Lab 7: Finding Moles and Molecules

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In this lab we are going to get more comfortable using moles by the determining the mass of some everyday substances, and then use this mass to calculate the numbers of moles and molecules/atoms the substance contains. We are also going to investigate electrolysis, a process that can be used to experimentally determine the value for mole (Avogadro’s number), as well as to demonstrate the molecular formula of water.

Experimental Title: Lab 7:  Investigating Moles and Numbers of Atoms/Molecules in Common Substances

Date of laboratory:  July 15, 2014

Purpose: The purpose of this laboratory is to determine the mass in grams, number of moles, and the number of molecules/atoms in small amounts of everyday substances. It is also to investigate electrolysis of water.

Introduction:

It is possible to relate to the mass of an object or substance, because it is tangible. You can feel how much mass a rock or a carrot has. Chemists often must have an idea how many atoms or molecules a substance contains, however, which is a number that is much more difficult to conceptualize.

The mole is used in chemistry to help make that transition. One mole of anything is 6.022 x 1023 items of that substance. It allows the chemist to use atomic masses from standard periodic tables (the mass of a single atom of an element is expressed in atomic mass units or amu, where 1 amu is 1/12 the mass of an atom of C12), simply by changing the unit from atomic mass units to grams. For example, one atom of hydrogen has an average mass of 1.008 amu, whereas one mole of hydrogen atoms has an average mass of 1.008 g. Wasn’t that easy?

Mole or Avogadro’s number = 6.022 x 1023

How did chemists arrive at Avogadro’s number? One way to do so experimentally is to examine the electrolysis of water. This video gives a quick explanation of how electrolysis can be used to supply data for the calculations:

The reaction for the electrolysis of water is
2H20 → O2 + 2H2

 

Special safety concerns for Lab 7:

  • If anything spills, please clean it up immediately with a paper towel and let your instructor know.
  • If glass breaks, do not pick it up with your bare hands. Notify your instructor immediately.
  • Be sure to wash your hands when you are finished with this lab
  • Avoid touching the battery with wet hands and don’t touch both terminals at once to avoid shocks.

Materials:

  • Empty aluminum can
  • Aluminum foil
  • Copper sample
  • Baking soda box with label
  • Magnesium sulfate
  • Sodium chloride
  • Sidewalk chalk
  • 9 Volt battery
  • Plastic cup
  • 2 metal tacks
  • 2 test tubes
  • Sink or tub to fill test tubes in
  • Rubber bands
  • Water
  • Graduated cylinders
  • Table top scales
  • Transfer pipette
  • Sharpie pen
  • Calculator
  • Periodical Table of the Elements with Atomic Mass listed

Procedures:

Note:  You can do the parts in any order, so go ahead to another part and come back to finish if you need to do so. No need to wait for materials.

Part 1. Moles, mass and number of Atoms: elements

Element:  Aluminum

1. Find the mass of one mole of aluminum using the value of atomic mass listed on the periodic table.

Value:

 2. Obtain an empty aluminum can. Weigh the can on a balance and record its mass in grams.

Mass:

3. Calculate how many moles of aluminum are in one aluminum can.

Number of moles:

4. Create a piece of aluminum foil that weighs as much a one mole of aluminum, using the balance. Save this sample to compare with other examples of one mole of a substance.

Element:  Copper

1. Find the mass of one mole of copper using the value of atomic mass listed on the periodic table.

Value:

2. Obtain a sample piece of copper. Weigh on a balance and record its mass in grams.

Mass:

3. Calculate how many moles of copper are in the sample.

Number of moles:

4. Using the number of moles, calculate the number of copper atoms found in the sample piece.

Number of atoms:

Element:  Sodium

1. Find the mass of one mole of sodium using the value of atomic mass listed on the periodic table.

Value:

2. Obtain a baking soda box. Looking at the label, how many mg of sodium are in one serving of baking soda?

Mass:

