Missing Mass
RESEED Training Session
12 Sep 14
Dick Healy
(after concept by Paul Conroy)
Background
• Module
– As presented, being discussed for use in 8th grade
in Lawrence as summary of chemistry module
• Intended for one or two class periods – data
analysis and discussion can be postponed
• Modification of experiment designed for RESEED by Paul Conroy as part of 2012 training
Warmup
Do Now
One of the mystery mixture reactions we tried combined
baking soda (sodium bicarbonate) with vinegar (acetic
acid) to release carbon dioxide. The formula is:
NaHCO3 + CH3OOH -> CH3OONa + CO2 + 2 H2O
Is this formula correctly balanced?
• Yes
• Almost
• Not even close
If you think it is not balanced, how would you correct it?
Experimental Background
• The combination of baking soda and vinegar releases
carbon dioxide in a fun way. The basic chemistry is
given by the formula
• NaHCO3 + C2H4O2 -> C2H3NaO2 + CO2 + H2O
Using the atomic weights given below, calculate the
atomic weight of each of the molecules:
Element
Atomic Weight
Hydrogen (H)
1
Carbon (C)
12
Oxygen (O)
16
Sodium (Na)
23
Experimental Background – Cont’d
NaHCO3 + C6H8O7 -> NaC6H7O7 + CO2 + H2O
Sodium Bicarbonate:
Acetic Acid:
Sodium Acetate:
Carbon Dioxide:
Water:
Mass on left side =
Mass on right side =
Conduct the Experiment
• Let’s do the experiment
• Please publish your results as:
Team Name
Mass of Sodium Bicarbonate
Mass of Vinegar
Missing Mass
Analyze the Data
• Once the experiment is complete and you have
multiple samples, what do you do?
• Follow-up questions
– What evidence do you have that a chemical reaction
occurred?
– What states of matter were involved in your experiment?
– Where is the sodium acetate?
– How much gas was released?
• Which plots to make?
– Added vinegar vs initial baking soda
– Missing mass vs initial baking soda
– Missing mass vs added vinegar
Data Graph
Missing Mass vs Initial Baking Soda
9
8
7
Missing Mass [g]
6
5
4
3
2
1
0
0
2
4
6
8
Sodium Bicarbonate [g]
10
12
14
16
Additional Data Graphs
Added Vinegar vs Initial Baking
Soda
Missing Mass vs Added
Vinegar
500
10
400
8
300
6
200
4
100
2
0
0.0
5.0
10.0
15.0
20.0
•Where is the data point if too
much/ too little vinegar is added?
•How does this change if we use 5%
or 8% vinegar?
0
0
100
200
300
400
•Where is the data point if
too little vinegar is added?
•Where is the point if too
much vinegar is added?
500
Possible Extensions
• Capture the escaping CO2 in a balloon and weigh
it (used in a conservation of mass demo by my
new teacher)
– Need a stoppered flask
– What errors are likely, and how to overcome?
• Determine volume of escaping gas using balloon
– Demonstrate measuring volume of irregular objects
– Relate to molarity using ideal gas law
Conclusions
• Ratio of missing mass to mass of sodium
bicarbonate is the critical measurement
– Very forgiving across large range of experimental
uncertainty
– Major error is the amount of dissolved CO2 in solution
– Very sensitive to precision of scale
• Great way to review balancing equations,
molarity, chemical reactions
• Involves 3 of 4 states of matter