Volumetric analysis part 2 Flashcards
solution=
solute + solvent
strength
concentration
concentration
the quantity of solvent dissolved in a certain amount of solute
how is concentration found?
by carrying out a titration
what unit does the titration formula give
moles per litre
what does %w/v mean
grams/100cm cubed
what does ppm mean
mg/litre
how do you find %w/v
find the grams in 100cm cubed
how do you find ppm
expres g/l in mg/l
g/l =
m/l x Mr
what is the Mr
all the big numbers added up
standard solution
a solution whose concentration is known exactly in m/l, g/l or ppm etc.
only base that can be weighed accurately
Na2CO3
what do you do once you put the clock glass on the mass balance
tare, adjust to zero
a mass balance should read…
… 0.01 grams of accuracy
what size volumetric flask should you use when preparing Na2CO3 solution
500 cm cubed
primary standard
a solute that can be weighed very accurately and you can add water to make a standard solution
4 characteristics of a primary standard
- obtainable in 100% form
- dissolves completely in water
- dry and remains dry during weighing
- does not decompose or evaporate during weighing
why can’t HCl be measured accurately?
it evaporates when opened
why can’t H2SO4 be measured accurately?
absorbs water from the atmosphere
2 ways to obtain a standard solution
- weigh out accurately a primary standard and make up to an exact volume with deionised water
- carrying out a titration on a solution of unknown concentration
what is a volumetric flask designed to do?
to hold a definite amount of liquid
what is a temperature figure on a volumetric flask?
liquids expand or contract at different temperatures, if the temperature figure is 20ºC then it can only be read accurately at that temperature
what do you rinse the volumetric flask with?
deionised water and the solution that it will contain
how do you fill the volumetric flask
first with a funnel and then using a dropper for the last 2 cm cubed s, until the bottom of the meniscus is at the calibration mark
how do you get a homogenous solution in a volumetric flask
add a stopper and invert 20 times
why do you invert a volumetric flask
the concentration of the solution might be lower in the neck than in the lower bulk of the flask
What is a pipette designed to do?
deliver an exact amount of volume of a liquid from one piece of glass apparatus to another
usual volume of a pipette
20/25 cm cubed
how is liquid sucked up into a pipette
using a pipette filler
how do you fill a pipette
to above the mark and let some out into the waste beaker until the bottom of the meniscus is at the mark
after filling the pipette
touch the tip of the pipette against the inside of the beaker to remove any drop of liquid sticking to the tip
why do you know blow out the last bit of liquid into the conical flask
it is manufactured to give accurate volumes only when when this last amount remains in the pipette
what should you rinse the pipette with before use?
deionised water and then the solution it will contain
why rinse with deionised water
to remove any impurities or dirt
why rinse with the solution it will contain
pushes out deionised water so that the liquid won’t be diluted
after use of pipette
rinsed with deionised water and will be allowed to empty
what is a burette
a long narrow tube with a tap at the end and has graduations down it’s full length
burette is read…
…from the top down
the burette reads to
0.1cm cubed accuracy
before use of a burette
check the tap to see if it turns freely, if not, grease it
what should you rinse the burette with
deionised water and then the solution it will contain
how do you fill a burette
with a short stems funnel, fill above the graduation mark and let flow until the bottom of the meniscus is at the zero mark, check to see if the area below the tap is full
how do you remove any drop of liquid sticking to the nozzle of the burette
touch it against the side of a beaker
why do you read a burette at eye level?
to avoid the error of parallax
how do you get a more accurate reading of a burette
put a white tile or white paper behind it
why is a conical flask, cone-shaped?
for swirling without losing some liquid
why do you swirl in a conical flask
to get a full reaction
how do you prevent acid/base sticking to the sides of a conical flask
wash them down with deionised water from a wash bottle
what do you rinse a conical flask with
deionised water but never the solution it is about to contain
why do you not rinse a conical flask with the solution it’s about the contain
it will change the volume in the conical flask, which matters, but the water doesn’t change the concentration you already measured
what can a graduated cylinder measure
approximate values only
what is the accuracy of a graduated cylinder
0.5 cm cubed
why is a graduated cylinder not very accurate?
it is too wide for accurate measurements
which instruments are very accurate
narrow instruments such as the pipettes, burettes etc.
purpose of a titration
to standardise a solution
find the exact concentration in moles per litre
why do you not pipette directly from the volumetric flask
impurities on the pipette would contaminate the entire solution in the volumetric flask
indicator used in Na2CO3 vs HCl titration
methyl orange
methyl orange in a basic solution
yellow
methyl orange in an acidic solution
red
methyl orange in a neutral solution
orange/pink
what should you not forget to do with the funnel?
take it out of the burette
what do you do with the 2 white tiles?
one underneath the conical flask and one behind the burette reading
end products of a titration
salt
water
CO2 gas
2 reasons not to put a base like NaOH in a burette
i. NaOH erodes glass causing taps to stick and leak
ii. NaOH reacts with CO2 in the air forming hardened Na2CO3 causing the tap to be stuck
why can a burette and pipette be washed down with the solution that they will contain?
the burette and pipette measure exact volumes of exact concentration
why can you not wash down a conical flask with the solution it will contain?
a conical flask is a container where an exact number of moles of one reactant coming from the burette meet and react with with an exact number of moles of the other reactant coming from a pipette
after standardising HCl with primary standard Na2CO3, what is it called?
a secondary solution
why should you not touch NaOH
it can give a severe burn
when obtaining NaCl crystals, why do you do another titration?
you do another one without the indicator
how do you obtain the NaCl crystals?
only NaCl and water is left in the conical flask
place on an evaporating dish over a hot plate
white crystals are formed
what do you titrate against vinegar (ethanoic acid)
NaOH solution
why do you dilute the ethanoic acid?
because otherwise you would get a very small reading which leads to greater percentage error
why would you not just use a higher concentration of NaOH
it’s not safe for a school laboratory
what indicator is used when standardising vinegar
phenolphthalein
when is phenolphthalein used
with a weak acid and a strong base
phenolphthalein in acid
colourless
phenolphthalein in base
pink
when do you use the pipette twice?
in titrating ethanoic acid (vinegar) because it is a liquid not a solid
ratio of vinegar dilution
1:5
how to find the original concentration of the vinegar
- find in m/l of dilute
- x5 = m/l of original
- find in g/l
- find g/100cm3 %w/v (divide by 10)
- eg. 7.8%, that’s it
what indicator used in finding % water crystallisation in hydrated sodium carbonate
methyl orange
how do you calculate % of water crystallisation in hydrated sodium carbonate
- find m/l
- find g of crystals in 1L of H20
- find g in 250cm3
- of the 5 g only x was Na2CO3
- 5-x = y
- y/5 x100 = percentage
eg. 39.2%
how to find ‘X’ in
Na2CO3–XH2O
100 - 29.2% of Na2CO3 = 60.8% of water in it 29.2% is 106g 60.8% is y grams find y by getting 1% and multiply, = 164 grams 1 H2O = 18g 164/18 X = 9.13 \_\_ 9
why do you not get a whole number when finding X?
experimental inaccuracy
just round off to the nearest whole number
number of moles of solute=
volume in cm3 X Molarity
___________________
1000
grams =
moles x Mr
formula used when calculating between different concentrations of the same substance
Vol.dil X M.dil =
Vol.conc X M.conc
