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Determination Of The Relative Atomic Mass Of Lithium

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Introduction

Determination Of The Relative Atomic Mass Of Lithium Assessed Practical 1 With the aim of determining the relative atomic mass of Lithium, two methods were used to measure this with greatest possible accuracy. > Reacting the Lithium with water and using the number of moles of H2 produced to calculate the number of moles of Lithium thus its relative atomic mass. > Titrating 0.1 molar HCl against the LiOH from method one to determine how many moles of HCl are needed to neutralise the LiOH, being able to measure the number of moles of LiOH thus calculating the relative atomic mass of the Lithium. Method One - Procedure 1. The required apparatus was set up as shown in the diagram below. 2. The Lithium was placed on a pan balance (having removed as much oil as possible), as this would need to be recorded for later calculations. 3. The measured mass of Lithium was placed in the conical flask and the bung put in place to collect the H2 gas produced. 4. Once the reaction was finished, a reading was taken from the measuring cylinder of the volume of H2 produced. ...read more.

Middle

and the titration repeated twice more for reliability and accuracy. Treatment of results - Method two Titration 1 Titration 2 Titration 3 Avg. Titre Start Vol. Of HCl 0cm3 0cm3 0cm3 End Vol. Of HCl 40.50cm3 40.55cm3 40.45cm3 Total Vol. Of HCl 40.50cm3 40.55cm3 40.45cm3 40.50cm3 The first titration was an approximate one where 0.50cm3 of HCl was added at a time to establish approximately what volume of HCl was required for neutralisation. Titrations two and three were carried out with greater precision so that a figure accurate to two decimal places could be obtained. To determine the R.A.M of Li from these results first the number of moles of HCl used and the balanced equation for the reaction must be examined so that it is possible to deduce the number of moles of LiOH involved in the reaction. LiOH(aq) + HCl(aq) --> LiCl(aq) + H2O(l) No. Of Moles = Vol(dm3) x Conc. No. Of Moles Of HCl = avg. titre x 0.100 M 1000 = 40.50 x 0.100 M = 0.004055 moles of HCl 1000 As there is a 1:1 ration of HCl to LiOH in this equation then in the 25.0cm3 portion of LiOH used there were 0.004055 moles of LiOH, to obtain the number of moles in the original 100cm3 this value must be multiplied by four. ...read more.

Conclusion

This would influence the input and consequently the output of the equation. The most efficient method to exclude this is to use a bung combined with a 'ladle'. The bung would be put in place and then the Li placed into the water thus allowing no gas to escape. Thirdly the problem of accurate reading of the measuring cylinder was encountered although it was possible to avoid this by filling the cylinder with air so the meniscus is at the uppermost measurement (rather than to the very top) so that the reading is as accurate as possible. It is further possible to remove this fault by collecting the gas in a gas syringe which can ensure no leaks and has much more refined degrees of measurement so readings are more accurate. Method two - Due to method two involving only titrating acid against the LiOH there are very few possibilities for error although human error must be taken into account when reading the level of the burette and visually checking for the point at which the solution turned from pink to colourless. This fault is almost impossible to eradicate. It was required that only 5 drops of indicator were added, this must be done carefully and accurately as it may affect the result if not. Determination Of The Relative Atomic Mass Of Lithium - Assessed Practical 1 January 2002 ...read more.

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