3-quantitive chemistry

Question 1

how to calculate relative formula mass of a compound

Answer 1

work out relative formula masses for all the elements in the compound
add up all the relative atomic masses

Question 2

how do you find percentage mass of an element in a compound

Answer 2

Ar x number of atoms of that element
————————————— x100
Mr of the compound

Question 3

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

Answer 3

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

Question 4

what is a mole

Answer 4

name given to an amount of substance

Question 5

what is avagadros constant

Answer 5

6.02 x 10^23

Question 6

explain what 1 mole of a substance

Answer 6

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

Question 7

why is avagadros constant 6.02 x 10^23

Answer 7

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

Question 8

how do you calculate number of moles

Answer 8

number of moles =
mass in g (of an element/compound)
———————————————-
Mr (of the element / compound )

Question 9

how is mass conserved in chemical reactions

Answer 9

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

Question 10

how can you show mass is conserved in a reaction from an equatiion

Answer 10

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

Question 11

why might you observe a change of mass of an unsealed reaction vessel

mass increase

Answer 11

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

Question 12

why might you observe a change of mass of an unsealed reaction vessel
decrease in mass

Answer 12

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

Question 13

what do the big numbers in front of reactants and products in chemical formulas show you

Answer 13

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

Question 14

how can you work out the balanced symbol equation for a reaction using masses of reactants and products

Answer 14

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

Question 15

reactions stop when one reactant is used up
explain how this works

Answer 15

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

Question 15

reactions stop when one reactant is used up
explain how this works

Answer 15

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

Question 16

how can you calculate mass of product formed in reaction by using mass of limiting reactant and balanced reaction equation

Answer 16

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

Question 17

what is the mass of the product known as

Answer 17

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

Question 18

one mole of gas occupies,…

Answer 18

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

Question 19

what is the formula to find the volume of a known mass of any gas at r.t.p

Answer 19

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

Question 20

for reactions between gases, how can you use the volume of 1 gas o find the volume of another

Answer 20

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

Question 21

what is concentration

Answer 21

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

Question 22

how can you measure concentration

Answer 22

volume of solvent(dm^3)

or number of moles of solute(mol)
————————————-
volume of

Question 22

how can you measure concentration

Answer 22

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

Question 23

what do titrations help you find

Answer 23

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

Question 24

how can you find concentration of a substance in mol/dm^3

Answer 24

no.of moles= conc x volume

Question 25

how do you convert mol/dm^3 to g/dm^3

Answer 25

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

Question 26

what does atom economy tell you

Answer 26

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

Question 27

how do you calculate atom ecomony

Answer 27

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

Question 28

what does 100% atom economy mean

Answer 28

all atoms in reactants have been turned into useful desired products
the higher the atom economy, the greener the prcess

Question 29

why is higher atom economy better for profits and the environment

Answer 29

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

Question 30

what are factors to consider when choosing which reaction to use to make a certain product

Answer 30

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

Question 31

what is percentage yield

Answer 31

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

Question 32

why should industrial processes have as high a percentage yield as possible

Answer 32

to reduce waste and costs

Question 33

how do you calculate percentage yield

Answer 33

mass of product made(g)
——————————- x 100
maximum theoretical mass
of product (g)

Question 34

in real life why will you never be able to get a 100% yield

Answer 34

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