lab report molarity

Creating Solutions of Standard Molarity

Lauren Ostler Peter Cestrone CHEM Lab 1251L-023 10/14/14

Trusted by over 1 million members

Try Scribd FREE for 30 days to access over 125 million titles without ads or interruptions! Start Free Trial Cancel Anytime.





Introduction :

This experiment was completed to determine and record the differences a series of standard diluted solutions and a made series s eries of diluted solutions had in comparison to. This was done by applying the fundamentals of spectroscopy and the Beer-Lambert law to an unknown 2+

solution. The solution used, in this case, was Cu specifically copper (II) sulfate pentahydrate. The purpose was to determine the accuracy of these methods through experimentation. The hypothesis was that the methods used would p rovide accurate results. A solution is a homogeneous mixture of two or more substances (can be a solid, liquid, or a gas). A solution is made up of a solvent- the largest amount of substance present- and a solute+2

dissolved, smaller amount of substance present. For instance, the 0.2 M Cu solution was made by dissolving solid copper (II) sulfate pentahydrate into water, the entire mixture was the solution, the solid copper (II) sulfate pentahydrate was the solute, and the water was the solvent. When there was a high amount a mount of solute in the solution the solution had a high concentration; moreover, when there was a low amount of solute in the solution the th e solution had a low concentration. The type of concentration focused on in this experiment was molarity. The molarity (M) of (M) of a solution is the number of moles of solute over liters of solution.

When a solution is prepared calculations usually need to be made to calculate the amount of solute or the volume of solution. There Th ere are two ways to prepare a solution: 1. Solution of specified concentration prepared b y dissolving a solid (of a specified amount





2. Dilution Dilution is when a less concentrated solution is prepared from a more concentrated solution. When a solution is diluted the concentration is decreased, but it does not change the number of moles of solute in the solution. So, the number of moles in the concentrated solution is equal to the number of moles in the diluted solution.

The fundamentals of spectroscopy, or the science of measuring levels of spectra when

electromagnetic radiation is emitted, were used in this experiment ex periment by using a “spec. 20” or spectrophotometer. This machine was used to measure the amount of light that passed through the solution in the form of percent of light transmitted or %T. When an object has ha s color, such as blue, the blue light that was reflected was also seen and the object absorbed all other wavelengths of light color. The greatest absorbance of color was the objects complementary complementar y color. The more concentrated an object was the less light can be absorbed. For example, more light passed through the clear solution rather than the dark solution. When a solution is colored, like copper (II), the higher the concentration the darker the color. So, when the solution was

diluted the solution’s color was lighter because the solution had a lower concentration of colored solute in comparison to the solvent. The copper (II) concentrates used in this experiment ranged from blue to pale blue (cyan). It had the greatest absorbance of o f orange (which was its





The absorbance of the unknown concentration was found by using the equation from the line of best fit found in the Beer-Lambert plot of data. The Beer-Lambert law is the direct relationship relationship between the amount of light absorbed by the colored solution and the concentration of the sample. The calibration curve is attained when wh en graphing the experimental data to determine the concentration of the solution of the same substance of unknown concentration.

Procedure :

For the first part of the experiment the spec. 20 was calibrated and turned on 15 minutes 2+

before use then set to the wavelength for maximum absorbance of the Cu solutions (wavelength of 600 is the maximum). Ten small test tubes were cleaned and labeled before b efore the standard solutions and the blank solution of water were poured in. The spec. 20 was calibrated by placing the blank solution tube into the machine. Moreover, the blank solution placed in the machine had a 100% transmittance (which was regulated by b y turning the knob on the right so that the machine read 100% transmittance) and a 0% transmittance once the blank solution was removed and the cap was closed (which was regulated by turning the knob on the left so that the machine read 0% transmittance). The blank solution test tube was then placed into the spec. 20 again to make sure the machine was working properly. For the second step, the % transmittance was then recorded of all of the standard solutions (0.500M, 0.200M, 0.100M, and 0.050M CuSO4) and used to calculate the absorbance.





The third part of the experiment consisted of prepa ring the copper (II) solutions of different concentrations. In a 100mL beaker, 2. 5 grams of solid copper (II) sulfate pentahydrate were dissolved in about 15 mL m L of water. Once the solid dissolved by mixing the solution back and for between the beaker and a 50 mL graduated cylinder, a Pasteur pipette was used to add enough water into the solution until the meniscus met the 20mL mark on the graduated cylinder. This was the first solution. Part of the solution was poured into one of the 4 of the 5 remaining test tubes and the rest was used for dilution. The concentration of the next nex t solution was 2+

determined by using the dilution formula: 8 mL of the 0.5 M Cu solution was used to create the 2+

2+

2+

0.2 M Cu solution, 10 mL of the 0.2 M Cu solution was used to create the 0.1 M Cu 2+

2+

solution, and 10 mL of the 0.1 M Cu solution was used to create the 0.05 M Cu solution. These solutions were added to each test tube labeled in accordance to the concentration and placed into the spec. 20 to find the % transmittance. The fourth and final step was filling the final test tub e with the copper (II) sulfate solution of unknown concentration and finding the % transmittance using the spec. 20.

