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Chemistry - AS Level > Amount Of Substance > Flashcards

Amount Of Substance Flashcards

1
Q

what is relative atomic mass

A

relative atomic mass is the average mass of an atom of an element compared to one twelfth the mass of an atom of carbon-12

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2
Q

formula

A

average mass of an atom of an element/1/12 mass of one atom of carbon-12

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3
Q

what is the molecular mass

A

swap atom for molecule

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4
Q

why should ionic compounds not be referred to as molecules

A

they have a giant lattice structure

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5
Q

find the mr of H2SO4

A

-hydrogen = 1 so H2 = 2
-sulphur = 32.1
-oxygen (4x16)= 64
add together = 98.1

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6
Q

what is the standard unit of measurement for chemists

A

the mole

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7
Q

what is the mole

A

6.022x10^23

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8
Q

what is the mole also referred to

A

avogadro constant

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9
Q

why can we not use volume for standard SI

A

equal masses of substances are unlikely to contain the same number of particles

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10
Q

what is one mole equivalent to

A

the ar or mr in grams

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11
Q

formula for no. of particles

A

moles x 6.022x10^23

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12
Q

formula for moles

A

no. particles/ 6.022x10^23

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13
Q

formula for moles using mass

A

mass/MR

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14
Q

formula for mass

A

moles x MR

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15
Q

what must every mass be in

A

grams

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16
Q

how to convert

A

leave in x10 form
mg = 10^-3
microgram = 10^-6
1 tonne = x10^6

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17
Q

formula for density

A

mass= density x volume

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18
Q

monatomic

A

exist as one, cannot form a compund

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19
Q

formula for nitric acid

A

HNO3

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20
Q

phosphoric acid

A

H3PO4

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21
Q

where to look for ions

A

group number

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22
Q

silver

A

Ag^+

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23
Q

zinc

A

Zn2+

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24
Q

carbonate

A

CO3^2-

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25
Q

how to form formula

A

cross over rule

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26
Q

why do some form positive ions

A

gain a H^+ ion which is a proton

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27
Q

What do metals form when mixed

A

they are alloys which is a mixture

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28
Q

how many moles are in 72.0g of Mg

A

mass/MR= 72.0/24.3

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29
Q

calculate moles of an ion in a compound

A

take total moles of compound and times by number of atoms

30
Q

what is the empirical formulae

A

the simplest whole number ratio of atoms of each element present in each compound

31
Q

what is true in ionic compounds

A

it is always the empirical formula

32
Q

what is the molecular formula

A

the true number of atoms of each element in each molecule

33
Q

how to work out empirical formulae

A

-moles =mass/mr
-the find closet ratio
-specific ratio corresponds to the formula

34
Q

how do you find the molecular formula from empirical

A

find original mr and see if it matches
then divide mr of molecular by mr of empirical

35
Q

what is a hydrated salt

A

Water of crystallisation is when some compounds can form crystals which have water as part of their structure

36
Q

what is a anhydrous salt

A

when water is driven off via heating and leaves a solid compound and one that does not contain water of crystallisation

37
Q

what do you do to find the formula of a hydrated salt

A

weigh the crucible
weigh out 2g of hydrated magnesium sulfate or any form of sulfate into the crucible and weigh
heat without lid gently using a bunsen
allow to cool and re-weigh
heat again for 2 min and re-weigh
repeat this until results are at a constant mass

38
Q

what results should you collect- hydrated salt

A

-exact mass of hydrated salt- mass of c and mgso4 - empty crucible
-exact mass of anhydrous-
-exact mass of water- first heating minus 2nd heating

39
Q

what is a formula for hydrated salt -magnesium

A

MgSO4*5H2O ⇌ MgSO4 + 5H2O

40
Q

how do you calculate percentage error

A

uncertainty / measurement(g) x100

41
Q

what are some sources of error

A

-convection current
-spitting - these will make the mass appear greater than expected
-not fully reacted-some water remains - x is less than expected
-anhydrous could decompose- eg sulfer dioxide produced which could decompose- x is greater than
-busen flame- incomplete combustion-soot forms - x is less tan expected

42
Q

how would you improve these factors

A

-make uncertainty smaller
-make measurement larger

43
Q

give some improvements to the experiment

A

-use a lid to prevent spitting
-heat to a constant mass

44
Q

to calculate number of moles of water which combine with one mole of anhydrous salt

A

mass of moles (water) =mass/mr
moles of salt = mass/mr
divide by smallest number
place in ratio
water number is number of moles of water eg number in front of H20 and behind the dot

45
Q

how to calculate percentage error using given value

A

look at how many calcs have been used to get mass and times the given uncertainty in question by this number
eg 0.005g uncertainty
find hydrated salt - 2 calcs before - e,pty crucible and crucible and hydrated salt
so times 0.005g by 2 and divide by mass
= 0.01/2.05= 0.49%
very small error

