Losing Your Marbles Lab



“Losing Your Marbles” Stoichiometry Lab- Working with “Molarity”:

Reacting Marble Chips (Calcium Carbonate: CaCO3)

With an Aqueous Solution of 1.00 M Hydrochloric Acid.

In order to better understand the terms:

“excess reactant”, “limiting reactant”, “theoretical yield”

Two chemical reactions will be taking place simultaneously:

Double Replacement: 1 CaCO3 (s) + 2 HCl (aq) ( 1 CaCl2 (aq) + 1 H2CO3 (aq)

Decomposition: 1 H2CO3 (aq) ( 1 H2O (l) + 1 CO2 (g)

Overall:

1 CaCO3 (s) + 2 HCl (aq) ( 1 CaCl2 (aq) + 1 H2O (l) + 1 CO2 (g)

Introduction:

Calcium carbonate is the predominant mineral in limestone and marble. As you know, marble has many indoor and outdoor decorative and structural uses; additionally, many of our countries historic monuments are made of marble.

Rain water in the northeastern U.S. is mildly acidic due to the reaction between the water itself and several gaseous industrial pollutants. This “acid rain” exposes our limestone and marble monuments to a mild acidic solution, and is a major contributor to the detrimental “weathering” of these monuments. By doing today’s lab, you will see how limestone and marble are affected by an acidic solution as you measure the mass of CaCO3 before and after exposing it to an aqueous HCl (hydrochloric acid) solution whose “molarity” (concentration expressed as “moles per liter”) is 1.00 M (pronounced “molar”).

The calcium carbonate that you start with today is an “excess” reactant. This means that, on purpose, the mass you start with is more than you need to react with all of the HCl that you will use. The HCl will, on purpose, get all used up! A reactant that on purpose gets all used up (or used up first) is called a “limiting” reactant.

At home if you are using a recipe (for example, to make cookies), you usually try to plan ahead so that you don’t run out of something (for example, sugar) – BUT, you could also decrease the amounts of all your other ingredients, and plan on making fewer cookies. Intuitively you know to re-calculate the number of cookies that you expect to produce if one ingredient will run out.

In chemistry, the similarity is that we have to re-calculate the amount of product that we expect to produce when we know we will run out of an ingredient. Whatever chemical we run out of first, is the chemical that we use in order to calculate our expected amount of product.

The “expected amount of product” is the “math answer” (from your calculations or your recipe); and in chemistry, we call it our “theoretical yield”. It doesn’t matter if the expected amount is a number of atoms or molecules, a number of grams, or a number of liters….We still just call it the “theoretical yield”.

The amount of product that we actually produce is called the “actual yield”. As an analogy, you might expect to make 4 dozen cookies… that is your theoretical yield. If you only actually make 3 dozen cookies… that is your actual yield.

SAFETY CONCERNS: Eye goggles are required. Acid solutions are caustic; they damage soft tissues quickly. If you get any hydrochloric acid solution in your eyes use the eyewash immediately. If an HCl solution gets splashed on your body, wash it off quickly. Do NOT waft anything.

Supplies:

Supplies found in the large glassware cabinet: 250-mL beaker, 50-mL graduated cylinder

Supplies found at your lab station: bottle of CaCO3 chips, bottle of 1.00 M HCl

Supplies found in the drawer at your lab station: metal spatula (scoop)

Name: ____________________________

Procedure:

1. Obtain between 5.00 -10.00 grams of CaCO3 chips. You may do this by placing the empty 250-mL beaker on top of the balance and then taring (zeroing) the beaker to = 0.00 grams, prior to transferring the CaCO3 into the beaker (as it remains on the balance). The reaction proceeds fastest with a large surface area, so try to use small pieces of CaCO3. While it is NOT necessary to use exactly any particular mass (as long as the mass is between 5 and 10 grams), you should always record exactly the amount that you do use. Record the exact mass here (with correct unit and correct substance indicated):

___________________________

2. For this next step, remember to read the volume at the very bottom of the meniscus: Use a 50-mL graduated cylinder to measure between 40.0 – 50.0 mL of 1.00 M HCl; then, carefully pour this acid solution into your beaker containing the CaCO3 chips. While it is NOT necessary to use exactly any particular volume, you should always record exactly the amount that you do use. Record the exact volume here (with correct unit and correct substance indicated):

___________________________

3. You will observe bubbles of CO2 as the CaCO3 reacts with the HCl. The reaction proceeds for quite a while (perhaps 10 minutes or so); every now and then swirl the beaker gently; careful not to splash the HCl out of the beaker. When you no longer observe bubbling as you swirl, you may conclude that the HCl reactant has been all used up; remember, the HCl is the limiting reactant. As you are waiting for the reaction to reach completion, you should work on questions 1, 4, and 5 (next page).

