Density Determination for Solutions



Density Determination for Solutions

Background Information:

The density of a sample of matter represents the mass contained within a unit volume of space in the sample. For most samples, a unit volume means 1.0 mL. The units of density, therefore, are quoted in terms of grams per milliliter (g/mL) or grams per cubic centimeter (g/cm3) for most solid and liquid samples of matter. Because the density does in fact represent a ratio, the mass of any size sample divided by the volume of that sample, gives the mass that 1.0 mL of the same sample would possess.

Densities are usually determined and reported at 20 0C (around room temperature) because the volume of a sample, and hence the density, will often vary with temperature. This is especially true for gases, with smaller (but still often significant) changes for liquids and solids. References books and tables always specify the temperature at which a density was measured.

Density can also be used to determine the concentration of solutions in certain instances. When a solute is dissolved in a solvent, the density of the solution will be different from that of the pure solvent itself. Handbooks list detailed information about the densities of solutions as a function of their composition (typically, in terms of percent solute in the solution). If a sample is known to contain only a single solute, the density of the solution could be measured experimentally, and then the handbook could be used to reference the unknown solution based on density.

The determination of the density of certain physiological liquids is often important screening tool in medical diagnosis. For example, if the density of urine differs from normal values, this may indicate a problem with the kidneys secreting substances that should not be lost from the body. The determination of density (specific gravity) is almost always performed during urinalysis.

For liquids, very precise values of density may be determined by pipeting an exact volume of liquid into sealable weighing bottles (this is especially useful for highly volatile liquids) and then determining the mass of liquid that was pipeted. A more convenient method for routine density determination for liquids is to weigh a particular volume of liquid as contained in a graduated cylinder. It is this second technique that will be used in this lab.

The concentration of solutions is often expressed in terms of the solution’s percentage composition of a weight basis. For example, a 5% sodium chloride solution contains 5 g of sodium chloride in every 100 grams of solution (which corresponds to 5 g of NaCl in every 95 g of water present.

Purpose: The purpose of this lab is to …..ect.

II. Materials:

10.00 mL Graduated cylinder Hanging Pan Balance

Transfer pipets

NaCl solutions at: 5%, 10%, 15%, 20%, 25%

Unknown solution A

Unknown solution B

III. Procedure: (Goggles and aprons must be worn at all times in lab)

Part A: Determination of the density of the solutions

1. Obtain a clean, dry, 10.00 mL graduated cylinder and take mass.

2. Place about ½ full of 5% NaCl solution. Measure the mass of the graduated cylinder and the solution.

3. Measure the exact volume of solution. Make sure you dry your graduated cylinder before using.

4. Determine density of the solution.

5. Repeat steps 1-4 using the 10%, 15%, 20%, and 25% NaCl solutions.

6. Be certain to record the temperature of the lab room the day of the experiment.

7. Repeat step 1-4 using the two unknown solutions.

IV. Data and Analysis:

Organizing the Data

1. Using the data from your table, construct a graph of the density of your solutions of known concentration versus the percentage of NaCl the solution contains. This graph will only contain the data from 5% - 25%.

2. Using the graph from #1 plot the density of the two unknown concentrations. This should allow you to determine the concentration of NaCl in the solution of unknown concentration.

Name of Table: Solutions

| | 5% | 10% | 15% | 20% | 25% |Unknown |Unknown |

| | | | | | |“A” |“B” |

|Temperature | | | | | | | |

|(0C) | | | | | | | |

|Mass (g) | | | | | | | |

|Volume (mL) | | | | | | | |

|Density (g/mL) | | | | | | | |

Question Analysis:

1. What is your independent variable (x-axis)?

2. What is the dependent variable (y-axis)?

3. What sort of relationship exist between the density and the % composition?

4. What is the concentration of Unknown solution A?

5. What is the concentration of Unknown solution B?

V. Conclusion:

1. Your graph of density vs. concentration should yield a straight line. At what point would you expect this graph to intersect the y-axis, if all data were perfect? Why?

The Handbook of Chemistry and Physics reports that following specific gravities for NaCl solution at 200C.

|% by weight NaCl | 5% | 10% | 15% | 20% |

|Density | 1.0359 | 1.0726 | 1.1105 | 1.1498 |

2. Use the information above to construct another graph Percent Solution Vs. Density. After constructing the graph plot your X and Y values for unknown 1 and unknown 2.

3. What did you get for the density of unknown 1 and unknown 2?

4. Was your value the same as the value you got when you constructed a graph using the densities you calculated in Part A? Explain.

5. If not the same, what is your percent error?

% error = [ Experimental Value – Accepted Value ]

Accepted Value X 100

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