Topic 3: Quantitive chemistry

Question 1

how do you work out relative formula mass (Mr)?

Answer 1

all the relative atomic masses (the big number) added together from all the atoms

Question 2

what is the relative formula mass (Mr) of MgCl2?

Answer 2

Mg=24
Cl=35.5
Mg+(2xCl)
24+(2x24)=95

Question 3

what is the formula to calculate % mass of an element in a compound?

Answer 3

Percentage mass of an element in a compound = (Ar x number of atoms in the element/ Mr of the compound) x100

Question 4

find the percentage mass of sodium in sodium carbonate Na2CO3?

Answer 4

Ar sodium=23
Ar of carbon=12
Ar of oxygen=16
Mr of Na2CO3 = (2x23) + 12 + (3x16)
(Ar x number of atoms/Mr of compounds) x 100
(23 x 2/106) x 100

Question 5

A mixture contains 20% iron ions by mass. what mass of iron chloride (FeCl2) would you need to provide the iron ions in 50g of the mixture? Ar of fe=56 Ar of Cl = 35.5

Answer 5

the mixture is 20% iron by mass so in 50g there will be 50 x 20/100 = 10g of iron in the mixture

percentage mass of iron = (Ar x number of atoms/ Mr of compound) x 100
(56/56+(2x35.5) )x 100 = 44.09%

iron chloride contains 44.09% iron by mass so there will be 10g of iron in (44.09/100)/10= 23g
so you need to have 23g of iron chloride to provide the iron in 50g of the mixture

Question 6

what is a mole?

Answer 6

a way of measuring particles by the number 6.02 x 10 to the power of 23. so if oxygen contained one mole of atoms it would contain 6.02 x 10 to the power of 23 atoms

Question 7

how do you work out a substances mole number from its Ar?

Answer 7

the atoms Ar is equal to the weight of mole in grams. for example Carbon has an Ar of 12 so one mole of carbon weighs 12g

Question 8

Nitrogen gas (N2) has an Mr of 28 so how much would one mole of nitrogen weigh?

Answer 8

one mole of N2 weighs exactly 28g

Question 9

what is the formula to calculate the number of moles?

Answer 9

moles = mass in g (of element/compound) / Mr (of the element or compound)

Question 10

how many moles are there in 66g of CO2?

Answer 10

Mr of CO2 = 12 + (16x2) = 44
no of moles = mass / Mr
66/44 = 1.5 mol

Question 11

what is the mass if there in 4 moles of carbon dioxide?

Answer 11

Mr of carbon = 12
you re arrange the formula to get:
mass = Mr x moles
4 x 12 = 48g

Question 12

in a reaction can atoms be lost?

Answer 12

NO ATOMS CAN BE LOST OR CREATED

so the mass of products = mass of reactants

Question 13

balance the equation LI + F2 –> LiF

Answer 13

2LI + F2 –> 2LiF

Question 14

why should equations be balanced?

Answer 14

because the law of conservation of energy states that no matter can be lost or made in a reaction

Question 15

why would the mass be lost in a chemical reaction?

Answer 15

mass cannot be lost in a chemical reaction so you probably didn’t get all the mass as it was lost as a gas

Question 16

what are the two explanations for the change of mass in an unsealed vessel?

Answer 16

1) if the mass increases it is because one of the REACTANTS is a mass (eg when a metal reacts with oxygen)
2) if mass decreases then one of the PRODUCTS is probably a gas

Question 17

in the formula Mg + 2HCL –> MgCl2 + H how many moles are there in all the reactants and products?

Answer 17

there is:
mg=1 mole
2HCL=2 moles
MgCl2=1 mole 
H=1 mole
this is because the big number in front of the reactants/products tells you the number of moles

Question 18

what steps do you need to take to balance an equation using the masses of reactants?

Answer 18

1) divide the mass of each substance by its relative formula mass to find the number of moles
2) divide the number of moles by the smallest number of moles in the reaction
3) if any numbers arnt whole multiply all the numbers by the same amount so they are all whole numbers
4) write the balanced symbol equation for the reaction by putting these numbers in front of the chemical formulas

Question 19

8.1g of zinc oxide (ZnO) reacts completely with 0.60g of carbon to form 2.2g of carbon dioxide and 6.5g of zinc write a balanced symbol equation for this reaction Ar(C)=12 Ar(O)=16 Ar(Zn)=65

Answer 19

1) work out the Mr
ZnO: 65+16=81 C:12 CO2:12+(2x16)=44 Zn:65

2) divide all the mass by its Mr to find out how many moles each substance has
ZnO:8.1/81=0.10mol C:0.60/12=0.050mol
CO2:2.2/44=0.050mol Zn:6.5/65=0.10

3) divide by thr smallest number of moles which is 0.050
ZnO:0.10/0.050=2.0 C:0.050/.050=1.0
CO2:0.050/0.050=1.0 Zn:0.10/0.050=2.0

4) all the numbers are whole so you can now write out the equation
2ZnO + C –> CO2 + 2Zn

Question 20

what is the limiting reactant?

Answer 20

the reactant that is used up in the reaction

Question 21

how do you calculate the mass of a product by using the mass of the limiting reactant and the balanced reaction equation?

