Determination of Specific Gravity and Density of a Solution

The objective of this lab is to determine the specific gravity and density of an unknown solution using a specific gravity bottle, also known as a pycnometer. The lab focuses on the use of tools and equipment, safe laboratory practices, understanding chemical composition, and recording of experimental results.

Density is defined as the mass per unit volume of a substance and is commonly expressed in grams per milliliter (g/mL). Specific gravity, on the other hand, is the ratio of the mass of a substance to the mass of an equal volume of a reference substance, usually water, at a constant temperature. Specific gravity is a dimensionless quantity.

The specific gravity bottle, or pycnometer, is a specialized glass bottle used to measure the specific gravity of liquids. In this lab, the specific gravity of the unknown solution will be determined by comparing its mass to the mass of an equal volume of water.

The lab provides the equations and calculations involved in determining specific gravity. Equation (2) shows the relationship between specific gravity, the mass of the solution, and the mass of water. Equation (4) relates specific gravity to the densities of the solution and water, while equation (5) shows that at the same volume, specific gravity is the ratio of the density of the solution to the density of water.

The procedure for the lab is outlined step by step. It begins with ensuring that the specific gravity bottle and plug are clean and dry before weighing. The empty bottle is weighed multiple times to calculate the mean weight of the bottle. The bottle is then filled with distilled water, excess water is blotted, and the weight is recorded. This rinsing step is repeated two more times.

Next, the bottle and plug are rinsed thoroughly with the unknown solution, discarding the washings. The bottle is filled with the unknown solution, excess solution is blotted, and the weight is recorded. This step is repeated two more times.

Finally, the bottle and plug are rinsed thoroughly with distilled water, and they are returned to the stockroom. It is important not to leave the plug in the bottle.

By conducting this lab, students gain experience in using a specific gravity bottle and analytical balance, practicing proper rinsing techniques, and calculating specific gravity and density. They also learn about the relationship between mass, volume, density, and specific gravity, and how to apply these concepts to determine the properties of an unknown solution.

Objective

·         Use the tools and equipment necessary for basic scientific analysis and research

·         Demonstrate safe practices in the Chemical Laboratory

·         Demonstrate an understanding of Molecules, Compounds and Chemical Composition

·         Demonstrate the proper use of Exponential Notation and Significant Figures

·         Record the results of investigation through writing

·         Use scientific reasoning to evaluate physical and natural phenomena

·         Identify the unifying themes of the scientific field of study

Materials

·         Pycnometer Bottle

·         Unknown Solution

Discussion

In the previous experiment, we determined the densities of solid zinc and aluminum. Similarly, the density of liquids can be determined experimentally. However, measuring the volume of a substance remains a challenging task, just as it is with solids. Density is defined as the mass per unit volume and is measured in grams per milliliter (g/mL). The equation representing density is as follows:

(1)                                                                             d = M/V

Specific gravity, on the other hand, is defined as the ratio of the mass of a substance to the mass of an equal volume of a reference substance, typically water, when the temperature is constant. Since specific gravity is a ratio of masses, it is a dimensionless quantity. To determine the specific gravity of an unknown solution, a specific gravity bottle (Figure 5) can be used conveniently. The procedure involves determining the weight of the empty bottle, followed by filling it successively with water and the unknown solution. The weight of each substance occupying the bottle is then determined.

The specific gravity can be calculated using the following equation:

    (2)                                               Specific gravity = (M solution/MH2 O (equal volume))

    (3)                                                        From Equation (1)       M = (d) (V)

By substituting the mass and volume equations (d = M/V) into the above expression, we obtain: 

    (4)                                                 Specific gravity = (((d solution)(V solution))/((d water)(V water)))

From this equation, it becomes evident that when the volumes of the two solutions are equal, the specific gravity of the solution is the ratio of the density of the solution to the density of water (at the same temperature):

   (5)                                                    Specific gravity = ((d solution)/((d water))

It's important to note that at the temperature where the density of water equals one (at 4°C), the specific gravity and density of the solution have the same numerical value. However, at all other temperatures, the specific gravity and density are not equal.

Procedure

In this experiment, the objective is to determine the specific gravity and density of an unknown solution. The following steps should be followed:

1.   Obtain a specific gravity bottle, also known as a pycnometer.

2.   Ensure that the bottle and plug are clean and dry. Before weighing, wipe the outside of the bottle with a towel. Weigh the bottle three times using an analytical balance to calculate the average weight of the empty bottle.

3.   Completely fill the bottle with distilled water, insert the plug, remove any excess water, and weigh it. Empty the water and repeat this rinsing step two more times.

4.   Rinse the bottle and plug thoroughly three times using the unknown solution. Discard the washings after each rinse. To do this, fill the bottle halfway with the unknown solution, insert the plug, and shake it. Fill the bottle with the unknown solution, insert the plug, and weigh it. Empty the unknown solution and repeat this process two more times.

5.   Rinse the bottle and plug thoroughly with distilled water, and then return them to the stockroom. Ensure that the plug is not left in the bottle.

Report Sheet for Determination of the Specific Gravity and Density of a Solution

Name_______________________________________                          Date___________________________

Lab / Section_________________________________

Unknown Number________________

Questions for determination of the Specific Gravity and Density of a Solution

State the purpose of this lab.

1.   When does the specific gravity of a solution equal the density of a solution?

2.   Define Density and provide its formula.

3.   When determining the density of the unknown, it’s not critical to keep the bottle and plug clean and dry. True  /   False

4.   Define Specific Gravity

5.   List the procedure taken in the laboratory to ultimately determine the specific gravity of your unknown.

6.   Identify the unknown?

Review Questions

1.       What is the objective of the lab?

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2.       Name three tools or equipment used in the lab.

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3.       Explain the concept of density and how it is calculated.

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4.       Why is it important to demonstrate safe practices in the chemical laboratory?

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5.       What are significant figures and why are they important in scientific calculations?

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6.       Describe the difference between the analytical balance, triple beam balance, and top loading balance.

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7.       What are the major features of an analytical balance? How should it be handled and maintained?

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8.       Explain the difference between determinant (systematic) errors and random errors.

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9.       How can you determine the volume of a regularly shaped solid object?

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10.    What is the purpose of the water displacement method in determining the volume of a solid object?

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11.    How is the density of a substance related to its mass and volume?

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12.    What is specific gravity and how is it different from density?

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13.    Describe the procedure for using a pycnometer to determine the specific gravity of a liquid.

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14.    Why is it necessary to rinse the pycnometer with water and the unknown solution multiple times?

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15.    How can the specific gravity of a solution be calculated using the densities of the solution and water?

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16.    What is the significance of the slope of a graph in relation to the density of a substance?

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17.    How can you distinguish one pure substance from another using density measurements?

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18.    Why is it important to accurately measure the dimensions of solid objects in determining their density?

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19.    How can error analysis help in evaluating experimental data and improving accuracy?

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20.    What are the potential sources of error in the determination of density or specific gravity?

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