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Pipetting, Weighing, Accuracy and Plotting

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The aim of this experiment was to determine the density of water and, of unknown solutions, by being as accurate as possible, by pipetting and weighing the solutions. Density if defined as “The mass per unit volume of substance; common unitg cm-3” (Lewis. R and Evans. W., 2011). As the density is an intensive property of all matter it depends on temperature and pressure. “The density of a mixture may also depend on its composition” (Nedelsky. L., 1983). Density of a matter can be calculated by: Density= Mass (g)Valume(cm3)=g cm-3

Pipette is a scientific tool, used in labs to drain off liquids to transfer it from a place/container to another for different purposes. There are different types of pipettes; ones which were used in this practical were serological and mechanical pipettes. Mechanical pipettes are volumetric pipettes which are used to transfer a specific volume of solution/liquid. The capacity of mechanical pipettes varies depending on the pipette itself. Serological pipettes are also measuring pipette which have gradients that continue down all the way in to the tip of the pipette. A pipette dispenser must be used to drain off liquid by serological pipette.

Mechanical pipette are used to measure small amount of liquids and are very accurate whereas serological pipette are used to measure large amount of solutions and are not as accurate as mechanical pipettes. During this experiments accuracy, consistency and precision was essential by recording the results in correct decimal places and significant figures and, by plotting the results on the right format of a graph. It was crucial to decide which type of pipette, either Serological or mechanical pipette to use for different amount of solutions, as one type of pipette, for example, P-200 could not be used to measure more than 200μl of solution. Therefore, one type of pipette could not be used to measure different amount of solution. Material

Water and the unknown solution, both were dispensed from different type of pipettes, serological pipette(P-200 to measure 50 μl, 100 μl and 200 μl and P-1000 for 1000 μl) and mechanical pipette (to measure 100 μl and 1000 μl). The solutions were weighed on electronic balance (which is proficient to weigh at least 0.001 gram) by pipetting the liquid into a plastic weighing boat. The readings were taken from the digital/ electronic balance in three decimal places. Methods

The practical was divided into three parts.
First part of the practical was about basic weighing using 1000 μl mechanical pipette and serological pipette to measure 1000 μl or 1 ml of water. 1000 μl of water were pipetted by serological pipette into the weighing boat on the scale three times, a value around 1.000 grams were obtained. The same procedure were repeated using P-1000 (1000 μl) mechanical pipette. The setting of the P-1000 mechanical pipette had to be on “100” before using it. The second part of the practical was about comparing different pipettors and their ability to pipette 100 μl. P-100, P-100 and serological pipettes were used and the experiment was repeated three times to maintain consistency and accuracy. The result was also compared to the lab partner’s result to maintain a good validity. The third part of the experiment involved measuring and pipetting water and an unknown sample with different type of pipettes and different amount of solution. The pipette has to be set to different amount according to how much solution needed to be measured. The experiments were repeated three times and the density of water and the unknown solution was calculated from these values. Another set of values were taken from another group which was experimented on a different sample, to calculate the density between two different unknown solution and water. Result

Table1
The table above is the results of ‘My data’ and ‘partners’ data result of pipetting of water. The values of both experiment is very similar which means there is a level of validity in the result.

Table2
The table above shows the result of the experiment for the unknown sample B. It states both sets of the result for ‘My data’ and Partners’ data. Again, the values of repeated experiments are very similar, and the standard deviations are very small, which shows both consistency and validity. Figure1. My data

Figure1. My data

Figure1 and figure2 shows the relationship between the weight (g) and volume (μl) of water and sample B. Figure1’s values were taken from ‘my data’ and figure2’s values were taken from ‘partners’ data. Both graphs show that the density of the unknown sample is higher than water as the lines on the graph for sample B is above the line for water. Both graphs are very similar, which concludes that the results have a high level of validity and accuracy.

Figure1 and figure2 shows the relationship between the weight (g) and volume (μl) of water and sample B. Figure1’s values were taken from ‘my data’ and figure2’s values were taken from ‘partners’ data. Both graphs show that the density of the unknown sample is higher than water as the lines on the graph for sample B is above the line for water. Both graphs are very similar, which concludes that the results have a high level of validity and accuracy.

Figure2. Partner’s data

Table3.

The table above shows the result which were taken from another group. An anonymous result were spotted and highlighted. It will be ignored and will not be used. Figure3.

The graph above shows the relationship between sample, sample B and water. Water and sample has the same density and sample A has a higher density. Discussion

This experiment determined pipetting, weighing, accuracy, plotting and precision. Part1 of the experiment shows that the mechanical pipettor is more accurate than the serological pipettor as the average values of the mechanical pipettor is close to 1000 μl then it is for serological pipettor. This conclusion can also be supported by the standard deviation, hence, the standards deviation for mechanical pipettor is smaller than it is for serological pipettor, meaning it has a greater central tendency and the data is more concentrated around the mean. Part2 of the experiment involved using P-200 pipettte and its ability to pipette 100 μl. Again, the result shows that the mechanical pipettor is more accurate and precise than serological pipettor.

Part3 of the practical determined the density of the solutions. The results for the unknown sample C was taken from another group and compared with the primary result of water and unknown solution B. From figure3, it can be concluded that the solution with highest density is sample A, with 1.96×10-3 g cm-3. Water and Sample B has similar density with 1.08×10-3 g cm-3 and 1.02×10-3 g cm-3respectively. The experiments were carried out with no control over the temperature of the solution which means that the density of both solutions could have been affected by the temperature. Lastly, the values of both people’s data were closely similar, concluding that the result has a high level of precision, reliability and accuracy. The data were gathered in three decimal places to improve its accuracy.

References

Lewis. R. and Evans. W. (2011). Chemistry, P5-6. United Kingdom. Palgrave Macmillan. Parker. P. S. (1983). McGraw-Hill Encyclopedia of Chemistry, P505. Philippines. McGraw-hill, Inc.

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