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GCSE Chemistry > Topic 3: Quantitive chemistry > Flashcards

Topic 3: Quantitive chemistry Flashcards

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1
Q

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

A

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

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2
Q

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

A

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

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3
Q

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

A

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

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4
Q

find the percentage mass of sodium in sodium carbonate Na2CO3?

A 
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
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5
Q

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

A

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

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6
Q

what is a mole?

A

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

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7
Q

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

A

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

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8
Q

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

A

one mole of N2 weighs exactly 28g

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9
Q

what is the formula to calculate the number of moles?

A

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

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10
Q

how many moles are there in 66g of CO2?

A

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

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11
Q

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

A

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

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12
Q

in a reaction can atoms be lost?

A

NO ATOMS CAN BE LOST OR CREATED

so the mass of products = mass of reactants

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13
Q

balance the equation LI + F2 –> LiF

A

2LI + F2 –> 2LiF

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14
Q

why should equations be balanced?

A

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

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15
Q

why would the mass be lost in a chemical reaction?

A

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

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16
Q

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

A

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

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17
Q

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

A 
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
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18
Q

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

A

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

19
Q

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

A

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

20
Q

what is the limiting reactant?

A

the reactant that is used up in the reaction

21
Q

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

A

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

22
Q

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

A

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

23
Q

what is the formula for percentage yield?

A

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

24
Q

why are yields almost always less than 100%?

A

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

25
Q

what is the formula for atom economy?

A

atom economy= Mr useful product / mr of reactants x100

26
Q

calculate the atom economy of:

CH4 + H2O –> CO + 3H2

A 
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%
27
Q

what are the drawbacks of low atom economy?

A

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

28
Q

how to use low atom economy?

A

1) find a use for the waste

2) recycle waste

29
Q

what is atom economy?

A

tit tells you the percentage of mass wasted in a reaction

30
Q

what is the formula for concentration?

A

CONCENTRATION = MOLES / VOLUME

31
Q

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

A

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

32
Q

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

A

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

33
Q

what does an acid and a alkeli form?

A

salt and water

34
Q

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

A

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

35
Q

what does the pH scale do?

A

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

36
Q

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

A

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

37
Q

REQUIRED PRACTICAL: how to increase accuracy in titration experiments?

A
  • you need several consistent readings

- calculate the mean taking out any anomalous results

38
Q

what is the formula for moles of gas?

A

MOLES = VOLUME / 24

39
Q

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

A

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

40
Q

what is the volume of 2 moles of CO2?

A

2 x 24 = 48dm3

41
Q

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

A

24 / 24 = 1

in 1 mole there are 6.02 x 10 to the power of 23 molecules

42
Q

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?

A

volume 60dm3
moles 2.5
mass 70
Mr 28

43
Q

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

A

30 as mass is conserved in a reaction

GCSE Chemistry (5 decks)

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