An Experiment to Determine the Water of Crystallization in Sodium Borate

Apparatus:

Safety spectacles

Weighing bottle

Balance capable of weighing to within 0.01g

Spatula

Stirring rod with rubber end

Dropping pipette

Wash bottle of distilled water

Funnel

Volumetric flask, 250 cm3

Beaker, 250 cm3

Retort stand

Burette clamp

Burette, 50cm3

Pipette, 25cm3

Pipette filler

Beaker, 100cm3

White tile

Conical flask, 250cm3

Reagents used:

Distilled Water

Hydrated Sodium Borate

1.0 M Hydrochloric acid

Screened methyl orange

Sodium Borate solution

Method:

Prepare a standard solution of hydrated sodium borate (250cm3 solution)

Place the weighing bottle on the balance and record the mass of the empty bottle.

Add approximately 4.80 grams of the hydrated sodium borate compound into the weighing bottle and record their weight together.

Transfer the measured hydrated sodium borate to the 250 cm3 beaker then find the mass of the weighing bottle once again to see how much of the compound was actually moved

Add small amounts of distilled water into the beaker while constantly stirring the water with the stirring rod and crushing the compound with the rubber end. Repeat until compound is fully dissolved (make sure the solution does not rise over 250cm3)

Rinse the volumetric flask with distilled water then transfer the hydrated sodium borate solution into it. Make sure you rinse out the stirring rod, the beaker and the funnel with distilled water and pour this water into the volumetric flask to reduce chances of error (left behind solution).

Make the solution up to 250cm3. To do this make sure that when the bottom of the meniscus is nearing onto the mark at eye level, use the dropping pipette to make sure that you do not pass the mark.

Shake the volumetric flask several times (about eight times) by turning it all the way around and waiting for the solution to move downwards, then turning it back up, etc.

Method:

Titrate 25cm3 portions of the solution against standard 0.1 mol dm-3 HCl (aq).

Using the funnel, rinse the burette with the hydrochloric acid and fill it with the same solution. Do not forget to rinse and fill the tip. Record the initial burette reading in the 'Trial' column of the results table

Rinse the pipette with some of the hydrated sodium borate solution.

Rinse a 250 cm3 conical flask with distilled water

Using a pipette filler, carefully transfer 25.0 cm3 of the solution to a clean 250 cm3 conical flask.

Add one drop of screened methyl orange to the conical flask's solution

Place the flask on a white tile below the burette.

Run hydrochloric acid from the burette into the flask, with swirling, until the solution just turns gray. (First flask can be used as a trial run as the point needed will probably be passed the first time)

Record the final reading on the burette in the "final" section of the results table.

Rinse the conical flask with distilled water and then redo the titration until 3 constant readings are observed

Results:

Mass of bottle and contents before transfer, m1 22.13 ± 0.01 g

Mass of bottle and contents after transfer, m2 17.33 ± 0.01 g

Mass of hydrated sodium borate, m = (m1 - m2) 4.80 ± 0.01 g

Mass of sodium borate without water 2.54 ± 0.01 g

Amount of sodium borate, n = m/M 0.0126 Mol

Uncertainty in pipette reading = ± 0.1

Uncertainty in burette reading = ± 0.06

Uncertainty in volumetric flask = ± 0.15

Pipette Solution Hydrated Sodium Borate 0.0504 mol/dm3 25.00 cm3 ± 0.06

Burette Solution Hydrochloric acid 0.10 mol/dm3

Indicator 1 drop screened methyl orange

B4072- (aq) + 2H+ (aq) + 5H20 4H3BO3 (aq)

B CaVa = A CbVb

0.1 x 25.2 = 2 x C x 25

2.52 = C

50

Concentration of hydrated sodium borate = 0.0504 mol dm-3

N (moles of hydrated sodium borate) = CV

N = 0.0504 x 0.025

N = 0.00126

0.00126 Moles of hydrated sodium borate in 25 cm3

N = 0.00126 x 10

N = 0.0126 Moles of hydrated sodium borate in 250 cm3

Moles x relative formula mass of Sodium Borate = Mass of Sodium Borate

0.0126 x 201.22 = 2.54 grams

Mass of sodium borate = 2.54 grams

Mass of hydrated sodium borate - mass of sodium borate = mass of water

4.8 - 2.54 = 2.26 grams

Mass of water = 2.26 grams

Mass , = Number of moles of water

Relative Formula Mass of water

2.26 . = 0.125 Moles

18.02

Na2B4O7 H2O

Mass 2.54 2.26

Number of moles 0.0126 0.125

Ratio 1 9.9

Rounded ratio 1 10

Conclusion:

Through this titration we have concluded that the ratio of water to the sodium borate in hydrated sodium borate is 10 is to 1 therefore the formula for hydrated sodium borate is:

Na2B4O7xH2O

Where X = 10

Na2B4O710H2O

Evaluation:

The indicator solution (screened methyl orange) is green in alkali, gray or cloudy when neutral and purple in acid.

An example of something that could have been done better to improve the accuracy of this experiment is:

The indicator used was not exact, to show neutrality it turned gray and there are many shades of gray so there was a region where different volumes of acid could have been thought to be neutralizing the alkali. A more precise indicator should have been used or a certain shade of gray (with something to compare it to) should have been used to maintain consistency.