CHM 130LL: Electrolytes Lab

Introduction
Electrolytes are substances that conduct electricity in solution. In this experiment, you will use a
conductivity tester to determine whether substances are strong, weak, or non-electrolytes. The
conductivity tester has red and green LEDs that will light up if a solution contains ions that will
conduct electricity. Ions can carry electricity across a solution between the electrodes.

A strong electrolyte will completely dissociate into ions in solution and will cause a strong or bright
light. A weak electrolyte will only dissociate to a small degree. Only a small percentage of the
compounds will dissociate into ions but most will stay together as intact molecules, and a weak light
will be seen. Non-electrolytes will not dissociate into ions at all and will not conduct electricity. It
is important to know the difference between ionic and covalent compounds in this lab. Ionic
compounds are generally metal cations bonded with nonmetal anions. Covalent compounds are
composed of nonmetal atoms covalently bonded together.

Background
The best everyday example of an electrolyte is Gatorade™ or any similar sports drink. When you
exercise, you lose electrolytes through sweat. In order to maintain normal cell function, it is crucial
that those electrolytes be replaced. Electrolytes are used by your body to regulate functions such as
heartbeat, brain function, and muscle control. The most common electrolytes that must be kept in
balance in your body are sodium, potassium, magnesium, calcium, chloride, and bicarbonate.

Sodium is responsible for regulating the electrical signals to your brain, muscles, and nervous
system. Most of your body’s fluid (outside the cells; e.g., blood and urine) is high in sodium. It is
possible to have too much sodium (hypernatremia) or too little sodium (hyponatremia) in your
system.

Potassium is the major electrolyte found in cells. One of the main roles of potassium is in the
regulation of your heartbeat and muscle function. It is common for athletes who are experiencing
muscle cramps to consume potassium (bananas are a great source). Like sodium, it is possible to
have increased potassium levels (hyperkalemia) or decreased potassium levels (hypokalemia).
Potassium is not lost as rapidly as sodium since it is inside cells.

Chloride is also a major electrolyte in

the body. Like sodium it is found
mainly in body fluids outside the cells.
Human body fluids have almost the
same concentration of chloride ions as
sea water. Bicarbonate acts as a buffer in
the body to help control the pH of your
blood and other fluids.

Figure 1 to the right illustrates the

difference between an electrolyte (left
picture) and a non-electrolyte (right
picture).
Picture from McMurry/Fay 5th edition,
Pearson
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Understanding solubility rules: Water is commonly used as a solvent in
solutions. When a substance is soluble in water, it is said to be aqueous. The
notation (aq) is used for the physical state and specifically means that substance
will dissolve in water. Solubility rules for ionic compounds are on your Periodic
Table. If an ionic compound will dissociate, or break into ions, it is soluble. If is
will not break into ions then it is said to be largely insoluble (very few ions
form.) When an ionic compound with a polyatomic ion dissociates and breaks
into ions, the polyatomic ion stays together. For example, in potassium sulfate,
K2SO4 which is soluble, the ions are two K+ and one SO42- ions. See the beaker above. Do not
separate the S and O atoms in sulfate ion.

Strong electrolytes: A strong electrolyte will completely dissociate (break

apart) into ions in solution and will cause a strong or bright light. Soluble ionic
compounds are strong electrolytes. One example is potassium fluoride (KF)
dissolved in water. If you check the solubility rules on your Periodic Table,
potassium ions are always soluble. This means that potassium compounds will
always break apart completely (100%) into ions in water. We can represent this
as shown in the picture to the right. (Also see the left image in Figure 1.)
Notice that once the solid is placed in water it breaks apart into separate ions that
have charges. These charges help us write the correct chemical formula for ionic compounds.
Strong acids and bases are also strong electrolytes.

Watch the movie at this link (http://www.wou.edu/las/physci/ch412/hydrolysis.htm) to see an

animation of sodium chloride dissolving in water.

Weak electrolytes fall between strong and non-electrolytes. Weak acids and
bases are the most common examples of weak electrolytes. Insoluble ionic
compounds are also weak electrolytes as they are very slightly soluble in water,
and dissociate into a few ions. As you might guess, these will usually give a
weak light when placed in a conductivity tester. The reason is that these
substances will dissociate into ions to a small degree (1-5%). Since most of the
compounds will stay together in molecular form, there aren’t as many ions
floating around in solution. Therefore, they can only conduct electricity to a
small degree, and that is why a weak light is seen. Notice in the drawing of
hydrofluoric acid on the right how only one molecule out of five (or 20%) is dissociated into ions.
In reality, most weak acids and bases only dissociate about 1-5%.

Watch the animation at this link to see a strong acid versus a weak acid ionizing in water.
http://www.mhhe.com/physsci/chemistry/animations/chang_2e/acid_ionization.swf

Non-electrolytes do not break apart into ions. These substances are usually
covalently bonded molecules (non-metal + non-metal like C6H12O6). Water
molecules are not strong enough to pull the compounds apart into ions and
therefore they will not conduct electricity. (See the right image in Figure 1.)
The image to the right illustrates what you would see if you could zoom in to
the atomic level. Notice that the molecules stay intact in the beaker: there are
no ions at all. They are not broken apart by water. Also notice that this is a
covalent molecule.

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Summary: In general:
Strong Electrolytes – strong acids, strong bases, soluble ionic compounds
Weak Electrolytes – weak acids, weak bases, insoluble ionic compounds
Non-Electrolytes – molecular or covalent compounds

Procedure: Half of the lab (in pairs) will measure electrolytes while the other half (again in pairs)
works on the post lab questions.

