Chemistry planning exercise

Finding out how much Acid there is in a Solution.

The aim of my investigation is to find the concentration of sulphuric (VI) acid from a sample. The solution is thought to have a concentration between 0.05 and 0.15 moldm-3.

To do this I will perform a titration involving sulphuric (VI) acid and sodium carbonate. A titration involves reacting a standard solution with another who’s concentration is unknown. I am given anhydrous sodium carbonate so will have to prepare a standard solution of this before attempting the titration. The indicator I will be using to indicate when the reaction is fully completed is methyl orange.(6) This is because I am using a strong acid and a weak alkali and methyl orange is the most appropriate indicator for this type of acid-base titration.The standard solution must be of a certain concentration for the titration to be a success. This is dependant upon the concentration of sulphuric (VI) acid. As this value ranges from 0.05 and 0.15 moldm-3 I can assume an average value of 0.10 moldm-3 as its concentration.

The reaction taking place will be

H2S04 + Na2C03 H2CO3 + Na2S04

As this equation is in a 1:1 molar ratio, the same concentrations of both sulphuric (VI) acid and sodium carbonate should be used.

Preparing a Standard Solution

A standard solution of sodium carbonate needs to be prepared.

Sodium Carbonate, Na2CO3, is a primary standard because it meets certain requirements: -

It is available in a highly pure state.
It must be stable in air.
It must be easily soluble in water.
It should have a high molar mass.
In solution, when used in volumetric analysis, it must undergo complete and rapid reaction.

The process involves weighing accurately a sample of sodium carbonate and use it to make a solution close to 0.10 mol dm-3. This will be used to determine the concentration of the sulphuric (VI) acid sample.

However, the weight of sodium carbonate I need to use is still unknown, so this will have to be found.

Using the formula n= Vc, where, n= moles of solute (mol.)

V= volume of solution (dm 3)

c= concentration of solution (mol dm-3)

The volume of solution to be made will be 250 cm3 as this will give me enough solution to complete the desired amount of titrations with but not too much as to have a surplus.

n=Vc

n= 0.25 x 0.1

n= 0.025 mol. (5)

Using this value I can ...

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However, the weight of sodium carbonate I need to use is still unknown, so this will have to be found.

Using the formula n= Vc, where, n= moles of solute (mol.)

V= volume of solution (dm 3)

c= concentration of solution (mol dm-3)

The volume of solution to be made will be 250 cm3 as this will give me enough solution to complete the desired amount of titrations with but not too much as to have a surplus.

n=Vc

n= 0.25 x 0.1

n= 0.025 mol. (5)

Using this value I can use the formula m=Mr*n to calculate the mass of sodium carbonate I will need. Mr= relative molecular mass

n= moles of solute (mol.)

m= mass of solute (g)

Mr(Na2CO3)=2*23+12+3*16

=106

m=Mr*N

m=106*0.025

m=2.65g (4)

Equipment Required.

Safety Spectacles
Weighing Bottle
Spatula
Sodium Carbonate Na2CO3
Access to a balance capable of weighing to within 0.001g
250 cm3 beaker
Stirring rod with rubber end
Volumetric flask, 250 cm3, with label
Filter funnel
Dropping Pipette.

Method

Transfer accurately about 2.65g of sodium carbonate into a weighing bottle and weigh it to the nearest 0.001g.
Put about 150 cm3 of water into a 250 cm3 beaker. Carefully transfer the bulk of the sodium carbonate from the weighing bottle into the beaker.
Reweigh the bottle with any remaining sodium carbonate to the nearest 0.001g. The mass difference is the mass of the solid used.
Stir to dissolve the solid.
Transfer the solution to the volumetric flask through the filter funnel. Rinse the beaker well, making sure all liquid goes into the volumetric flask. (The solid may be transferred directly into the volumetric flask, through the filter funnel but only if the solid will dissolve easily and if the funnel has a wide enough stem to prevent blockage).
Add distilled water until the level is within 1 cm of the mark on the neck of the flask. Insert the stopper and shake to mix the contents.
Using the dropping pipette, add enough water to bring the bottom of the meniscus to the mark on the volumetric flask. Insert the stopper and shake thoroughly 10 times to ensure complete mixing.
Label the flask with the contents, your name and the date.

