Preparing Salts

Ira Gupta

X - B

Chemistry Lab report

Preparing salts

Aim: To carry out a neutralization reaction involving a certain volume of a known concentration of acid (sulphuric acid) and an alkali (sodium hydroxide) to determine its concentration.

The equation: H2SO4 + 2NaOH Na2SO4 + 2H2O

Apparatus:

A burette (±0.05cm³) and its holder
2 measuring cylinders (±0.5cm³)
Sodium hydroxide of unknown concentration
Sulphuric acid of conc. 0.1mol/dm³
Phenolphthalein and universal indicator
A beaker
A filter funnel
Digital balance (±0.005g)
A crucible or dish to allow evaporation
Tripod stand
Bunsen burner

Procedure:

Measure out 10cm³ of sulphuric acid of concentration 0.1mol/dm³ and pour it into a beaker.
Setup a pipette above the beaker and pour 50cm³ (till it reaches the zero mark) of sodium hydroxide of unknown concentration in it. You may use a filter funnel while pouring the alkali into the beaker.
Add few drops of phenolphthalein to the acid in the beaker. It will be colourless.
Now, start adding small drops of sodium hydroxide to the beaker with acid under the pipette.
With every drop that falls into the beaker, there will be a ‘pink splash’ and then it will disappear. Keep swirling the beaker for thorough mixing.
Carefully observe the drops falling because when the drop of sodium hydroxide does not create a pink splash that is when the neutralization has happened. Or when the solution suddenly turns pink, it can be assumed that alkali just became surpassed the amount needed to neutralize the acid and one drop before was when neutralization was complete.
After making this observation note the amount of alkali used up to guess its concentration.
If time allows, the experiment may be repeated with universal indicator to reinforce results.
After establishing the concentration of alkali and thus the exact volume needed, repeat the experiment without the universal indicator.
After doing that, heat the neutralized solution obtained to obtain dry sodium sulphate.
Measure the mass of the sodium sulphate obtained by scraping it off the crucible if necessary and calculate the percentage yield of the salt by comparing it to the theoretical values that you work out once the concentration is established.

Modifications:

Using both indicators to ensure accuracy and diversity in experiment.
Doing the experiment thrice, with and without the indicator.

Safety Procedure:

Handle the acid and alkali very carefully as these are corrosive.
Handle pipette with care as it is very breakable.
Do not leave the neutralized solution burning to obtain salt without supervision.

Experimental Setup:

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Modifications:

Using both indicators to ensure accuracy and diversity in experiment.
Doing the experiment thrice, with and without the indicator.

Safety Procedure:

Handle the acid and alkali very carefully as these are corrosive.
Handle pipette with care as it is very breakable.
Do not leave the neutralized solution burning to obtain salt without supervision.

Experimental Setup:

Observation:

Qualitative: When sodium hydroxide drops fell into the acid, brief change in colour from transparent to pink could be seen where it fell, but as the beaker was swirled it disappeared. The drop that signified neutralization did not cause this brief colour change and thus in my opinion the results were very close to accurate.

However, in one of the experimental trials with phenolphthalein, the volume of sodium hydroxide used was measured when the solution turned pink (signifying alkalinity). This presents the possibility that the alkali volume was about a drop more than needed and thus the result of 21 cm³ as compared to the other more accurate readings.

Quantitative:

Table 1: Table showing the amount of sodium hydroxide used to neutralize 10cm³ of sulphuric acid completely for three different trials.

From the pattern of the results obtained, it could be easily deduced that the concentration of the sodium hydroxide was also 0.1mol/dm³ as the volume is double the volume of sulphuric acid and if the equation is referred to, it is obvious that the concentration is same as the acid.

However, to neutralize 10ml of the acid, the average of the results obtained for the alkali, which is 20.5, was used.

Table 2: Table showing the mass of salt obtained from heating the neutralized solution without the indicator.

Calculation of theoretical yield of salt:

1 mole of acid gives 1 mole of salt

∴ Concentration (mol dm-3) × volume (dm³) = no.of moles of acid

∴ 0.1 × 10/1000 = 0.001 mol

∴ 0.001 × 2(23) + 32 + 4(16) = 0.142g (theoretical yield of salt)

(formula mass of sodium sulphate)

Table 3: Table showing the percent yield of each of the trials in terms of mass of salt obtained

Conclusion: Thus, it can be concluded that the concentration of the sodium hydroxide given to us was 0.1mol/dm³ and the average percentage yield of salt from the trials proved to be quite low, 39.9%.

Evaluation: Overall, the experiment was successful at helping the determination of the concentration of sodium hydroxide. However, the yield of salt was very low and there may have been many reasons for that.

Why the yield was low: The yield may have been low because the acid and the alkali may not have reacted completely. There may have been impurities that did not react.

How to improve: The reactants should have been swirled very thoroughly to allow complete reaction to take place. A stir could have been used too. Try to obtain purer samples of the acid and alkali.

Otherwise, some things that could have been done to make the experiment to make it more accurate: