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Finding the Exact Concentration of Sulphuric Acid in a Solution Through a Titration

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Finding the Exact Concentration of Sulphuric Acid in a Solution Through a Titration

Aim: To find the exact concentration of sulphuric acid in a solution
through a titration. The titration is between sulphuric acid and
sodium carbonate has to be in liquid for in order to carry out the
experiment so it is dissolved into distilled water to a concentration
of 0.1 mol dm³


Introduction-
-------------

During the extraction of a metal from its ore, sulphuric dioxide is
often produced. This is converted to sulphuric acid and is sold as a
useful by-product. I am going to be given a sample of sulphuric acid,
which is thought to have a concentration between 0.05 and 0.15-mol
dm³.

The purpose of this experiment is to find the accurate concentration
of the sulphuric acid. I will do this by carrying out a titration
between sulphuric acid and sodium carbonate solution. Therefore this
is an acid-alkali titration (which is the determination of
concentration by adding measured amounts of standard reagents to a
known volume until the end point is reached).

* Sulphuric acid is considered a strong acid, as it is completely in
the form of ions in dilute solution.

* Sodium carbonate is a weak alkali as it only partially forms ions
in dilute solution.

* For this titration methyl orange is the best indicator for the end
point between a strong acid and weak alkali.

[IMAGE]Na2CO3 (aq) + H2SO4 (aq) Na2SO4 (aq) + CO2 (g)

* One mole of sodium carbonate is needed to neutralise one mole of
sulphuric acid so it in a 1:1 ratio for moles



APPARATUS AND EQUIPMENT
=======================


Chemicals used

Amount

Sodium carbonate

2.65g

Methyl orange

3 drops

Sulphuric acid

Will vary in total

Distilled water in washer bottle

150cm³ (for sodium carbonate solution)

DIAGRAM

[IMAGE]

Lab equipment

Use

Spatula

To place sodium carbonate in boat

Boat

To place sodium carbonate on scales

Digital weighing scales

To measure sodium carbonate

250 cm³ beaker + glass stirring rod

To dissolve the sodium carbonate in to distilled water

250 cm ³Conical flask

To further shake sodium carbonate solution in

Burette and clamp

To place sulphuric acid in and efficiently pour in to beaker

Pipette and pipette filler

Mentioned in method

Funnel

Volumetric Flask


METHOD

1. Weigh mass of boat set scales to 0 then add 2.65g of anhydrous
sodium carbonate to the boat using a spatula

2. Put sodium carbonate in to beaker rinse boat using distilled
water

3. Put 150cm³ of distilled was to the same beaker

4. Stir well with glass rod till solution goes clear and sodium
carbonate is completely dissolved

5. Transfer the dissolved sodium carbonate to a volumetric flask.
Wash out any remains on the funnel using distilled water to ensure
accuracy of sodium carbonate solution

6. Add distilled water to the volumetric flask until the solution is
close to the graduation mark. Then use as a pipette to make sure
the bottom of the meniscus touches the graduation mark. This
ensures the sodium carbonate solution concentration is kept
accurate. Shake well one more time.

7. Set up the burette with clamp on the floor put paper on the
reading so it can be read easily. Fill it with sulphuric acid to
the top (0) using funnel and pipette to get it accurate as you get
close to the zero. Place beaker underneath to avoid spillages.

8. Use pipette filler and pipette to transfer 25cm³ of sodium
carbonate solution from the 250cm³ volumetric flask to the conical
flask. Again wash out pipette using distilled water. Do not hold
the pipette above the liquid as it sucks air back up.

9. Add 3 drops of methyl orange solution to the conical flask. Place
conical flask under the burette. Set this on the table. I am using
3 drops as this gives a darker red/ orange colour and the colour
change can be detected easier.

10. Add the sulphuric Acid solution from burette to the conical
flask by turning the stopcock on the burette vertical note the
initial final reading before the end point.

11. Keep the flow rapid till the colour changes to a lighter red,
but before this turn the stopcock slightly horizontal as the acid
comes out drop by drop.

12. Read off the value on the burette find out the volume of
sulphuric acid needed to react with sodium carbonate.

13. Carry out the titrations Until you have recorded three results
within 0.1cm³

CONTROL VARIABLES

Ensure the same sodium carbonate solution and sulphuric acid batch is
used for all titration to ensure a fair test and the results will be
accurate and reliable.

Try to carry out the titrations on the same day to ensure there isn’t
a temperature change and that they are done in one uniform
temperature. If the temperature is increased, the liquid will expand
making the test inaccurate.