After finishing the standard solution an acid-base titration must be done to determine the concentration of a sample of sulphuric (VI) acid by titrating against a standard solution of sodium carbonate (0.1 mol.)

Using the standard solution of sodium carbonate (Na2CO3) as a primary standard and titrating it against sulphuric (VI) acid the following reaction takes place: -

H2S04 + Na2C03 H2CO3 + Na2S04

To show when this reaction is complete (the equivalent point) you use an indicator called methyl orange. This changes from yellow to orange and finally to red in a solution becoming more acidic. The point at which the solution turns red is called the end-point and shows that the reaction is complete.

Equipment Required.

Safety Spectacles.
Filter funnel (small)
Burette, 50 cm3 , and stand
2 beakers, 100 cm3
Sulphuric (VI) acid solution approximately 0.1 mol.
Pipette, 25 cm3
Pipette filler
Standard solution of sodium carbonate
4 conical flasks, 250 cm3
Methyl orange indicator
White tile
Wash bottle of distilled water

Method

Using the funnel, rinse the burette with the sulphuric (VI) acid solution and fill it with the same solution. Don’t forget to rinse and fill the tip. Record the initial burette reading in the trial column.
Using a pipette filler, rinse the pipette with some of the sodium carbonate solution and carefully transfer 25.0 cm3 of the solution to a clean 250 cm3 conical flask.
Add 2-3 drops of methyl orange indicator.
Run sulphuric (VI) acid solution from the burette into the flask, with swirling, until the solution turns red. This first flask is used as a trial run, because the end-point will probably over estimated. Record the final burette reading.
Refill the burette with the sulphuric (VI) acid, and again record the initial burette reading to the nearest 0.05 cm3 (one drop).
Using the pipette, transfer 25.0 cm3 of the sodium carbonate solution to another clean conical flask. Add 2-3 drops of the methyl orange indicator to the solution.
Carefully titrate this solution to the end-point, adding the acid drop by drop when you think the colour is about the change. Before adding the final drops rinse the sides of the conical flask with distilled water from a wash bottle.
Repeat steps 5-7 at least twice more.
Empty the burette and wash it carefully after the titration, especially if it has a ground glass tap.

Accuracy

Burette readings should be recorded to the nearest 0.05 cm3 (approximately one drop). Consecutive titrations should be concordant, within 0.10 cm3 and the titration should be repeated until this is achieved. The mean of the two (preferably three) concordant readings should be calculated to the nearest 0.05 cm3. Using the three significant figures ensures that the third significant figure is more reliable. (1)

Safety

When completing the experiment certain safety procedures have to be taken into account: -

Safety spectacles must be worn at all times to ensure no chemicals, or broken glassware enters the eye.
A lab coat must be worn as to stop chemicals getting on clothing.
You must not run in the lab as to not knock over any chemicals or equipment.
0.1 molar sulphuric (VI) acid is an irritant so if any gets on the skin it must be washed off immediately. (2)
Methyl orange is slightly hazardous in the case of skin or eye contact (irritant). Care should be taken, if any gets on the skin it should be washed off immediately. (3)

If the plan is carried out to the highest level of accuracy as stated in the plan and measurements are also accurate; this should provide precise results. If concordant results are used this ensures the reliability of the results. The materials used must be used accurately, such as the weighing scales, to 0.001g and the glassware, so that the bottom of the meniscus is on the appropriate reading. The titration must be carried out as the plan states, the burette must be filled inside the scale and the conical flask accurately. The equipment must be cleaned properly before use with the appropriate chemical or distilled water to ensure precise and accurate results. The standard solution must be 0.1 molar to ensure an accurate titration.