as chemistry amount of substance

Home

Get App

Create

how can we find out the formula of a substance such as magnesium oxide through an experiment
- find the mass of a crucible and its lid
- place magnesium in the crucible and reweigh the crucible with the lid
- heat the crucible and when the magnesium starts to glow place the lid over the crucible
- keep lifting the lid to allow air to replace
- continue until there is significantly more glowing
- allow to cool and reweigh
- work out the mass of magnesium by doing the following calculation : mass of magnesium + crucible + lid - mass of crucible + lid
- work out the mass of oxygen by doing the following calculation : mass of magnesium + oxygen + crucible + lid - mass of magnesium + lid + crucible
- calculate the formula from the masses
we may have a formula that is different to the actual formula of magnesium oxide this may be because
- in our experiment there was more magnesium than oxygen this may be because : when the lid was lifted magnesium oxide escaped in the form of smoke
- in our experiment there was more oxygen than magnesium this may be because there was a weighing error or perhaps the magnesium was oxidised before the experiment
empirical formula

formula showing the simplest ratio of atoms of each element present in a compound
molecular formula

formula showing the actual numbers of atoms of each element present in one molecule e
examples of compounds with different molecular and empirical formulas
- ethane
- molecular formula C2H6
- empirical formula CH3
examples of compounds with the same empirical and molecular formula

water - H20
how to calculate the empirical formula
- find the masses of each of these elements present in a compound (by experiment)
- work out the number of moles of atoms of each element use n = m/Mr
- convert the number of moles of each element into a whole number ratio
how to calculate molecular formula
- work out empirical formula
- work out molar mass of empirical formula
- find the ratio of empirical formula mass to Mr in terms of 1:y
- x the number of atoms of each element of in the compound by y
we want 250cm^3 of sodium chloride solution which has a concentration of 0.1 moldm^3 calculate the mass of sodium chloride to be weighed
- number of moles = mass/Mr
- number of moles = concentration x volume
- number of moles = 0.1 x 250 x 10^-6 = 0.025
- 0.025 = x/58.5
- 0.025 x 58.5 = x = 1.46g
concentration (mol dm^-3) =
- moles (mol)
- --------------
- volume (dm^3)
1dm^3 =

1000 cm^3 so when converting from cm^3 to dm^3 divide bt 1000
there are actually two different units for concentration
- g dm^-3
- mol dm^-3
if given the concentration in g dm^-3 and mol dm^-3
- use moles = mass/Mr to find mass
- and then concentration/mass
if given the concentration in mol dm^-3 and want g dm^-3
- use moles = mass/Mr to find mass
- and then mass x concentration
it is possible to calculate the limits of accuracy of different measuring instruments in an experiment but you cannot calculate the exact effect of

errors which are not quantifiable
you can measure the volume of a liquid with a

measuring cylinder , pipette or burette
....... are least accurate

measuring cylinders
percentage error =
- 1/2 garduation (error)
- --------------------------- x 100
- reading
measuring cylinder error

+ or - 0.5 cm^-3
pipette error

+ or - 0.05cm^3
burette error
- + or - 0.05 cm^3 x 2 because you take readings at the start and the end so total error = + or -0.10 cm^3
- if you are using a burette to do a titration there may be another error of one or two drops which is due to your judgement of when the indicator changes colour . This means that in a titration you may have an error of + or - 0.2 cm^3
thermometer error

+ or - 0.5 degrees centigrade

Author

ghoran

243135

Card Set

as chemistry amount of substance

Description

revision

Updated

2013-10-27T18:39:27Z

Show Answers