Titration Experiment Write Up
There is a link between the volume of alkali needed to neutralise a certain volume of acid of a known concentration. We can perform a titration of the acid and alkali, and therefore use the volume of alkali added to determine its concentration. In this investigation, we are neutralising 25 cm3 of 1 mol/dm3 of hydrochloric acid (HCl) with sodium hydroxide (NaOH) of an unknown concentration. Depending on the amount added, we can calculate its concentration using a balanced equation and other formulas.
Equipments used: Chemicals used:
- Conical flask - Hydrochloric acid (1 mol/dm3)
- 25 cm3 pipette - Sodium hydroxide (Conc. Unknown)
- Pipette filler - Phenolphthalein
- Burette clamp
- Retort stand
- White tile
- Small Pipette
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Using a pipette filler, fill the pipette up to the mark (25cm3) - since the value needs to be very accurate, make sure it fills up to where the bottom of the concave meniscus touches the mark, observing from eye level.
Then released the acid into the conical flask. Pipette 3 drops of phenolphthalein in to flask, swirl it to make sure it has mixed completely.
Using a burette clamp, secure it onto a retort stand. Rinse the burette with the alkali (NaOH) and, with a beaker underneath, let the sodium hydroxide run through the bottom of the burette by leaving the tap on, make sure there are not any bubbles present - this is to avoid any inaccurate reading later on.
Fill the burette with NaOH up to where the bottom of the meniscus hits 0, again reading from eye level to avoid errors in measurements. Do this using a funnel to avoid any spills, but remember to take it out so there won’t be any droplets which can affect the final reading. Record the start value as 0 on the table.
Place the conical flask under the burette (with a white tile underneath in order to determine when exactly the end point is reached since it would be easier to spot a very pale pink), open the tap and remember to keep swirling in the process in order to constantly mix the acid and alkali together, otherwise it would not be clear when the end point it immediately.
As end point is approaching (when it takes longer for solution to turn clear as it mixes) the sodium hydroxide should be added drop by drop to determine the titer more accurately - because if we add a lot at a time, there is a high possibility that it would go over the end point by more than it should.
As the solution turns pale pink, the titration is complete, read the end value at the bottom of the meniscus from eye level and calculate the titer.
The first titration will produce the rough titer, the end point would usually occur around that value. This is used to indicate about where the end point would be in further titrations.
Pour out the solutions in the conical flask and rinse it to remove the remainders - this is to avoid the previous solution reacting with those in the next titration
Repeat the titrations 2 to 3 times (steps 4 through 7), until two consecutive titer, each within 0.1 cm3 of each other, is achieved. This is when the titration is accurate enough that it produces two values close to each other.
Below are the results of my titration:
To calculate the concentration of NaOH used, write down a balanced equation:
NaOH (aq) + HCl (aq) -------> NaCl (aq) + H2O (l)
1 : 1
The one to one ratio between sodium hydroxide and hydrochloric acid means one mole of NaOH reacts with 1 mole of HCl to produce 1 mole of NaCl and 1 mole of water.
Therefore, the number of moles of HCl (with concentration of 1 mol/dm3) present in 25 cm3 would be number of moles of NaOH used to neutralise it.
Number of moles (HCl) = (CxV)/1000
= 0.025 moles <------- number of moles of NaOH used
Rearranging the formula to work out the concentration of NaOH with known volume and number of moles.
Concentration (NaOH) = 1000n/V
= 0.97 mol/dm3
There are some things that could have been improved to increase accuracy of my result. For example, for my rough titration, I have forgotten to remove the funnel before titrating, so I probably used more alkali then what I recorded. However, it does not affect the end result much since I didn’t include it into my mean titer calculation, but it might be an unreliable value for my future titration to be based on. Next time, I will remember to check whether the funnel is still there before titration begins.
To improve accuracy, I should add the sodium hydroxide drop by drop when it’s near the end point, taking extra care while doing so, because for both my actual titration, I accidentally released too much NaOH and the solution turned into a dark pink instead of the pale pink it’s meant to be. This means the titration is actually pass its end point a lot rather then just over when it is completely neutralised. Also, if I do this experiment again, I would repeat the titration a couple more times and calculate the mean titer from there.
Other then the mistakes above, I have done as much as I can to make sure the result is as accurate as possible, therefore I think the concentration I calculated is fairly close to the actual concentration of the solution.