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Intro to Chemistry Chem1020 Lab. Identification of an Unknown Liquid. Chemistry Department Minneapolis Community & Technical College. Overview. Part I Introduction Part II Solubility determination Part III Density measurement Part IV Boiling point determination Part V Identification.

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Identification of an unknown liquid l.jpg

Intro to Chemistry Chem1020 Lab

Identification ofan Unknown Liquid

Chemistry Department

Minneapolis Community & Technical College


Overview l.jpg
Overview

  • Part I

    Introduction

  • Part II

    Solubility determination

  • Part III

    Density measurement

  • Part IV

    Boiling point determination

  • Part V

    Identification


Part i introduction l.jpg
Part I. Introduction

In this experiment, you will be given a pure unknown liquid and the goal is to determine which one of the eight known substances this mysterious liquid is most likely to be.

In order, you will determine its three physical properties (meaning that you are not changing its chemical composition throughout the experiment). They are:

  • solubility in water and ethyl alcohol,

  • density, and

  • boiling point.

    Then you will compare these properties with the properties of those of eight candidates. The best match will suggest the identify of your unknown liquid.

Make sure to record this unknown number on your data sheet.

Caution: All procedures must be performed in the fume hood. Make sure to cover the containers whenever they are moved out of the fume hood.


Part ii solubility l.jpg
Part II. Solubility

Your instructor will first demonstrate that when two liquids are mixed, how will it look like when they are “soluble” or “insoluble”.

Then you will determine the solubilities of your unknown liquid in water and ethyl alcohol, respectively.

Test tube 1:

20 drops of unknown

+

20 drops of water

Test tube 2:

20 drops of unknown

+

20 drops of ethyl alcohol

Soluble or Insoluble

Soluble or Insoluble


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Use the medicine dropper on your bench for the unknown liquid only.

Precautions!!!

Do not let the tip of any dropper touch the test tube, especially the inside. Put the dropper back to its original container after each use.

Before recording the observation, mix the two liquids by gently flicking the bottom of the test tube.

Dispose the wastes from both tubes into the designated waste container.


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Part III. Density liquid

  • You will use the same method as you did in the previous “Measurement” lab to determine the density of this unknown liquid.

  • Instead of a volumetric pipette, a 10-mL graduated cylinder will be used to accurately determine the volume of ~5 mL liquid.

  • Record the mass measurements as XX.XX ± 0.01 g.

  • Record the volume measurements as X.XX ± 0.01 mL.


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1. liquid

2.

3.

  • Determine and record the mass of an empty 10-mL graduated cylinder.

  • Use the medicine dropper to transfer approximately 5 mL of liquid into the cylinder.

  • Determine and record the volume of the liquid. (Often being forgotten)

  • Determine and record the total mass of cylinder and the liquid.

  • Pour the liquid into the large test tube on your bench for later boiling point measurement.

  • Repeat steps 2-5 for a second trial.

4.

5.


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Sample Calculation of Density liquid

Daverage= (D1 + D2 )/2 = (0.791 + 0.782)/2 = 0.787 g/mL

  • Since the difference between these two density determinations

    (0.791 – 0.782) = 0.009 g/mL is small enough (< 0.03 g/mL),

    a third trial is not necessary.

  • Follow the rules of significant figure calculation strictly.


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Part IV. Boiling Point liquid

Lower the test tube into a water bath sitting on a hot plate. Make sure all the unknown liquid is submerged in the water bath.

Drop a piece of white rock, called boiling chip, into the large test tube. The chip ensures a smooth boiling and prevents “bumping”.

1 cm

Turn the knob on the right to dial “6” and start heating.

Insert a split-hole stopper with a thermometer into the test tube which should have ~10 mL of the unknown liquid.


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Part IV. Boiling Point liquid

  • Watch the unknown liquid inside the large test tube closely. When there are abundant strings of bubbles arise from the boiling chip, read the thermometer immediately. Record this temperature as the boiling point in the form of XX.X ± 0.1 ºC. Waiting too long will boil off too much liquid resulting in inaccurate determination.

  • Raise the large test tube so that it is out of the water bath.

  • (Often forgotten)

  • Once cooled, dispose of the remaining unknown liquid and

  • the boiling chip into the designated waste container.


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Part V. Identification liquid

Fill in the table on p.4 of the protocol with data of your unknown liquid. See the example shown in blue.

First cross out unlikely candidates based on solubility.

The remaining four candidates unfortunately all have density around 0.79 g/mL, so we have to look for the one with a boiling point closest to 68.1 ºC , that of our

0.787

68.1

soluble

soluble

unknown. It looks like methyl alcohol has the closest one. We got a winner! Acetone has the second closest boiling point, so can be put down as the 2nd most likely candidate.

If our unknown had a boiling point of 61.0 ºC, then methyl alcohol and acetone would be equally possible. No conclusive determination can be made.


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Used small test tubes and medicine droppers

Used large test tubes and graduated cylinders

Vials with remaining unknowns


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