Formulae and Equations

Question 1

what is the molecular formula

Answer 1

formula that shows the number and type of each atom in a molecule

Question 1

what is empirical formula

Answer 1

simplest whole number ratio of atoms of each element present in one molecule or formula unit of a compound

Question 2

how to work out molecular formula

Answer 2

calculate empirical mass
divide relative formula mass by empirical mass
Multiply each number of elements by this number

Question 3

water of crystallisation

Answer 3

when some compounds can form crystals which have water as part of their structure this is called a hydrated compound.

Question 4

PV = nRT

Answer 4

p = (Pa)
V = (m3)
n = (mol)
R = 8.31 Jmol-1K-1
T = (K)

Question 5

what is a precipitation reaction

Answer 5

reaction of two aqueous solutions to form products which include one solid

Question 6

what is avogadros constant

Answer 6

number of particles equivalent to the relative atomic mass or molecular mass of a substance

Question 7

moles from PV = nRT

Answer 7

n = PV/RT

Question 8

how to work out number of moles with molar gas volume

Answer 8

volume / molar gas volume

Question 9

molar gas volume in room and standard temp

Answer 9

at room temp it is 24.0
at standard it is 22.4

Question 10

what is volumetric analysis

Answer 10

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

Question 11

how can you measure volumes

Answer 11

volumetric or graduated pipette and a burette

Question 12

how can you make a standard solution

Answer 12

Weigh the sample bottle containing the solid on a (2 dp) balance.

Transfer solid to beaker and reweigh sample bottle.
Record the difference in mass.

Add distilled water and stir with a glass rod until all the solid has dissolved.

Transfer to a volumetric flask with washings.

Make up to the 250cm° mark with distilled water.

Shake flask.

Question 13

what are common errors when making volumetric solution

Answer 13

Common errors in this method include systematic errors on the balance, lost substance in transfer processes and overfilling of the volumetric flask. These can be reduced using washing methods and by reading volumes from the bottom of the meniscus.

Question 14

how to preform the titration

Answer 14

Measuring a known volume (usually 20 or 25 cm3) of one of the solutions with a volumetric pipette and placing it into a conical flask

The other solution is placed in the burette
To start with, the burette will usually be filled to 0.00 cm3

A few drops of the indicator are added to the solution in the conical flask

The tap on the burette is carefully opened and the solution added, portion by portion, to the conical flask until the indicator starts to change colour

As you start getting near to the end point, the flow of the burette should be slowed right down so that the solution is added dropwise

You should be able to close the tap on the burette after one drop has caused the colour change

Multiple runs are carried out until concordant results are obtained

Concordant results are within 0.1 cm3 of each other

Question 15

uncertainty in burettes

Answer 15

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

Question 16

what is the concentration of a solution

Answer 16

amount of solute dissolved in a solvent to make 1 dm3 of solution

Question 17

what is a dilute concentration

Answer 17

solution with a low concentration of solute

Question 18

what can affect a random error

Answer 18

How easily the instrument or scale is to read
The person reading the scale poorly
Changes in the environment

Question 19

what is a systematic error

Answer 19

errors that occur as a result of a faulty or poorly designed experimental procedure

Question 20

examples of systematic error

Answer 20

forget to zero an electronic balance (using the tare button) the mass weighings will always be higher than they should be

If you don’t read the volume in a burette at eye level, the volumes will always be smaller than they should be due to a parallax error

Question 21

formula for percentage uncertainty

Answer 21

absolute uncertainty / measured value x 100

Question 22

what happens when you multiply or divide experimental measurement

Answer 22

you add together percentage uncertainties and then calculate absolute uncertainty from sum of percentage uncertainties

Question 23

what happens when you add or subtract two measurements

Answer 23

you add together the absolute measurement uncertainties

Question 24

in reactions why do not all reactants react to form products

Answer 24

Other reactions take place simultaneously
The reaction does not go to completion
Products are lost during separation and purification

Question 25

how can you work out percentage yield

Answer 25

actual yield / theoretical yield x 100

Question 26

how can you work out actual yield and theoretical yield

Answer 26

number of moles or mass of product obtained experimentally
number of moles or mass obtained by a reacting mass calculation

Question 26

how can you work out actual yield and theoretical yield

Answer 26

number of moles or mass of product obtained experimentally
number of moles or mass obtained by a reacting mass calculation

Question 27

what is atom economy

Answer 27

how many of the atoms used in the reaction become the desired product

Question 28

atom economy formula

Answer 28

molecular mass of desired / sum of molecular mass of all reactants x 100

Question 29

what is meant by concordant results

Answer 29

titration results that are within 0.2 (cm3) of each other

Question 30

what is meant by molar volume of a gas

Answer 30

volume space occupied by one mole of a gas at a specified temperature and pressure/rtp/st/standard conditions