"Finding out how much acid there is in a solution"

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Anna Galloway

As Level Investigation

“Finding out how much acid there is in a solution”


        I will carryout an acid-base titration to determine the concentration of hydrochloric acid (HCl); I will do this by making up a solution of sodium carbonate (Na2CO3) of known molarity. I will then citrate the unknown molarity of acid into the sodium carbonate; from these results it will enable me to calculate the molarity of the unknown acid.

        The reaction:

Sodium carbonate + hydrochloric acid        sodium chloride +water + carbon dioxide

Na2CO3(aq) + 2HCl(aq)                          NaCl(aq) + H2O(l) + CO2

        Planning to make the sodium carbonate solution:

        I already know that the approximate concentration of the hydrochloric acid is around 0.2mol/dm, from the above balanced chemical equation I know that 2 moles of hydrochloric acid react with 1 mole of sodium carbonate; therefore I will make a solution of sodium carbonate of 0.1 mol/dm^3. Half that of the approximate molarity of the hydrochloric acid, I have made it 0.1 mol/dm^3 so to keep the volumes of solutions being titrated of a sensible amount as regard to the size of glass wear available.


Sodium carbonate salts relative formula mass:

Na2CO3  . 10H2O   =  106 . 180 =  286

N.B there is 1 mole of sodium carbonate crystallised with 10 moles of water in this


Volume of sodium carbonate required per titration + 25cm^3, total volume required for experiment = 250 cm^3 of 0.1 mol/dm

Number of moles = (molarity * volume) / 1000

Number of moles = (0.1 * 250) / 1000

                             = 0.025 moles

Amount of substance = relative formula mass * number of moles

Amount of substance  = 286 * 0.025

                                = 7.15g

I will need to measure out 7.15g of sodium carbonate salt to dissolve in to 250cm^3 of water.

        How do I make up the solution of sodium carbonate?

1).Accurately using 2-place balance measure 7.15g of sodium carbonate

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2).Tip the crystals into large beaker

3).Add 100 cm^3 of distilled water

4).Gently heat over Bunsen flame (careful not to allow boiling)

5).Once solution turns clear/fully dissolved remove from heat

6).Transfer contents to volumetric flask

7).Wash beaker out with distilled water, transfer washings to volumetric flask (repeat 2x)

8).Carefully make up volume in volumetric flask with distilled water (slowly adding until 1cm from graduation mark then use pipette slowly add the water until meniscus just touches graduation mark)

9).Insert bung/stopper and invert several times.

        The titration:

1).Set up resort stand and burette clamp

2).Wash burette out with ...

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