46
Q

what do you do in a reacting mass calc

A

-find moles of the thing you know mass of
-then look at ratio - if 2 : 1 then divide moles by 2
-then find mass via moles x mr

47
Q

what do you do if given two reactants

A

work out which one is in excess and which one is the limiting reagent

48
Q

how would you find the limiting reagent

A

-find moles of both
-then smaller one is the limiting
-larger is in excess
-you would then use the limiting reactant for the calc

49
Q

how to find LR and IE if one is a 2 or 3

A

you would x the one that is singular by value of other one
-then see if it is larger
-if not then the smaller is limiting
-if it is then multiple is the limiting

50
Q

example

A

Step 1: The symbol equation is:

2Mg (s) + O2 (g) → 2MgO (s)

Step 2: The relative formula masses are:

Magnesium : 24 Oxygen : 32 Magnesium Oxide : 40

Step 3: Calculate the moles of magnesium used in reaction

Step 4: Find the ratio of magnesium to magnesium oxide using the balanced chemical equation
Therefore, 0.25 mol of MgO is formed

Step 5: Find the mass of magnesium oxide

mass = mol x Mr

mass = 0.25 mol x 40 g mol-1

mass = 10 g

Therefore, the mass of magnesium oxide produced is 10 g

51
Q

for reacting mass of volume and conc

A

moles=concxvol
moles=vol(dm3)/24 - if in g then 24000

52
Q

what makes up a solution

A

a solute(solid) and solvent(liquid)

53
Q

how to calculate conc from an equation

A

-calculate moles by mass/mr
-then use equation to see ratio for unknown
-the use conc= mol/vol

54
Q

what is the definition of titration

A

Volumetric analysis is a process that uses the volume and concentration of one chemical reactant (a standard solution) to determine the concentration of another unknown solution

55
Q

what equipment is used

A

a volumetric or graduated pipette and a burette

56
Q

what is the reaction that occurs

A

neutralisation

57
Q

why is the precise equipment used in titration

A

Burettes are usually marked to a precision of 0.10 cm3
Since they are analogue instruments, the uncertainty is recorded to half the smallest marking, in other words to ±0.05 cm3
The stoichiometric point or equivalence point occurs when the two solutions have reacted completely and is shown with the use of an indicator

58
Q

what is the method that is used

A

The steps in a titration are:
Measuring a known volume (usually 20 or 25 cm3) of one of the solutions with a volumetric or graduated pipette and placing it into a conical flask
The other solution is placed in the burette
A few drops of the indicator are added
The tap on the burette is carefully opened and the solution added, portion by portion, to the conical flask until the indicator just changes colour(this is the end point)
Multiple trials are carried out until concordant results are obtained

59
Q

why is acid used to wash the burette

A

to make sure it is clean so conc/moles arent affected

60
Q

why should u not keep the funnel in

A

it can cause the measurement to be less than it should be due to drips

61
Q

why should u make sure there is no air bubbles

A

can cause the titre to be more than it actually was

62
Q

titration calc example

A

Example 1: Calculations from titration results

In a titration, 25.00 cm3 of 0.05 mol dm-3 hydrochloric acid was neutralised by 8.50 cm3 of sodium hydroxide solution. Calculate the concentration of the sodium hydroxide solution.

Answer

Step 1: Find the number of moles of acid

moles of acid = concentration x volume in dm3

moles of acid = 0.05 x 25/1000 = 1.25 x 10-3 mol

Step 2: Deduce the number of moles of alkali

The equation for the reaction shows the mole ratio is 1:1

HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)

∴ moles of alkali = 1.25 x 10-3 mol

Step 3: Work out the concentration of the alkali

concentration = moles/volume in dm3

concentration = 1.25 x 10-3/0.0085 = 0.15 mol dm-3

63
Q

Titration calc

A

find moles of things that you know off

64
Q

How to find percentage yield

A

Actual/theoretical x 100

65
Q

Method for preparing solution from mass of a solid

A

Weigh out accurate mass if solid in weighing boat and dissolve in a suitable vol of deionised water in a beaker
Stir with glass rod and rinse the weighing boat into beaker with deionised water
Rinse glass rod into beaker
Plans glass funnel into top of volumetric flask and pour prepared solution down glass rod into beaker
Rinse glass rod again
Rinse funnel
Remove funnel and add deionised water to the volumetric flask until just below line
Add deionised water using pipette up to the line so meniscus is on
Place stopper in and invert 10x

66
Q

What is a back titration

A

Reacting insoluble solid with known excess of HCl
Then titrate excess’s did with alkali eg NaOH
Colourless to pink

67
Q

Gas calc

A

No of moles = gas vol / 24 dm3

68
Q

What is the ideal gas equation

A

pV=nRT
R is always 8.31 kJ
P=Pa
V = m3
N = no of moles
T= kelvin

69
Q

Converting from degrees to kelvin

A

+273

70
Q

Convert from cm3 to m3

A

Divide by 1000000

Chemistry - AS Level (15 decks)

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