AOS

Question 1

Explain how ions are detected and relative abundance measured (2)

Answer 1

• ions hit detector / negative plate & and gain an electron
• relative abundance proportional to size of current

Question 2

what would improve chances of getting concordant results?

Answer 2

invert volumetric flask several times after adding distilled water

Question 3

suggestion to reduce overall measurement uncertainty in titration

Answer 3

use less concentrated/larger volume alkali in burette

Question 4

carbonates and monoprotic acids always react in a

Answer 4

1:2 ratio

Question 5

Method to calculate value of water of crystallisation x in MgSO4.xH2O; (5)

Answer 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

Question 6

what is relative atomic mass

Answer 6

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

Question 7

what is relative molecular mass

Answer 7

the average mass of a molecule relative to one twelfth of the mass of an atom of carbon-12

Question 8

History of the atom:

Answer 8

• 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

Question 9

isotopes are:

Answer 9

• atoms of the same element w diff no. of neutrons and diff masses
• all react the same way because same electron configuration

Question 10

when an acid and a base react, the reaction ratio is determined by

Answer 10

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

Question 11

how to calculate atom economy

Answer 11

Mr of desired / Mr of all products x 100
* the big numbers count

Question 12

formula for % uncertainty of mass

Answer 12

uncertainty / mass added x 100

Question 13

importance of % yield

Answer 13

important in getting as much desired product as possible in the reaction

Question 14

importance of atom economy

Answer 14

to ensure production of by-products is minimised

Question 15

reactivity

all isostopes have:

Answer 15

the same reactivity
* all have same electron configuration

Question 16

Some antacid tablets contain carbonate ions. Suggest why this may be a disadvantage when used as indigestion tablets.

Answer 16

They produce CO2 gas, which may cause ‘wind’

Question 17

Hydrogen bonding must:

Answer 17

• be between molecules
• have a 180 bond angle between the two molecules
• show all lone pairs and partial charges

Question 18

Describe a method to add a known mass of solid to a beaker to make a solution (2)

Answer 18

• 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.)

Question 19

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

Answer 19

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.

Question 20

Explain why C-Br is a polar bond but CBr4 is not a polar molecule

Answer 20

• the molecule is completely symmetrical (tetrahedral)
• the dipoles cancel out

Question 21

What is empirical formula

Answer 21

simplest ratio of atoms of each element in a compound

Question 22

Make a standard solution using acid: (6)

Answer 22

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