3. Calculate how many moles of sodium are in a single serving of baking soda.

Number of moles:

4. Using the number of moles, calculate the number of sodium atoms found in one serving of baking soda.

Number of atoms:

5. If a recipe for biscuits calls for 3 teaspoons of baking soda, how many moles of sodium are in the biscuits?

 

Part 2. Moles, mass and number of Molecules: Compounds

Compound:  Water

1. Find the mass of one mole of water using the value of atomic masses listed on the periodic table.

Molar mass of water:

2. Tare a graduated cylinder. Add 10 mL of water and weigh.

Mass of 10 mL of water:

3. Calculate how many moles of water are in 10 mL.

Number of moles:

4. Using the number of moles, calculate the number of water molecules found in 10 mL.

Number of molecules:

5. Weigh out one mole of water using a graduated cylinder and a scale. Compare the relative size/volume to one mole of aluminum (foil). Save your sample in a cup or beaker to compare to other examples of one mole of a substance.

Compound:  Sodium chloride

1. Find the mass of one mole of sodium chloride using the value of atomic masses listed on the periodic table.

Molar mass of sodium chloride:

2. Obtain a sample of sodium chloride. Determine the mass of the sample in g using the scale.

Mass of sodium chloride sample:

3. Calculate how many moles of sodium chloride are in the sample.

Number of moles:

4. Using the number of moles, calculate the number of sodium chloride molecules found in the sample.

Number of molecules:

5.How many individual ions (both cations and anions) are in the sample?

6. Weigh out one mole of sodium chloride. Compare the relative size/volume to one mole of aluminum (foil) and one mole of water.

 

Compound: Sidewalk chalk

Determine how many moles of sidewalk chalk it takes to write your name on the sidewalk. Note: Sidewalk chalk is made out of calcium sulfate (CaSO4) rather than the calcium carbonate (CaCO3) used for chalkboard chalk.

1. Find the mass of one mole of calcium sulfate using the value of atomic masses listed on the periodic table.

2. Weigh a piece of sidewalk chalk.

Weight of chalk before writing your name:

3. Go outside and write your name on the sidewalk. Come back inside and weigh the chalk again.

Weight of chalk after writing your name:

Grams of sidewalk chalk required to write your name:

How many moles of calcium sulfate was needed to write your name?

test-tubes

Part 3. Electrolysis of Water

1. Obtain a plastic cup, 9 volt battery, 2 metal tacks, a Sharpie pen, 2 test tubes and some magnesium sulfate or Epsom salts. (Baking soda would also work.)

2. Press the 9 volt battery against the bottom of the cup and mark where the terminals touch.

3. Press the tacks through the bottom of the cup at those points, so the terminals of the battery can touch the flat tops of the tacks and the points are projecting into the cup.

4. Fill the cup about 3/4 full with water. Add a teaspoon of Epsom salts and stir.

5. Take the apparatus to the sink. Fill the sink with enough water to be able to submerge the test tubes. You will want to put your thumb over the filled test tube under water and then transfer the tube to the cup so the open end is submerged and no bubbles are trapped in the top. Hold the tube upright.

6. Repeat with the second test tube. Now position the two tubes so that one is over each thumbtack. Wrapping the testube with a rubber band wwill help keep them together and upright.

7. While one person holds the cup and tubes, lift it up so the second person can touch the battery to the two tacks.

Record your observations:

This video from ScienceFix shows the setup in the first part.

He shows it sitting by itself, but you will probably need to at least hold the test tubes upright.

Conclusions:

Once you have completed the parts, sit down and write a sentence or two to explain the results of each.

Discussion:

Record any thoughts you have about the experiments, including:

  • Possible improvements to the procedures or how to tweak techniques
  • How the results differed from your expectations
  • Suggestions for other experiments
  • What key concepts you learned about mass, moles, numbers of molecules and the electrolysis of water

Please leave a comment or send an e-mail if you have any questions before our meeting.

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