Lime water is a saturated solution containing about 1 g of
calcium hydroxide per dm3 of solution. Its concentration
could be determined by titrating 20 cm3 of lime water with 0.01M
hydrochloric acid. Answer questions a-d to help work out a
theoretical titre for this titration.
Ar values; Ca=40, O=16, H=1
Eqn: Ca(OH)2(aq) + 2HCl(aq) = CaCl2(aq)
+ H2O(l)
a) What is the molarity of a lime water solution
containing 1g/dm3?
Give your answer to 3 sig fig.
Is this 0.00100?
b) How many moles of Ca(OH)2 are there in 20 cm3 of this
solution?
Moles of Ca(OH)2: C x V/1000 = 0.01 x 20/1000 = 0.002
mol ?
c) How many moles of HCl are needed to react with this number of
moles of Ca(OH)2?
1:2; Moles of HCl: 0.002 x ½ = 0.001 mol ?
d) What volume of 0.01 M HCl would contain this number of
moles?
Volume = Moles/Concentration; 0.01/0.01 = 0.1 dm3
?
e) i) Why would this not be a suitable volume for the
titre?
Help here as well please.
ii) Suggest how this titre value could be
reduced by half.
Is this doubling the concentation of HCl?
f) Suggest a suitable indicator for this titration. Give
the colour change expected at the end point.
Is this Phenolphthalein? Pink? I don't know much
about experiment as I don't get classroom time, so I'm thinking
this is more appropriate than methyl orange in this case because
this would give a darker pink which would be easier to see than if
methyl orange was used?
g) Name the apparatus which you would use to accurately measure
the 20 cm3 of calcium hydroxide solution.
Again, I'v got no clue.
Please help me. Thank you.
a) No, molarity = mole / volume (dm3). You have to calculate the
moles of Ca(OH)2 present in 1g, then divide by the 1dm3 volume to
obtain molarity.
b) moles (in 20cm3) = 20/1000 x molarity
c) yes, simply twice the moles of Ca(OH)2, since the base is
diprotic, while the acid is monoprotic.
d) correct formula used, wrong values (due to error carried
forward)
e) the titre value is too large, exceeding the burette
capacity.
f) since this is a strong acid against strong base titration,
any indicator will do. Just correctly state its end-point colour
change.
http://en.wikipedia.org/wiki/PH_indicator
g) pipette