Experiment #2: Measuring Volume and Weight



Ateneo de Manila University

School of Science and Engineering

Department of Chemistry

General Chemistry I Laboratory (Ch 8)

Ms. Joanna Maria N. San Pedro

Experiment #1: Measuring Volume and Weight

MODIFIED PROCEDURE

This modified procedure is outlined for the convenience of everybody. For the purpose of uniformity, a data table is already supplied at the end of this modified procedure. All data will be inputted in an Excel® database for easy counter-checking of results. Be sure to answer all the questions regarding the procedure (the why questions) in the results and discussion part of the post lab.

Part I. DENSITY OF PURE ACETONE/WATER AND MIXTURES (with different %Acetone composition). Do everything in duplicate.

A. For pure water and pure acetone:

1. Obtain the dry weight of the 50-mL Erlenmeyer flask and 10-mL graduated cylinder.

2. In the graduated cylinder, add water up to the 1 mL mark. Record the weight.

3. Fill up to the 8 mL mark. Record the weight.

4. The mass of water can be calculated by subtracting the weight of the cylinder containing 1 mL water from the weight of the cylinder containing 8 mL water. In effect, we are removing the amount of water occupying the graduated cylinder below the 1 mL mark.

5. You can calculate for density right away or proceed to your mixtures. Use the formula for density below:

Where,

Mf is the final mass of soln + mass of grad cyl

M1mL is mass of g.cyl. + 1mL solution

Vf is the final volume reading.

6. Repeat steps 2 to 5 for acetone.

B. For 25% acetone – 75% water mixture:

1. Measure 6.0 mL of water in the graduated cylinder (make sure you dried it first). Transfer it to your dry flask and weigh.

2. Measure 2.0 mL of acetone in the DRY graduated cylinder. Transfer it to the same flask and weigh again.

3. Cover the mouth of the flask with a watch glass to minimize the evaporation of acetone (since it’s very volatile). Swirl your solution so that your density will be uniform throughout the mixture.

4. Transfer 1 mL of the mixture to your DRY graduated cylinder, and weigh. Transfer the rest of the liquid mixture and weigh again. Obtain the final volume reading. Take note that volumes are not additive, so you DO NOT get a volume exactly 8 mL. Do you expect to get a volume above or below 8 mL? Why? (Assuming you measured almost exactly 2.0 ml and 6.0 mL for acetone and water respectively).

5. You will not be able to transfer all the liquid into your graduated cylinder. Will this affect your results? Why? Find the density and average concentration (expressed as %acetone) of your mixtures. Also, write the formula for % acetone using your data parameters. Obtain the average value for your density and %acetone.

Where,

Mfl+acet +water= mass of flask + acetone + water

Mfl, H2O = mass of the dry flask + water

Mfl+acet+H20 = mass of flask, acetone and water

C. For 50% acetone – 50% water

Repeat steps 1 to 5 except that you will use 4.0 mL each of water and acetone.

D. For 75% acetone – 25% water

Repeat steps 1 to 5 except that 6.0 mL of acetone and 2.0 mL of water will be employed.

Part II. OBTAINING THE DENSITY OF A MIXTURE WITH AN UNKNOWN %ACETONE COMPOSITION. Do everything in duplicate also.

1. Get your unknown from the stock room. Don’t forget to bring a medium-sized test tube.

2. Measure density in the same way as you did for the pure components.

Part III. FINDING THE %ACETONE COMPOSITION OF THE UNKNOWN.

1. Plot your data points in a graph of average density (y-axis) vs. average % acetone (x-axis). A guide on how to deal with graphs using Excel® is already available at the Reserved Section of the Rizal Library.

2. Be sure to display the equation of the line (y = mx + b) and the linearity coefficient (R2) on your data scatter plot. Is your slope greater or less than zero? Why?

3. Given the average density of your unknown, use the equation of your line to calculate for the % acetone of your unknown.

Other Questions:

1. How can you use your data to predict the density of a 40/60 acetone/water mixture? What conditions should hold for you to be able to do this?

2. Using your graph, can you predict the average densities at another temperature?

Data:

|  |  |D A T A |

| |Empty weight of flask

(g) |Empty weight of grad cyl (g) |Wt. Of flask + water (g) |Wt. Of flask + water + acetone (g) |Wt. grad cylinder + 1 mL

(g) |Final wt. grad cyl. + liquid (g) |Final Volume of Liquid (mL) | |Water |Trial 1 | | |  |  | | | | | |Trial 2 | | |  |  | | | | |Acetone |Trial 1 | | |  |  | | | | | |Trial 2 | | |  |  | | | | |Acetone/H20 25/75 |Trial 1 | | | | | | | | | |Trial 2 | | | | | | | | |Acetone/H20 50/50 |Trial 1 | | | | | | | | | |Trial 2 | | | | | | | | |Acetone/H20 75/25 |Trial 1 | | | | | | | | | |Trial 2 | | | | | | | | |UNKNOWN |Trial 1 | | |  |  | | | | | |Trial 2 | | |  |  | | | | |

-----------------------

Mf - M1mL

D =

Vf -1mL

Mfl+acet+H2O – Mfl+H2O

% Acetone =

Mfl+acet+H20 – Mdry,fl

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