Hey
I don't understand this multiple choice question from the 2014 Catholic Trials.
An experiment was performed to determine the volume of carbon dioxide gas released during the fermentation of a glucose solution. The reaction took place over several days in a flask containing yeast that was immersed in a water bath at 28 degrees and stoppered with cotton wool. Which of the following factors would most significantly compromise the validity of this experiment?
A) The evaporation of water
B) The temperature of the water bath
C) Measurement error associated with determining the mass lost
D) Lack of oxygen due to the presence of the cotton wool plug
(The answer is A)
Thanks heaps!
Hey Angie!
Rui has asked me to supplement his answer so please have a read what I wrote below
A is the correct option simply because all we are trying to measure is
the volume of CO2 produced. If water evaporates (especially since we are carrying out the experiment under 28 degrees here, so a large chance that some water would have evaporated) then we obtain
a measurement that includes a combined mass loss of CO2 and H2O. Validity means
"are we measuring what we want to measure" and this is clearly not the case if water has also evaporated. Keep in mind, WE ONLY WANT TO MEASURE THE MASS OF CARBON DIOXIDE.
Explaining this one step further. When you conducted this experiment what you would have done is to measure the mass of fermentation mixture before fermentation has taken place, and the mass of fermentation mixture after fermentation has taken place. The loss in mass after fermentation you would assume to be CO2 (You would probably have carried out the experiment with a limewater flask connected to the fermentation mixture as well). But keep in mind that even at room temperature, water evaporation still occurs, meaning that whilst CO2 is being evaporated, water is too! So when you measure the lost weight, thats a combined weight of evaporated CO2 and WATER. So evidently we are not measuring JUST the mass of CO2 and therefore the experiment becomes invalid.
So, how do we improve validity? We can do one of the two things:
1. Keep a carton of water under the same temperature as your fermentation mixture. Make sure the volume of the water is the same as the volume of your fermentation mixture. Measure the loss in the amount of water at the same time when you measure the loss in mass of the fermentation mixture, subtract this from the measured weight loss of the fermentation mixture. Why are we doing this? Because we want to see how much water has evaporated under the same conditions and in an equal amount of time as fermentation takes place. This way we can measure only the mass of CO2 produced.
2. This is an easier method. Simply connect your fermentation mixture to a conical flask filled with limewater. When CO2 is released it reacts with Ca(OH)2 (lime) to form a milky liquid CaCO3. By measuring the increase in the weight of the limewater and divide this weight by the molar mass of CO2 we can work out how many moles of CO2 has been produced and therefore calculate the volume of CO2 produced. Make sure when you do this experiment that everything is enclosed to prevent CO2 from escaping into the air. Why do we not worry about water evaporation in this case? Because it doesnt affect anything! It does not react or enter the limewater and therefore all we are measuring here is the amount of CO2 produced!A bit long (quite long tbh) for a multiple choice answer, but I thought I would just make everything clearer for you.
Best Regards
Happy Physics Land