if Gas produced in the reaction.
Gases expand to fill any container. If vessel unsealed, gas expands out from vessel, escapes into air
a mixture contains 20% iron ions by mass.
What mass of iron Chloride (FeCl₂) would be needed to provide the iron ions in 50 g of the mixture. Ar of Fe=56 Cl=35.5
-Find mass of iron in mixture-
Mixture contains 20% iron by mass, in 50g will be 20%x 50 = 10 grams of iron.
-Calculate %bymass of Fe in FeCl₂
56 / 56+2(35.5) X 100= 44.09% iron by mass
-Calculate how many grams of FeCl₂ will provide 10 grams of Iron
FeCl₂ has 44.09% iron chloride by mass,
There will be 10 g Fe in 10/44.09% =23 grams
steps to work out balanced symbol equation
find no. of moles of each substance as whole number ratio using equation: Mass Mr Mol
why is an excess of 1 reactant commonly used
what is the limiting reactant, why
The amount of product formed is directly proportional to
WHY
-ensure all of other reactant is used
-reactant that completely used up, limits amount of products
-the amount of limiting reactant, (eg 1/2 the limiting reactant= 1/2 the products formed. Doubling limiting reactant = double the product formed AS LONG AS ITS STILL LIMITING)
-reactant particles=more product particles
Calculate Mass of Aluminium Oxide formed when 135g of aluminium is burned in air
5 steps
-write balanced equation- use crossover method and then balance
4Al +3O₂ —–> 2Al₂O₃
-calculate relative formula masses
Al:27 Al₂O₃:102
-Find mols of substance that you know mass of
135g of Al.
Mols= mass/Mr = 135/27 = 5 MOLS
-use balanced equation to work out moles of other substance (use ratios) 4: 2 but we have 5 Mols so…. 5: 2.5
-Use moles and Ar/Mr of other substance to work out its MASS
mass=molsxMr = 2.5x102
= 255 g of aluminium oxide
at the same …… and ……. equal numbers of mols of any …… will occupy the same ……..
at the same temperature and pressure equal numbers of moles of any gas will occupy the same volume
at room temp and pressure , 1 mole of ANY gas occupies ……
Formula to find vol of known mass of gas
What is room temp + pressure
24 dm³
Volume= (mass of gas / Mr of gas) x 24
room temp=20*c
room pressure= 1 atm
How much Carbon Dioxide formed when 30dm³ of Oxygen reacts with carbon Monoxide.
2CO + O₂ —> 2CO₂
1 mol of O₂ = 30dm³
1O₂ : 2CO₂
1:2 ratio
30dm³ : 60dm³
ANSWER= 60dm³
what is the relative formula mass of 2KBr
Ar of K=39 Ar of Br=80
39+80=119
DON’T use LARGE NUMBERS to calculate Mr
relative atomic mass=
sum of (mass of isotope x isotope abundance)/ 100
%mass of an element in a compound=
(Ar x no. of atoms of that element/ Mr of compound) x100
concentration=
2 equations
mass of solute(g) /volume of solvent(dm³)
number of moles of solute (mol) /volume of solvent(dm³)
number of mols=
mass/relative formula mass
mean rate of reaction=
quantity of reactant used/ time
quantity of product formed/time
Rf value=
distance moved by substance/distance moved by solvent
overall energy change=
energy used to break bonds- energy released when bonds are made
percentage yield=
(mass of product made/ maximum theoretical mass of product) x100
atom economy
(relative formula mass of desired product/ sum of relative formula masses of all reactant ) x100
volume of gas=
number of moles of gas x 24
concentration is measured in….
The more solute there is in a volume, the more…
Concentration =
How do u go from cm³ to dm³
mol/dm³ or g/dm³
concentrated the solution
mass OR number of mols of solute/volume of solvent
Divide by 1000
atom economy tells us
Why is a high atom economy important
100% atom economy means
the amount of starting material that end up as useful products. - the mass of wasted reactant
for sustainable development and economic reasons
-all atoms in reactants turned into DESIRED products
why is low atom economy reactions bad x3
how can this be changed
-makes lots of waste- must be disposed of
-use recources quickly- unsustainable if materials non renewable.
-not profitable- raw materials are costly and waste products costly to RESPONSIBLY remove + dispose of
-should find way to use waste product-There is more than 1 way to make a product so make reaction create useful ‘‘by products’’ instead of ‘‘waste products’’
what affects why particular reaction chosen to produce product
5 things
atom economy
yield
rate
equilibrium position
usefulness of by products
why is it not possible to always get the % yield that was calculated
x3
what is yield
what is % yield
-reaction may not go to completion bcse its reversible, products can turn back into reactants
-some product may be lost when separated from reaction mixture
-some reactants may react in different ways to expected
YIELD- amount of product obtained
yield compared w/ maximum theoretical yield
why might some reactants react in ways differently than expected
x2
react w/ gases in the air
react w/ impurities in the mixture
end up forming extra products
-how might some liquid/ solid be lost when separated from reaction mixture
When filtering a liquid to remove solids- u lose abit of liquid/ solid
-if you want to keep the liquid- youll lose the bit that remains w/ the solid + filter paper
-if you want to keep the solid- some will get left behind when scraped of filter paper
-solid/liquid lost when transferred from 1 container to another- left on inside surface of old container