Part I – Determining strengths of electrolytes

1. Make sure the well plate is clean and dry. (It is crucial that the wells are clean and dry because
any contamination will give incorrect results.)
2. Notice that each row on the well plate is labeled with letters and each column is labeled with
numbers. This makes it possible to identify which substance is in which well (ex: A3 or C2). In
your data table (on the next page), write which well you will place each substance in.
3. For the solid salt (NaCl) and solid sugar (C12H22O11) use those bottles to carefully fill two wells
about ½ full of each solid making sure no crystals spill over into a different well. Do NOT add
water to these two solids. All the other substances you will test are already dissolved in DI
water, so they are aqueous solutions. For these solutions place 20 drops of each solution into a
well. You will test silver chloride solution in a separate vial at the instructor station, take your
conductivity tester to the instructor station – make sure you test the silver chloride in the brown
vial at the instructor station, do NOT pour it into your well plate.
4. Connect the 9-volt batter to the conductivity tester and make sure it is snapped into place (you
should hear a click when it is fully connected). Turn the tester “On” and notice the LED lights
are not lit. (When you are finished today make sure you disconnect the battery)
5. To test each solution or solid, place both electrodes in the well. Record your observations of
both LED lights in your data table. For a very strong electrolyte, you should see both red and
green LED’s light up. You may need to use your hand to shade the LED’s from the overhead
lights to be sure.
6. After testing one substance, turn the tester off, rinse the electrodes with DI water into a waste
beaker, and carefully wipe the electrodes with a Chem wipe. Now you may test the next one.
7. Save your substances until you are sure of your results. You might have to measure
conductivities several times to determine the appropriate value on the scale.
8. Once you have double-checked your LED lights, assign conductivity values and scale numbers
on your report sheet according to the table below. The back of the conductivity tester also shows
the conductivity reading based on the LED brightness.
9. When you are finished, dump your well plate into the waste jar. Then rinse it with tap water
followed by a little DI water. Dry the well plate as well as you can.
Use the following conductivity scale to rate each substance.
Red LED Green LED Conductivity Scale Electrolyte
Off Off Very low/none 0 Non
Dim Off Low 1 Non
Medium Off or Dim Medium 2 Weak
Bright Dim High 3 Weak
Very bright Medium Very high 4 Strong

Part II – Post lab questions. Draw the solutions based on the solubility rules, not based on your
observations today, as some solutions may give a false positive result due to CO2 from air dissolving
into the solutions creating ions.

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CHM 130LL: Electrolytes Name: ________________________________
Lab Report Partner: _______________________________
Section Number: ______________

Part I Data:
Well
Substance mixed with DI Location Red LED Green LED Conductivity Scale
water unless noted
Solid sodium chloride,(s)
Solid sugar, C12H22O11(s)
Tap water
Sodium chloride
Sugar, C12H22O11(aq)
Hydrochloric acid, HCl(aq)
Sodium hydroxide
Acetic acid, HC2H3O2(aq)
Potassium iodide
Calcium carbonate
Fructose, C6H12O6(aq)
Silver chloride Vial

1. Which of the substances in Part I of today’s lab are strong electrolytes?

2. Which of the substances in Part I of today’s lab are weak electrolytes?

3. Which of the substances in Part I of today’s lab are non-electrolytes?

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Post-lab Questions:

1. Briefly explain why soluble ionic compounds are strong electrolytes.

2. Briefly explain why insoluble ionic compounds are weak electrolytes.

3. Describe what types of chemicals tend to be each of the following:

Strong electrolytes: ________________________________________________________________
Weak electrolytes: _________________________________________________________________
Nonelectrolytes: ___________________________________________________________________

4. For each substance below, write the formula of the substance on the line and then draw what you
would see if could zoom in to the atomic level of the substance in DI water. (Similar to the drawings
on page 2.) Use solubility rules, not your experimental results, to determine solubility.

sodium chloride: _______________ sugar: __________________

DI water alone: ____________________ hydrochloric acid: ________________

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sodium hydroxide: ____________ barium bromide: __________________

silver chloride,______________ lead (II) iodide: __________________

potassium iodide,______________ calcium carbonate: __________________

fructose ,_________________ lithium sulfate: __________________

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5. For each of the following compounds, write the formula then predict whether it would be a
strong, weak, or non-electrolyte when placed in DI water. For the ionic compounds only, put (s) or
(aq) after the formula.

Formula Strong, weak or non electrolyte?

a. calcium hydroxide _______________________ _______________________

b. silver carbonate _______________________ _______________________

c. lead(II) sulfate _______________________ _______________________

d. phosphorus trifluoride _______________________ _______________________

e. sodium phosphide _______________________ _______________________

f. barium sulfate _______________________ _______________________

g. strontium acetate _______________________ _______________________

h. zinc nitrate _______________________ _______________________

6. Write the name for the following chemicals.

a. Na2SO4 ___________________________ e. Mg(NO3)2 ______________________________

b. KI _____________________________ f. AuCl3_________________________________

c. P2S5 _____________________________ g. Cu(NO2)2 ______________________________

d. N2O3 ____________________________ h. Li2SO3 ________________________________

7. Write the formula for the following chemicals.

a. Calcium phosphate _________________ e. Trinitrogen tetraoxide _____________________

b. Potassium sulfide __________________ f. Magnesium acetate _______________________

c. Carbon dioxide ____________________ g. Nickel(III) cyanide _______________________

d. Cobalt(II) chloride _________________ h. Silver sulfate ___________________________

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