Coursework help - rep given!

im doing my AS OCR chemistry coursework where i have to plan an experiment to determine the concentration of limewater.

I've done most of the plan, (method for titration etc.) but got confused over how to end it.

i have been told that 'you are provided with 250cm^3 of limewater which has been made such that it contains approimately 1g/dm^3 of calcium hydroxide. - isnt this what im meant to be doing the experiment to find out?

if not, what do i do once i've done a concentration between 0.02 mol/dm^3 HCl and the limewater, to find the answer?

Please help,it's due tomorrow and im really confused!

Reply 1

nataloulou

Titration
1 2 3
Initial burette reading (cm3) 1.85 21.25 1.65
Final Burette reading (cm3) 21.25 41.15 21.45
Titre (cm3) 19.40 19.90 19.80

Mean of concordant titres (2&3) = 19.85cm3
Calculation
Finding the concentration of the hydrochloric acid after dilution
Took 25cm3 of 0.3 mol/dm-3 HCL
And 250cm3 of distilled water

Ratio: 0.3 0.0
25: 250
1: 10
Therefore, 0.3/ 10 = 0.03 mols/dm3

Concentration of limewater solution
Ca(OH)2 (aq) + 2HCL(aq) -> CaCl2(aq) +H2O(l)
Mol reacting 2mol 1 mol
Vols used 25cm3 19.85cm3
0.025dm3 0.01985 dm3
Moles= conc* volume
Moles of HCL= 0.03*0.01985
= 0.0006 mol
Therefore, because half as many moles of Ca(OH)2 react with HCl.
Mols of Ca(OH)2 = 0.0003mols

Conc of Ca(OH) 2 = 0.0003/0.025
=0.012mol/dm-3

erm yes. i can't actually find the method for the experiment. but maybe the calculation will help you?

Reply 2

Good bloke

19

Your experiment is to determine the exact concentration. They give you that information so that you can plan to use appropriately concentrated reagents.

Reply 3

*BCM*

11

EmmaK90

im doing my AS OCR chemistry coursework where i have to plan an experiment to determine the concentration of limewater.

I've done most of the plan, (method for titration etc.) but got confused over how to end it.

i have been told that 'you are provided with 250cm^3 of limewater which has been made such that it contains approimately 1g/dm^3 of calcium hydroxide. - isnt this what im meant to be doing the experiment to find out?

if not, what do i do once i've done a concentration between 0.02 mol/dm^3 HCl and the limewater, to find the answer?

Please help,it's due tomorrow and im really confused!

i think that the approximate conc of Ca(OH)2 is given to you so that you know roughly how much HCl you have to add to neutralise this solution - your task is to find the exact conc of the solution ie to 3 d.p.

if you titrate 25 cm^3 portions of the Ca(OH)2, youll have about 0.025 g of Ca(OH)2 in that portion ie 0.000338 moles of Ca(OH)2 - this will require 0.000676 moles of HCl to neurtalise, so you must use the appropriate volume of HCl solution, depending on its conc - if you are using 0.02 M HCl youll need about 34 cm^3 of it

hope that helped, your second question wasnt v clear