Results :

Concentrations of

%T

T

Absorbance

0.05

76.0

0.76

0.119

0.1

68.0

0.68

0.167

0.2

58.0

0.58

0.237

Standards (M)





+2

Table1: Shows the measured values of the known (standard) Cu solution obtained from the spec 20. The equation used to calculate the absorbance was A=2-log (%T) where A was the absorbance and %T was the percent of transmittance.

CALIBRATION CURVE OF STANDARD ST ANDARD COPPER(II) SULFA SULFATE TE SOLUTIONS 0.523

0.6 0.5 e c n 0.4 a b 0.3 r o s b 0.2 A 0.1 0

0.119

0

0.167

0.237

y = 0.8966x + 0.071 R² = 0.9975 0.1

0.2

0.3

0.4

0.5

0.6

Concentration (mol/L)

Figure 1: The Beer-Lambert plot shows the calibration curve of the absorbance and concentration of the solutions from table 1. The line of best fit was y= 0.8966x+0.071 where y was the absorbance and x was the concentration.

Concentration of the

%T

Absorbance

Actual Concentration

0.05

76.0

0.119

0.054

0.1

67.0

0.174

0.115

0.2

56.0

0.252

0.202

0.5

30.0

0.523

0.504

Made Solutions (M)







absorbance and %T was the percent transmittance. The equation used to calculate the actual concentration was y=0.8966x+0.071 when y was the absorbance and x was the concentration.

Solution

%T

Absorbance

Concentration (M)

Unknown

30

0.523

0.504

Table 3: This table shows the concentration of the given unknown copper solution obtained from the calibration curve and the equation y=0.8966x+0.071 from graph 1 where y was the absorbance and x was the concentration.

Solution Number

Concentration of

Concentration of

Calculated % Error

Standard Solution

Made Solution

1

0.05

0.054

8%

2

0.1

0.115

15%

3

0.2

0.202

1%

4

0.5

0.504

0.8%

Table 4: This table shows the comparison compa rison of the concentration values of the made concentration





3

0.237

0.252

6.33%

4

0.523

0.523

0%

Table 5: This table shows the comparison of the absorbance values of the made solutions and the absorption of the standard solutions as well as their % error b y using the equation

%error = │measured value - actual value│/ actual value * 100%

Discussion :

The purpose of this experiment was to determine d etermine the accuracy of methods used u sed through experimentation. The methods used included applying fundamentals of spectroscopy, the beerlambert law, and measuring concentration and dilution of the solutions. The data from the experiment showed the significance of how accurate the methods used were. The significance of the data shown in figure one from table one was depicted through the 2

calibration curve. The r value of 0.9975 showed that the data is practically linear. This means that the data of the standard solutions were accurate; moreover, the equation y= 0.8966x+0.071 is a reliable indicator of the standard solutions. From the eq uation the concentration of the unknown was found to be 0.5 M (shown in table 3). In table 4, the percent error of concentrations conc entrations of standard solution against the





In table 5, the percent error of absorbance of standard solution against the absorbance o f made solutions proved to vary from as little as 0 % to 6.33% error. These calculations were more accurate than those in table 4, 4 , however, table 4 was also a lso fairly accurate because of the variation in percent error. This variation also proves that there was evidence of error that could have resulted from the fault of the spec. 20, contaminated solutions, or the precision of measurements obtained by the graduated cylinder. The limitations of this lab include the small amount of time allotted for the experiment, the quality of the machine used (spec. 20), the limited precision of materials used for measurements (graduated cylinder), and the reliability of how cle an the materials used were. These limitations that may have been the cause of errors could have been avoided if the spec. 20 was in better quality, if all of the materials were reliably clean, and if a burette would have been used to measure the amount of water used rather than a graduated c ylinder. The strengths of this experiment were that there were standard solutions to compa re the made solutions to and that most of the experiment was quantitative so that measurements could be followed that helped improve accuracy.

Conclusion





could have resulted from the fault of the spec. 20, contaminated solutions, or the precision of measurements obtained by the graduated cylinder.

Sample Calculations :

The absorbance was calculated using the percent transmittance.

Example: 0.119 = 2- log (76)

Dilution calculations

Example: 0.5 * V = 0.2 (0.020) V= 8mL





Grams of copper(II) sulfate pentahydrate:

Example: 0.5M = moles/ 0.020 L Moles= 0.01 0.01 (249.6g/1mol) = 2.5g

Percent error

Example:

lab report molarity

Recommend Documents