chemistry

Chemistry 310

Laboratory Exercise #7

Stoichiometry and % Composition

Your name: Date:

Introduction:

Stoichiometry or ‘atom counting’ allows us to predict the amounts of substance used or

produced in a reaction when we know information about a different substance in the same

reaction. By relating formula through a balanced chemical equation we can count how many

molecules of substance A are needed to create substance B.

aA + bB dD + eE

If we use a molecules of A we’ll get d molecules of D, e molecules of E and need an

additional b molecules of B. The ratio of A:B needed to create D is always a:b (the

stoichiometric ratio), and is an exact conversion. We can use stoichiometry by counting

molecules, counting mols (6.022x1023 particles/mol), or liters of gas.

Decomposition is a type of reaction that allows atoms in a molecule to rearrange their

bonding to a more stable configuration upon heating. The products of decomposition reactions

are generally an ionic salt and a gas. The loss of the gas is favorable by entropy, and the

formation of a small, stable covalent compound is favorable by enthalpy.

Many ionic compounds undergo decomposition reactions: (with M is metal ion)

Bicarbonates:

MHCO3 (s) + heat à M2CO3 (s) + H2O (g) + CO2 (g)

Carbonates:

M2CO3 (s) + heat à MO (s) + CO2 (g)

Chlorates:

MClO3 (s) + heat + MnO2 (catalyst) à MCl (s) + O2 (g)

Bisulfates:

MHSO3 (s) + heat à M2SO3 (s) + H2O (g) + SO2 (g)

Must read the paragraphs below:

A common bicarbonate is sodium bicarbonate or baking soda (NaHCO3). Upon heating

above 75oC sodium bicarbonate decomposes to sodium carbonate or washing soda (Na2CO3)

and produces water and carbon dioxide gases as side products. Further heating of the sodium

carbonate can produce sodium oxide (Na2O) and carbon dioxide, but the decomposition to

sodium oxide typically occurs above 850oC, which is not easily attainable at home.

This lab will use the decomposition of a known amount of sodium bicarbonate to

measure the stoichiometry and find the mass lost during the decomposition. We will then use

the ratios found in the first portion of the lab to determine the percentage of sodium

bicarbonate in an unknown sodium bicarbonate/sodium chloride mixture. (Sodium chloride

does not decompose with heat.

Sodium bicarbonate is a common rising agent used in baking cookies, cakes, and nonyeasted breads. Sodium carbonate is used in water softening, home-made laundry detergents, and cleaning products. Sodium bicarbonate is a neutral compound in water solution, but caution should be used with sodium carbonate as it is strongly basic. Part of its effects as a cleaning agent are converting oils to soaps and it can do the same thing to the skin on your hands in large quantities!

Learning outcomes:

• Identify decomposition reactions.

• Calculate the stoichiometry of a decomposition reaction.

• Calculate the percentage of an unknown.

Experimental Procedure: Note: While it’s possible to do this at home, all the data you need is

included in the lab. No! I don’t ask you to conduct this experiment at home.

Part 1: Decomposition of Known Sodium Bicarbonate:

1) Weigh out 50-60 g of sodium bicarbonate by difference into a heat safe container.

2) Weigh out a second 50-60 g sample of sodium bicarbonate by difference.

3) Place the two samples on a cookie sheet and heat at 350oF in an oven for. 3-4

hours.

4) Carefully stir the samples using a stir rod or heat safe spoon every hour to ensure

even heating.

5) After 3-4 hours of heating remove the cookie sheet from the oven and allow to cool

in a dry area. Note: USE OVEN MITTS. HOT!

6) Once the samples have cooled to room temperature. Weigh each sample again.

7) Calculate the mass of remaining sodium carbonate and complete the calculations in

the lab report sheets.

Part 2: Decomposition of Unknown Sodium Bicarbonate – Sodium Chloride Mixture

1) Weigh out 50-60 g of unknown mixture by difference into a heat safe container.

2) Weigh out a second 50-60 g sample of sodium bicarbonate by difference.

3) Place the two samples on a cookie sheet and heat at 350oF in an oven for. 3-4 hours.

3

4) Carefully stir the samples using a stir rod or heat safe spoon every hour to ensure

even heating.

5) After 3-4 hours of heating remove the cookie sheet from the oven and allow to cool

in a dry area. Note: USE OVEN MITTS. HOT!

6) Once the samples have cooled to room temperature. Weigh each sample again.

7) Calculate the mass of remaining sodium carbonate-sodium chloride residue and

complete the calculations in the lab report sheets.

8) Residue may be deposited in the trash.

Experimental data:

Part 1: Decomposition of Known Sodium Bicarbonate:

Before heating:

After heating for 4 hours at 350 ⁰F

Part 2: Decomposition of Unknown Sodium Bicarbonate – Sodium Chloride Mixture

Before heating

After heating for 4 hours at 350 ⁰F

Chemistry 310

Laboratory Exercise #9: Report Sheet

Stoichiometry and % Composition

Your name: Date:

Experimental Data and Calculations

Use correct sig figs and units for all measurements and calculations

Part 1: Decomposition of Known Sodium Bicarbonate:

Question 1: Looking at the experimental procedure, fill in the Table below:

	Mass of Weigh Boat (empty container)	Mass of sample + container	Mass of sample only	Mass of residue + container	Mass of residue
Sample 1
Sample 2

Question 2: Write the balanced equation for the decomposition of sodium bicarbonate:

Question 3: Samples are heated at 350 ⁰F what is this in ⁰C? (show your work for credit)

Question 4: convert the mass of sodium bicarbonate to moles sodium bicarbonate (show your calculation for credit)

Sample 1 = ______________ mol NaHCO3 Sample 2 = ______________ mol NaHCO3

Question 5: Convert the mols of sodium bicarbonate to mols of water and mols carbon dioxide lost (show your calculations for credit).

Sample 1 = ______________ mol H2O and _____________ mol CO2

Sample 2= ______________ mol H2O and _____________ mol CO2

Question 6: Convert the mols of water and mols carbon dioxide lost to mass of water and mass of carbon dioxide (show your calculations for credit).

Sample 1 = ______________ g H2O and _____________ g CO2

Sample 2 = ______________ g H2O and _____________ g CO2

What error might make the mass of the residue too high? Give a two specific experimental

errors.

1) _________________________________________________________

2) _________________________________________________________

Part 2: Decomposition of Unknown Sodium Bicarbonate – Sodium Chloride Mixture

Question 1: Looking at the experimental procedure, fill in the Table below:

	Mass of Weigh Boat (empty container)	Mass of sample + container	Mass of sample only	Mass of residue + container	Mass of residue
Sample 1
Sample 2

Since we don’t know the original mass of the sodium bicarbonate, we have to work from the

products.

Question 2: How much mass is lost during heating? (show your calculations for credit).

Sample 1 = ______________ g lost

Sample 2 = ______________ g lost

Question 3: If all the sodium bicarbonate in the sample has decomposed what percent of the mass is theoretically lost as gas? Show your work.

Question 4: If we assume that the sample has completely decomposed, we can use the mass lost and the mass percent of gas to calculate the grams of sodium bicarbonate present. (show your calculations for credit.

Sample 1 = ______________ g NaHCO3 present

Sample 2 = ______________ g NaHCO3 present

Question 5: Use the mass of sodium bicarbonate calculated to find the original percent by mass of sodium bicarbonate in the unknown mixture. (show your calculations for credit).

Sample 1 = ______________ % NaHCO3 originally present

Sample 2 = ______________ % NaHCO3 originally present

Question 5: What is the average of % NaHCO3 originally present?