AOS Flashcards
Explain how ions are detected and relative abundance measured (2)
- ions hit detector / negative plate & and gain an electron
- relative abundance proportional to size of current
what would improve chances of getting concordant results?
invert volumetric flask several times after adding distilled water
suggestion to reduce overall measurement uncertainty in titration
use less concentrated/larger volume alkali in burette
carbonates and monoprotic acids always react in a
1:2 ratio
Method to calculate value of water of crystallisation x in MgSO4.xH2O; (5)
- use a 2 d.p balance to weigh an empty boiling tube
- add apporx 2g of hydrated salt and weigh boling tube again. record.
- heat test tube evenly using bunsen
- after every 30 sec of heating reweigh boiling tube and record
- continue heating until mass stays constant
what is relative atomic mass
the average mass of an atom of an element relative to one twelfth of the mass of an atom of carbon-12
what is relative molecular mass
the average mass of a molecule relative to one twelfth of the mass of an atom of carbon-12
History of the atom:
- dalton: thought all matter made of indivisible spheres
- thompson: plum pudding model, discovered electron
- rutherford: gold foil, mass of nucleus, +vely charged
- bohr: electrons orbit at fixed distances
- chadwick: neutrons
isotopes are:
- atoms of the same element w diff no. of neutrons and diff masses
- all react the same way because same electron configuration
when an acid and a base react, the reaction ratio is determined by
how many H+ ions disassociate from the acid. eg if its HCl + NaOH then since there is 1 H+ from the acid, the ratio is 1:1
how to calculate atom economy
Mr of desired / Mr of all products x 100
* the big numbers count
formula for % uncertainty of mass
uncertainty / mass added x 100
importance of % yield
important in getting as much desired product as possible in the reaction
importance of atom economy
to ensure production of by-products is minimised
reactivity
all isostopes have:
the same reactivity
* all have same electron configuration
Some antacid tablets contain carbonate ions. Suggest why this may be a disadvantage when used as indigestion tablets.
They produce CO2 gas, which may cause ‘wind’
Hydrogen bonding must:
- be between molecules
- have a 180 bond angle between the two molecules
- show all lone pairs and partial charges
Describe a method to add a known mass of solid to a beaker to make a solution (2)
- measure the mass of the weighing boat and solid
- Add the solid to a beaker and then reweigh the weighing boat
(and subtract to find the mass of solid added.)
The student uses this method to complete a titration.
* Rinse a burette with distilled water.
* Fill the burette with sodium hydroxide solution.
* Use a measuring cylinder to transfer 25 cm3 of the citric acid solution into a conical flask.
* Add 5 cm3 of indicator.
* Slowly add the sodium hydroxide solution from the burette into the conical flask.
* Add the sodium hydroxide solution dropwise near the end point until the indicator just changes colour.
* Repeat the titration to get concordant results.
The method used by the student includes three practical steps that will lead to an inaccurate final result.
For each of these three steps
* identify the mistake
* explain why it is a mistake
* suggest how the mistake can be overcome.
(6)
QoL
Stage 1
a. Problem – using a measuring cylinder
b. Explanation – large uncertainty / not accurate enough
c. Improvement – use a (volumetric) pipette (Not dropping pipette)
Stage 2
a. Problem – too much indicator
b. Explanation – may react and affect the endpoint reading
c. Improvement – use a smaller volume (2-6 drops)
Stage 3
a. Problem – rinsing the burette with distilled or deionised water
b. Explanation – will slightly dilute the alkali solution
c. Improvement – rinse the burette with alkali solution
Stage 4
a. Problem – adding alkali solution until the indicator “just” changes colour
b. Explanation – acid may not have fully reacted (as mixture not swirled)
c. Improvement – add alkali solution until a permanent colour change is seen.
Explain why C-Br is a polar bond but CBr4 is not a polar molecule
- the molecule is completely symmetrical (tetrahedral)
- the dipoles cancel out
What is empirical formula
simplest ratio of atoms of each element in a compound
Make a standard solution using acid: (6)
M1 weigh out sample in bottle
M2 transfer to (conical) flask / beaker and wash all sample in
M3 dissolve sample in (deionised / distilled ) water
M4 add into volumetric flask with washings
M5 make up to mark / 250 cm3 in volumetric flask
M6 shake / invert
