To determine the concentration of a lime water solution

Application

• There is a standard equation for a reaction of a metal hydroxide and hydrochloric acid:

M(OH)2 + 2HCL                MCl2 + 2H20

(M = metal)

• Therefore the equation for calcium hydroxide is:

Ca(OH)2  + 2HCL                CaCl2 + 2H20

• In order for us to be able to do the reaction we need to dilute the 2.00 moldm-3 hydrochloric acid to a much more usable concentration – 0.1 mol. To make this I shall use a dilution factor of twenty, that is, twenty parts de-ionised water to 1 part acid. So If I am to make 500cm3 of 0.1moldm-3 HCl I shall need 25ml of 2.00moldm-3 HCl.
• The equation says that the ratio of calcium hydroxide to HCl is 1:2. Therefore we need two moles of HCl to neutralise 1 mole of Calcium hydroxide.
• Number of moles of HCl:

No. Of moles = (volume/1000) x concentration

= (100/1000) x 0.1

= 0.01 mol

• The number of moles of calcium hydroxide is half the amount than that of the HCl so therefore:

0.01mol x 0.5

= 0.005 moles of Ca(OH)2

• The concentration therefore of the calcium hydroxide is:

(0.005 x 1000) / 50cm3 = concentration

= 0.1 moldm-3

Mr of Calcium hydroxide = 74

gdm-3 = 74 x 0.1

= 7.4 gdm-3

• I will use methyl orange indicator because the limewater is a weak base so methyl orange has an endpoint of 5 – 8 on the pH scale where as phenolphthalein indicator has a much higher end point this means that the end point would be reached before the equivalence.

Variable Control

• You need t ensure that you take your chemicals from the same batch so as not to get two different concentrations each time you test them.
• Temperature and pressure should have no effect on the reaction

Organisation

1. Set up the equipment as shown in the diagram below, ensuring that the burette valve is closed.
2. Run de-ionised water through the burette and wash all of the equipment with it
3. First of all dilute the hydrochloric acid using by adding slowly 25ml of 2.00moldm-3 HCl to 475ml of de-ionised water in the 500ml beaker
4. Then, using a pipette, put 100ml of the HCl solution into the burette
5. Put 50ml of limewater into a conical flask using the pipette then add 5 drops of methyl orange to the solution, this should turn the solution a pale orange
6. Open the burette valve and allow the HCl to flow into the limewater siolution, close the valve as soon as the limewater solution turns pink. Record the volum of HCl used.
7. Repeat steps 4,5 and 6 but close the before the end-point and add the HCl one drop at a time to get a more accurate reading
8. Repeat this a further 3 times and record all of your results in a table

Apparatus

• You will need:

• 25cm3 Pipette
• 500cm3 Volumetric Flask
• 250cm3 Conical Flask
• Burette
• White tile
• Clamp and Stand

• In addition to this you will need:

• 250ml of Calcium Hydroxide solution
• 25ml of 2.00mol Hydrochloric acid
• De-ionised Water
• Indicator (methyl orange)

• Apparatus set up:

Risk Assessment

• Hydrochloric acid is corrosive and so it is necessary to wear goggles, lab coats and gloves
• Calcium hydroxide can also be dangerous to the eyes so again safety goggles are important
• Also care needs to be taken when diluting the hydrochloric acid as they react violently so the acid should always be added to the water slowly and not the other way around.
• If any chemical comes into contact with the eyes then they need to be washed using an eyewash. With the low strengths in this experiment medical attention does not necessarily need to be immediately sought unless serious.