RISK ASSESMENT

* Wear safety goggles to prevent acid from entering the eye

* Stools must be tucked underneath the table ensure bags are out of
the way to prevent people from tripping

* Long hair should be tied back

* Ensure you are standing throughout the experiment so you are able
to quickly move out of the way in case of spillages etc. Also to
accurately record readings at eye level

* Use a funnel when pouring acid to prevent splashes and leakages

* Watch hands if the come in contact with any of the chemicals

* Wipe away immediately any spillages

* Don’t fiddle with the experiment too much as this may affect the
temp thus the results.

* Inform the teacher if any glassware id broken

* Ensure there is no running in the lab as this can cause hazards


Bibliography
------------

Chem ideas

www.ocr.org.co.uk

Chem storylines

Elements of life activity sheet 2.1

Results

Titration readings

Rough

1

2

3

4

5

Average titre

Final

29.7

27.7

28.6

28.7

29.1

28.5

28.43

Initial

0

0

0

0

0

0

0

Amount of

H2 SO4 used (ml)

29.7

27.7

28.6

28.7

29.1

28.5

28.43

Calculations

Concentration of sodium carbonate solution:

No. Of moles

[IMAGE]Concentration=

Volume

* 2.65g of anhydrous sodium carbonate

RFM of Na2 CO3 = 2×23+12+3×16

=106

Moles of Na2 CO3 = 2.65/106

= 0.025 moles (3 vs.)

* Combined with 0.25dm³ of water therefore concentration of solution
=

Conc. of sodium carbonate/conc. of water=

[IMAGE]0.025

=0.1 moldm³ (3.s.f.)

0.25

Concentration of 28.43 ml of H2SO4:

[IMAGE]Na3CO3 (aq) + H2SO4 (aq) Na2SO4 (aq) + Co2 (g)

1 mole of Na2CO3 neutralises 1 mole of H2 SO4

Therefore the ratio of sulphuric acid to sodium carbonate is 1:1

25cm² of sodium carbonate is needed to neutralise 28.43cm² of
sulphuric acid

Moles = concentration of solution × volume of solution (dm³)

0.1mol dm³ × (28.43/1000)-0.02843dm³=0.002843 moles of H2SO4 in 25cm³
of sodium carbonate solution

[IMAGE]Conc. Of H2 SO4 =0.002843 moles

0.025dm³

= 0.11 moldm³

Conclusion

The results obtained from my titration suggests that the concentration
of the sulphuric acid is 0.11 moldm³ whilst this value may not be
accurate it appears to agree with the statement that the acid is
thought to have a conc. between 0.05-0.15 moldm³. This is strong
evidence that the investigation results are quite satisfactory as they
are in the expected range. The accuracy of my results is discussed in
my evaluation.

Evaluation

My measurements were quite precise and reliable when I weighed out the
anhydrous sodium carbonate using the digital balance; I used a spatula
to measure exactly 2.65g. Also to ensure the distilled water was
exactly on the graduation mark I used a pipette to ensure the meniscus
sat exactly on the graduation mark. The pipette filler was extremely
accurate and precise it is designed to release exactly 25cm³. Also I
repeated my experiment 5times and three of my results are within 0.1ml
of each other. The concentration of sodium carbonate was accurate
because I used distilled water to wash all the sodium carbonate off
the weighing boat and funnel in to the volumetric flask.

However the most inaccurate part of the experiment was deciding when
the colour change of the indicator had occurred. This was hard because
although the colour change was fairly rapid, it was not always
instantly obvious. It was important to keep a specific colour in mind,
and stop the titration when the colour was reached. Fortunately I kept
the colour change point constant, although this could have been a
cause of any anomalies. To improve the investigation a pH metre can be
used instead of using a pH indicator to see when neutralisation
occurs. This would eliminate the most significant source of error,
which is largely due to human error.

I could have further improved the experiment by checking the pipette
filler for air gaps. Also using a magnetic stirrer would have ensured
all the sodium carbonate was completely dissolved. I could have used
more accurate scale –analytical balance which measure to 0.0001g so it
has a high degree of precision

Percentage error

I am going to work out the percentage error using the following
equation:

(True value- experimental value) × 100

[IMAGE] True value

Quantity measured

Percentage error

Mass of sodium carbonate using balance

(0.05×100)/2.65=0.19%

Volume of distilled water using volumetric flask

(0.5×100)/250=0.2%

Transferring sodium carbonate using pipette filler

(0.5×100)/25=2%

Carrying out titration using a burette

(0.005×100)/28.43=0.02% (to 2.s.f)

In order to minimise measurements errors larger quantities of all
solution could be used although concentration would remain the same.
For example

[IMAGE]250 cm³ graduated flask 0.2 × 100 = 0.08%

250

1 dm³ flask 0.2 ×100 = 0.02%

[IMAGE] 1000

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