Amount of substance Flashcards
define relative atomic mass
the average mass of one atom compared to 1/12th of the mass of one atom of carbon-12
define relative molecular mass
average mass of a molecule compared to 1/12th of the mass of one atom of carbon-12
unit for volume in ideal gas equation
m cubed
unit for volume in concentration?
dm cubed
how do i convert K to degreed celsius
+ 273
converting between units
cm cubed - dm cubed ( divide by 1000)
cm cubed - m cubed ( divide by 100,000)
dm cubed - m cubed (divide by 1000)
how do you calculate number of particles / atoms?
moles x avagrdos constant
define empirical formula
the simplest whole number ratio of atoms of each element in a compound
define molecular formula
actual number of atoms of each element in the compound
method for water of crystallisation
The water of crystallisation in calcium sulfate crystals can be
removed as water vapour by heating as shown in the following
equation.
CaSO4
.xH2O(s) → CaSO4
(s) + xH2O(g)
Method. *Weigh an empty clean dry crucible and lid . *Add 2g of hydrated calcium sulfate to the crucible and weigh
again
*Heat strongly with a Bunsen for a couple of minutes *Allow to cool *Weigh the crucible and contents again
*Heat crucible again and reweigh until you reach a constant mass (
do this to ensure reaction is complete).
why is using a lid for the crucible important?
lid improves the accuracy of the
experiment as it prevents loss of solid
from the crucible but should be loose
fitting to allow gas to escape
why should you not use large amounts of salt in the experiment?
Large amounts of hydrated calcium sulfate, such as 50g,
should not be used in this experiment as the decomposition is
likely to be incomplete.
why cant you use a wet crucible?
The crucible needs to be dry otherwise a wet crucible would
give an inaccurate result. It would cause mass loss to be too
large as the water would be lost when heating
why cant you use very small amounts of solid in the experiment
Small amounts of the solid, such as
0.100 g, should not be used in this
experiment as the percentage
uncertainties in weighing will be too
high.
how to make a standard solution?
- Weigh the sample bottle containing the required mass of solid
on a 2 dp balance - Transfer to beaker and reweigh sample bottle
- Record the difference in mass
- Add 100cm3 of distilled water to the beaker. Use a glass rod to
stir to help dissolve the solid.
*Sometimes the substance may not dissolve well in cold water so
the beaker and its contents could be heated gently until all the
solid had dissolved. - Pour solution into a 250cm3 graduated flask via a funnel. * Rinse beaker and funnel and add washings from the beaker
and glass rod to the volumetric flask. * Make up to the mark with distilled water using a dropping
pipette for last few drops. * Invert flask several times to ensure uniform s
why is a volumetric pipette more accurate than a beaker
Using a volumetric pipette is more accurate
than a measuring cylinder because it has a
smaller uncertainty
hazards and safety risks with making the solution
Irritant - dilute acid and alkalis- wear googles
Corrosive- stronger acids and alkalis wear goggles
Flammable – keep away from naked flames
Toxic – wear gloves- avoid skin contact- wash hands after use
Oxidising- Keep away from flammable / easily oxidised materials
units of ideal gas equation
Unit of pressure (P):Pa
Unit of volume (V): m3 Unit of temp (T): K
n= moles
R = 8.31 JK–1mol–1
errors of using a gas syringe
*gas escapes before bung inserted
*syringe sticks
* some gases like carbon dioxide or sulfur dioxide are
soluble in water so the true amount of gas is not
measured.
equation for % yield
actual/theoretical x 100
equation for % atom economy
mass of useful products / mass of all reactants x 100
why do chemists want high % yield?
Chemists want a high
percentage yield as means there
has been an efficient conversion
of reactants to products
why do chemists want high % atom economy?
Chemists want a high
percentage atom economy so
that the maximum mass of
reactants ends up in the desired
product (so minimising the
amount of by-product).
How do you carry out a titration?
*rinse equipment (burette with acid, pipette with alkali, conical flask
with distilled water)
*pipette 25 cm3 of alkali into conical flask
*touch surface of alkali with pipette ( to ensure correct amount is
added)
*adds acid solution from burette
*make sure the jet space in the burette is filled with acid
*add a few drops of indicator and refer to colour change at end point
*phenolphthalein [pink (alkali) to colourless (acid): end point pink
colour just disappears] [use if NaOH is used]
*methyl orange [yellow (alkali) to red (acid): end point orange]
[use if HCl is used]
*use a white tile underneath the flask to help observe the colour
change
*add acid to alkali whilst swirling the mixture and add acid dropwise at
end point
*note burette reading before and after addition of acid
*repeats titration until at least 2 concordant results are obtained- two
readings within 0.1 of each other
why is a conical flask more preferable than a beaker?
A conical flask is used in preference to a
beaker because it is easier to swirl the
mixture in a conical flask without spilling
the contents
why can distilled water be used?
Distilled water can be added to the conical
flask during a titration to wash the sides of
the flask so that all the acid on the side is
washed into the reaction mixture to react
with the alkali.
It does not affect the titration reading as
water does not react with the reagents or
change the number of moles of acid
added.
how can you decrease an apparatus uncertainties
To decrease the apparatus uncertainties
you can either decrease the sensitivity
uncertainty by using apparatus with a
greater resolution (finer scale divisions ) or
you can increase the size of the
measurement made
reducing uncertainties equipment
Replacing measuring cylinders with pipettes or burettes which have
lower apparatus uncertainty will lower the % uncertainty.
To reduce the % uncertainty in a burette reading it is necessary to make
the titre a larger volume. This could be done by: increasing the volume
and concentration of the substance in the conical flask or by decreasing
the concentration of the substance in the burette.
reducing in measuring mass
Using a balance that measures to more decimal places or using a larger mass will reduce the % uncertainty in weighing a solid. Weighing sample before and after addition and then calculating difference will ensure a more accurate measurement of the
mass added.
