Chemistry Practical: Finding the water content of Na2 CO3 .xH2O by Titration

Step 1: making up the solution

Step 1. Weigh about 4g of sodium carbonate hydrate crystals into a dry weighing bottle. Place bottle, lid and powder onto the balance and weigh using an accurate balance to record weight to 3 d.p. Note: remember not to expose crystals to air for too long to avoid water loss from crystals.

Step 2. Transfer the powder to a beaker (100 ml+ in size)

Step 3. Reweigh the bottle and lid (and subsequently any powder left in the bottle that didn't fall out).

Hence find the weight of sodium carbonate transferred to the beaker by subtracting it from the weight of the bottle, lid and powder.

Step 4. Dissolve the powder in distilled water by stirring the solution.

Step 5. Transfer the solution to a 250cm3 volumetric flask washing the beaker out with distilled water into the flask to ensure as much of the solution is washed into the flask as possible.

Step 6. Add distilled water to the solution to make it up to 250cm3 ensure exactly the correct amount is in the flask and that the bottom of the meniscus rests on the calibration mark.

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Stage 2: titration

Step 1. Wash out:

i.) the pipette, with a little of the sodium carbonate solution

ii.) the burette, with a little of the hydrochloric acid solution

iii.) the conical flask, with a little distilled water.

Step 2. Fill the burette with the hydrochloric acid solution, running some of the solution through the tap, until the bottom of meniscus is just on the zero level. Ensure that there is no air bubbles trapped.

Step 3. Using the pipette filler place 25cm3 of the sodium hydroxide solution in the clean conical flask.

Step 4. Add 4 or 5 drops of methyl orange indicator to the contents of the flask. Note the colour.

Step 5. Using the tap on the burette run the acid into the solution of sodium carbonate in the conical flask (shaking the flask as the acid drips in) until the solution changes colour. At this point note the reading on the burette and the colour of the indicator. This first titration is 'rough' so only an approximate volume of acid is required.

Step 6. Repeat steps 2, 3 and 4

Step 7. This time:

iv.) Run the acid into the solution until the volume of HCl is to within 2 cm3 of what it was in the rough titration (step 5).

v.) At this point run the acid into the solution at a slower rate (dripping it in slowly and shaking the solution in the process) so you can read the volume of HCl required to neutralise the solution to one drip (0.05 cm3). Note: take an accurate reading to within 0.05 and take the reading at the first colour change in the solution.

Step 8. Repeat steps 12 and 13 until satisfactory results are obtained:

vi.) Two results are obtained within 0.05 of each other

vii.) Three results within 0.10 of each other

Results

For 25cm3 of Na2 CO3 .xH2O the following results were obtained by titration using HCl.

Rough

Accurate

Accurate

Burette Reading end (cm3)

28.75

28.85

28.90

Burette Reading beginning (cm3)

0.00

0.00

0.00

Volume of HCl added (cm3)

24.15

24.10

24.30

Average Titre (cm3)

28.875

Colour change at end point: slightly pinkie orange

Calculations

1. Number of moles in 28.875 cm-3 of 0.1 moldm-3 of HCl =(28.875 x 0.1)/1000= 2.89x10-3

2. Number of moles of sodium carbonate in 25 cm-3=2.89x10-3/2=1.44x10 -3

3. Number of moles of sodium carbonate in 250 cm-3=1.44x10 -2

4. Mass of anhydrous sodium carbonate Na2 CO3 in 250 cm-3=1.44x10 -2 x 106=1.53g

5. Mass of water in the original hydrated crystals=3.99-1.53=2.46g

Empirical Formula

Na2 CO3

H2O

Mass

1.53

2.46

Mass/Mr

0.0144

0.13666

Ratio

1

9.5

Formula: 2Na2 CO3 .19H2O

Therefore x =19.

Compared to target value

I will now compare my value for the percentage water content of Na2 CO3 .xH2O with the target value of 62.9%.

My percentage value was 61.56%:

2.46 x100 = 61.56%

3.99

Therefore there was 2.13% difference between my value and the target value:

1.34 x100 = 2.13%

62.9

Errors from apparatus

% error in the concentration of the solution of sodium carbonate:

1. Maximum percentage error in the mass of sodium carbonate weighed (error 0.01g):

0.01/3.99 x 100= 0.25%

2. Maximum percentage error in the volumetric flask (error is 0.3 cm3):

0.3/250 x100= 0.12%

3. Maximum percentage error in the pipette volume (error is 0.15cm3):

0.15/25 x 100= 0.6%

4. Maximum percentage error in the burette volume (error is 0.10cm3):

0.10/28.875 x100= 0.35%

Total maximum percentage error from apparatus = 1.32%

I will now compare the difference between my value and the target value with the total maximum error from apparatus to see how much of it is down to the apparatus and how much is down to my own inaccuracies i.e. human error.

The difference between my value and the target value = 2.13%

Total maximum percentage error from apparatus = 1.32%

This comparison shows that I could have been more accurate when conducting my experiment:

One inaccuracy may be due to the fact that when titrating I shook the flask too vigorously and not all the drops landed directly in the solution but on the inside of the flask which would have resulted in an over estimate of the volume of HCl. This problem could be resolved by reducing the rate at which I did the titration which would enable me to shake the flask more steadily.

I could also repeat the titration more times- by doing this and averaging the closest few I would be more likely to find the precise volume of HCl required to neutralise the solution.

I could also shake the volumetric flask before taking the 25 cm3 sample from it as the dissolved crystals settled to the bottom of the flask after a while resulting in a varied concentration throughout(it was not homogenous and would subsequently give varied results in titration).