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AS Chemistry > Amount of substance > Flashcards

Amount of substance Flashcards

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1
Q

What is Avogadro’s Constant?

A

There are 6.022 x 10^23 atoms in 12g of carbon-12

- therefore 1 mole of any specified entity is 6.022 x 10^23 of that entity

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2
Q

What is the mole?

A

amount of substance in g that has the same no. of particles as there are atoms in 12g of Carbon-12

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3
Q

What is relative atomic mass (Ar) ?

A
  • the average mass of 1 atom compared to 1/12 of the mass of 1 atom of Carbon-12
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4
Q

What is relative molecular formula (Mr) ?

A
  • the average mass of 1 molecule compared to 1/12 of the mass of 1 atom of Carbon-12
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5
Q

Formula of moles for pure solids and gases

A

Moles (mol) = Mass (g) /Mr

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6
Q

Formula of moles for gases

A 
PV=nRT
P pressure in Pa
V volume in m^3
n moles
R gas contant (given)
T temp in Kelvin
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7
Q

Formula of moles for solutions

A

Conc (mol dm^-3 or M) = moles/ vol (dm^3)

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8
Q

Conversion for centimetre cubed to decimetre cubed

A

Divide by 1000

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9
Q

Convert Celcius to Kelvin

A

+ 273

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10
Q

What number of sig fig do you answer with?

A
  • use the smallest given in Q
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11
Q

No. of particles formula?

A

No. of particles = Moles x Avogadro’s constant

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12
Q

Density formula?

A

Mass/ Vol
- mass in g
- vol in cm3
units for density g cm^-3

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13
Q

What’s an empirical formula?

A
  • simplest ratio of atoms of each element in the compound
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14
Q

General method for calculating empirical formula

A

1 divide each mass (or % mass) by their atomic mass
2 for each of the answers divide them by the smallest one of those numbers
3 sometimes numbers calculated need to be multiplied to be whole numbers

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15
Q

What’s a molecular formula (from empirical formula)?

A
  • the actual number of atoms of each element in the compound
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16
Q

How do you find Mr from mass spectrometer?

A
  • the molecular ion (peak with highest m/z) will be equal to Mr
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17
Q

What’s the percentage yield?

A

actual yield / theoretical yield X 100

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18
Q

What’s percentage atom economy?

A

Mr of desired products / sum of Mr of all products X 100

19
Q

Reason for low atom economies?

A
  • inefficient, wasteful processes - atoms wasted & not made into useful products
20
Q

why are high atom economies good?

A
  • it’s an efficient process and is important for SUSTAINABLE DEVELOPMENT as they have fewer natural resources & create less waste
  • also less unwanted products made
21
Q

1 mole of any gas at room temp & room pressure will have what volume?

A
  • volume of 24 decimetre cubed
22
Q

State room pressure and temp

A

25 degrees Celsius

1 atm

23
Q

Method : Water of crystallization

A

1 weigh empty dry crucible & lid
2 Add 2g of hydrated calcium sulphate to the crucible & weigh again
3 Heat strongly with bunsen burner for a couple of mins & allow to cool
4 weigh crucible and contents again
5 heat crucible again & reweigh until you reach a constant mass (ensures reaction is complete)

24
Q

Why shouldn’t large masses e.g. 50 g of hydrated calcium sulphate be used?

A
  • decomposition is likely to be incomplete
25
Q

Why does crucible need to dry in the water of crystallization experiment?

A
  • wet crucible would give an inaccurate result

- it would cause mass loss to be too large as water would be lost when heating

26
Q

Why is the lid kept on the crucible?

A
  • improves accuracy as prevents loss of solid from crucible

- BUT should be loose fitting to allow gas to escape

27
Q

Why shouldn’t a small amount of solid be used in water of crystallization experiment?

A
  • errors in weighing will be too high
28
Q

What care should you take when drawing a gas syringe diagram?

A
  • make sure you don’t leave gaps in diagram where gas could escape
  • when drawing gas syringe draw some measurement marking on barrel
29
Q

Potential errors in using a gas syringe?

A
  • gas escapes before bung is inserted
  • syringe sticks
  • some gases like carbon or sulfur dioxide are soluble in water so the true amount of gas isn’t measured
30
Q

What’s a standard solution?

A
  • a solution for which the concentration is accurately known
31
Q

Describe how to make up a volumetric solution

A

TRANSFER KNOWN MASS OF SOLID

  • weigh sample bottle containing solids on a (2 dp) balance
  • transfer to beaker & reweigh sample bottle
  • record difference in mass

DISSOLVES IN WATER

  • add distilled/deionised water
  • stir or swirl with GLASS ROD
  • until all solid has dissolved

TRANSFER, WASHING & AGITATION

  • transfer to VOLUMETRIC flask
  • with washings
  • make up to 250cm^3 mark with water
  • shake for uniform concentration
32
Q

Why is using a volumetric pipette more accurate than a measuring cylinder?

A
  • because it has a smaller uncertainty
33
Q

Why shouldn’t you use heat or put hot solutions in a volumetric flask?

A

because heat would cause flask to expand and give incorrect volume

34
Q

What’s uncertainty?

A
  • estimate attached to a measurement that gives the range of values within which the true lies
35
Q

Percentage error calculation

A

uncertainty / quantity measured x 100

36
Q

percentage uncertainty calculation

A

2 x uncertainty in each measurement/quantity measured X 100

37
Q

What makes a balance more accurate?

A
  • the more decimal places a balance reads, the SMALLER the percentage error
38
Q

When weighing out a smaller mass, the percentage error is what?

A
  • more significant
39
Q

How to reduce percentage error in water of crystallisation?

A
  • record mass to 2 dp

- larger mass of hydrated crystals

40
Q

Why would using a larger mass of solid in making up a standard solution be more accurate?

A
  • a larger mass of solid dissolved gives a smaller weighing error so it will be a more accurate solution
41
Q

Titration method

A

1 rinse burette with solution you’re going to fill it with - discard rinsings & fill burette
2 rinse pipette with solution you’re going to pipette into conical flask. Using pipette & pipette filler transfer 25.0cm^3 of this solution into a conical flask
3 Add 2-3 drops of a suitable indicator to the conical flask (use white tile underneath to observe colour change)
4 Add the solution from burette with constant SWIRLING until the indicator just changes colour
5 Repeat to achieve 2-3 concordant results, adding the solution in DROPWISE near the endpoint
6 calculate mean titre from the concordant results

42
Q

What are concordant titre results?

A
  • those that agree to within 0.1cm^3
43
Q

Theoretical reasons for why percentage yield may be less than 100%

A
  • side reactions occur, by-products produced instead of expected products
  • reagents used may be impure
  • reaction is incomplete
44
Q

Practical reasons for why percentage yield may be less than 100%

A
  • some lost in the purification steps e.g. washing, separating in the separating funnel & in transfer between apparatus
  • some product is lost in distillation

AS Chemistry (4 decks)

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