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Determine the unknown concentration of three acid solutions using a standard known concentration of alkali.

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Introduction

Aim The aim of this experiment is to determine the unknown concentration of three acid solutions using a standard known concentration of alkali (which is also known as 'base'). The three acids that will be utilised in this practical are hydrochloric acid, ethanoic acid and sulphuric acid. The concentration of the acid will be determined by titrating the acid against the base. In this scenario sodium hydroxide will be titrated against the three acids named above. Theory In water, sodium hydroxide is a strong base. It separates into sodium and hydroxide ions. The most common strong bases are the water-soluble hydroxides that belong to sodium, potassium & lithium; all three can be referred to as ionic solids. A solution containing 0.5mol of dissolved NaOH per litre of solution will also contain 0.5mol dm-3 of OH- ions. This is due to the ratio being on a 1:1 basis. Please refer to the example below. NaOH Na+ + OH- When bases react with acids the reaction that takes place is known as neutralisation. In a 1:1 ratio of acid if there are 0.1 mols of hydrogen ions (H+) ...read more.

Middle

solution; the burette was filled (using a beaker) with the NaOH solution to a point between 0 and 1 on the scale. The middle of the meniscus was used to take a reading. The conical flask was placed below the burette. Using one hand to hold the conical flask, the other hand was used to control the tap of the burette. Slowly the tap was opened to deliver the alkali to the acid. As the alkali met the indicator in the acid, there was a slight colour change. The conical flask had to be swirled as the alkali was let into the acid. This was done so that the solution became thoroughly mixed. It was important to keep an eye on the flask as the colour could have changed at any time. As the end-point of the titration grew closer the colour in the conical flask would remain in the flask for longer length of time before disappearing. As the colour become more permanent, the addition of the alkali had to be slowed down (preferably to a few drops after a swirl). Until a permanent colour change was present the alkali had to be added carefully. ...read more.

Conclusion

1. HCl +NaOH NaCl + H2O (ratio = 1:1) 2. 0.1 x 24.1 = 2.41 x 10-3 x 1000 = 0.0964 mol dm-3 1000 25 3. CH3COOH + NaOH CH3COONa + H2O (ratio = 1:1) 4. Mol of NaOH = 0.1 x 25.05 = 2.50 x 10-3 = 0.1002 mol dm-3 1000 2 5. 6. H2SO4 + 2NaOH Na2SO4 + 2H2O (0.1 x 55.20 = 5.52 x 10-3) 7. Ratio = 1:2 � � � 2.76 x 10-3 x 100 = 0.1104 mol dm-3 25 Please note that as the ratio was 1:2, 5.52 had to be divided by 2. 8. 9. Contamination was one error source that had occurred; this was due to some people sticking the pipette directly into the acids, which meant that there could have been a chance that the acid had been contaminated. In the future this can be avoided by the proper use of a conical flask to transfer to the pipette. 10. Weak acid - substances capable of donating hydrogen but do not completely ionize in solution. Strong acid - an acid that completely dissociates to produce an H+ ion and the conjugate base 11. 12. ...read more.

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