4. Decant (pour off) the aqueous solution into the sink. Do NOT pour out the remaining calcium carbonate chips. The aqueous solution that you just discarded contained the products of your reaction; but, we’re not interested in the products today! What you will measure in a little while is the amount of extra left-over un-reacted CaCO3 remains, in order to determine how much CaCO3 has reacted.

5. Rinse the excess CaCO3 chips several times (5 times), each time using plenty of the distilled water from the squirt bottle, each time, discard the aqueous phase into the sink. Multiple rinsing (and discards) like this is the correct way to remove any of the products that might be still sticking to the chips.

6. Figure out a way to get the un-reacted marble chips out of the beaker and then carefully dry the marble chips with a paper towel. What I’ve found works well is to dump the chips onto a clean countertop, and pat them dry with a paper towel. Or, after removing most of the water with a paper towel, you may put them into the oven for a few minutes to remove the last traces of water. Record the mass of the excess CaCO3 chips (with correct unit and correct substance indicated): To do this, you may want to dry out your beaker, tare it again to = 0.00 on the balance, and put the dry chips back into the dry beaker.

___________________________

7. Clean up: Rinse the graduated cylinder in this manner – fill it up with tap water and dump it out 5 times. Dry out the beaker and wipe off the spatula with a paper towel. Return everything to their original places. Station should look tidy and dry when you leave.

Name: _________________________

Data/Calculations/Results: Show all work; include units and chemical substances after each number written.

1. Write the balanced chemical equation for the chemical reaction (you will find this above the lab intro). You do not need to include the phase notations (states).

2. Transfer your data to here:

Exact mass of CaCO3 that you chose to start with: ___________________

Exact volume of 1.00 M HCl that you chose to start with: ___________________

Convert the volume from mL to L: ___________________

Exact mass of leftover excess DRY CaCO3 chips: ___________________

3. Calculate the mass of CaCO3 that has reacted. [A subtraction problem.] ___________________

4. Using the mass of CaCO3 that has reacted (and your road map), calculate the mass, the volume, and the number of molecules of CO2 gas that you produced. The density of CO2 at “NTP” (normal temperature and pressure) is

1.842 grams / Liter. NTP is NOT the same as STP! Show all work below for the 3 dimensional analysis problems.

Mass of CO2 produced:

Volume of CO2 produced:

Number of molecules of CO2 produced:

5. Now, using the volume and molarity of the HCl solution that has reacted, (and your road map), again calculate the mass, the volume, and the number of molecules of CO2 gas that you produced. density CO2 = 1.842 grams / Liter. Show all work for all 3:

Mass of CO2 produced:

Volume of CO2 produced:

Number of molecules of CO2 produced

Hint – your answers for question 5 should be similar to your answers for question 4. If they are double instead, you haven’t used the coefficients from the balanced equation correctly in your “moles to moles” factors.

Name: __________________________________

You are a civil engineer for a major chemical producing company, and you have just been asked to evaluate the contributions of your team members. Rank each team member separately (specifying only their initials) on the following attributes. For each score of a 1, 2, 4 or 5 you must justify the score by noting specific example(s) in which the team member contributed positively or negatively. Use a scale of 1 – 5, where 1 is worst, 3 is average, and 5 is best. Do NOT rank yourself!

Persons’ initials: _A.____________ _B.______________

-Contributed by reading the instructions, knowing “where we are” in the procedure; rather than looking at other groups.

Person A______________________________________________________________________________________

Person B______________________________________________________________________________________

-Contributed by explaining the instructions, helping to solve “technical” problems, using own knowledge, common sense, and understanding; rather than asking the teacher for clarification repeatedly.

Person A______________________________________________________________________________________

Person B______________________________________________________________________________________

-Contributed by maintaining a non-critical positive attitude; rather than complaining about problems.

Person A______________________________________________________________________________________

Person B______________________________________________________________________________________

Contributed by gathering supplies, lending a hand, doing the work necessary; rather than waiting for others.

Person A______________________________________________________________________________________

Person B______________________________________________________________________________________

-Contributed to safety awareness by reminding others to keep goggles on; rather than taking own goggles off to “see”.

Person A______________________________________________________________________________________

Person B______________________________________________________________________________________

-Contributed to the focus of the group by remaining on task, encouraging others to do the same; rather than telling stories, wandering around, or having no awareness of time remaining.

Person A______________________________________________________________________________________

Person B______________________________________________________________________________________

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