Answer 21

1) write out the balanced equation
2) work out the relative formula mass (Mr) of the reactant and product you want
3) find out how many moles there are o the substance you know the mass of
4) use the balanced equation to work out how many moles there will be of the other substance in this case that’s how many moles of product will be made of this many moles of reactant
5) use the number of moles to calculate the mass

Question 22

calculate the mass of aluminum oxide formed when 135g of aluminum is burned in the air

Answer 22

1) write out the balanced symbol equation
4Al + 3C02 –> 2Al2O3

2) calculate the relative formula mass
Al:27 Al2O3:(2x27)+(3x16)=102

3) calculate the number of moles of aluminum in 135g
moles = mass/Mr = 135/27 = 5

4) work out the mole ratio
4 moles of Al react to produce 2 moles of Al2O3 half the number of moles are produced so 5 moles of Al will react to produce 2.5 moles of Al2O3

5) calculate the mass of 2.5 moles of aluminum oxide
mass = moles x Mr = 2.5 x 102 =255g

Question 23

what is the formula for percentage yield?

Answer 23

percentage yield = mass of product actually made/ maximum theoretical mass of product x 100

Question 24

why are yields almost always less than 100%?

Answer 24

1) not all the reactants react
2) there might be side reactions
3) you lose some product when you separate it from a mixture

Question 25

what is the formula for atom economy?

Answer 25

atom economy= Mr useful product / mr of reactants x100

Question 26

calculate the atom economy of: | CH4 + H2O --> CO + 3H2

Answer 26

``` 1) what is the desired product: hydrogen 2) work out the Mr for the reactants and useful product(s): 16 + 18--> 34 3) use the formula: 6/34 x 100 = 17.6% ```

Question 27

what are the drawbacks of low atom economy?

Answer 27

1) use resources quickly 2) more waste to be disposed 3) unsustainable 4) expensive

Question 28

how to use low atom economy?

Answer 28

1) find a use for the waste | 2) recycle waste

Question 29

what is atom economy?

Answer 29

tit tells you the percentage of mass wasted in a reaction

Question 30

what is the formula for concentration?

Answer 30

CONCENTRATION = MOLES / VOLUME

Question 31

a student started with 30 cm3 of sulfuric acid (H2SO4) of unknown concentration in a flask. she found that it took an average of 25 cm3 of 0.100 mol/dm3 sodium hydroxide (NaOH) to neuralise the sulfuric acid. find the concentration of the acid in mol/dm3. the balanced symbolo equation is: 2NaOH+H2SO4-->Na2SO4+2H2O

Answer 31

1) find the number of moles in the unknown substance (moles = conc. X volume) 0.100 mol/dm3 x (25/1000) dm3 = 0.00250 moles of NaOH 2) use the reaction equation to work out how many moles of the unknown stuff you had using the balanced equation you can see that two moles of sodium hydroxide reacts with one mole of sulfuric acid. 20 0.00250 moles of NaOH must have reacted with 0.00250 / 2 = 0.00125 moles of H2SO4 3) work out the concentration of the unknown stuff concerntartion = moles / volume 0.00125 / (30/1000) dm3 =0.0416 mol/dm3

Question 32

what is the concentration in g/m3 of 0.017 m/dm3 of sulfuric acid (H2SO4)

Answer 32

1) work out Mr H2SO4 = (2x1 ) +32 + (4x16) = 98 2) convert the concentration in moles (that we already know) into grams mass in grams = moles X Mr =0.04166 x 98 = 4.08333.. so the concentration in g/dm3 = 4.08 g/dm3

Question 33

what does an acid and a alkeli form?

Answer 33

salt and water

Question 34

25cm3 of HCL of concentration of 3.65mol/dm3 neutralizes 37.5cm3 of NaOH fin the consternation of NaOH?

Answer 34

1) convert the volume into dm3: 25cm3 = 0.025dm3 37.5cm3=0.0375dm3 2) work out moles: HCL has 3.65 x 0.025 = 0.09125 moles and as they are in the same mole ratio then NaOH also has 0.09125 moles 3) use the formula concentration =moles / volume to work out the concentration 0.09125 / 0.0375 = 2.43

Question 35

what does the pH scale do?

Answer 35

tells us how acidic or alkali a substance is using universal indicator

Question 36

REQUIRED PRACTICAL: how to use titrations to work out the concentration of a solution?

Answer 36

1) add a set amount of substance to a conical flask using a pipette and pipette filter. add 3 drops of phenolphthalein 2) use a funnel to fill a burette with some acid of unknown concentration. record the initial volume of the acid 3) using a burette add the acid to the alkali a bit at a time - giving the conical flask a swirl. go slowly near reaching the end point 4) the indicator changes SUDDENLY changes color when all the alkali has been neutralized 5) record the final volume of the acid in the burette and use this to work out the amount needed to neutralise the alkali

Question 37

REQUIRED PRACTICAL: how to increase accuracy in titration experiments?

Answer 37

- you need several consistent readings | - calculate the mean taking out any anomalous results

Question 38

what is the formula for moles of gas?

Answer 38

MOLES = VOLUME / 24

Question 39

what is the rule about how much space a gas will occupy?

Answer 39

one mole of any gas will occupy the same volume under the same temperature at 20 degrees one mole of gas will occupy 24dm3

Question 40

what is the volume of 2 moles of CO2?

Answer 40

2 x 24 = 48dm3

Question 41

how many molecules of gas are present in 24dm3 of CO2?

Answer 41

24 / 24 = 1 | in 1 mole there are 6.02 x 10 to the power of 23 molecules

Question 42

a car has an air bag that is inflated by 70g of nitrogen N2 when activated. what volume would the N2 occupy at room temperature and pressure?

Answer 42

volume 60dm3 moles 2.5 mass 70 Mr 28

Question 43

in a balanced equation if the RFM of one side is 30 what is the RFm of the other side?

Answer 43

30 as mass is conserved in a reaction