What is the Correct Method for Measuring CO2 Gas Volume?

[duke656]

Hi!
I am trying to evaluate the amount of CO2 gas produced from the reaction of CaCO3 and HCl. Theoretically the amount of gas that can be achieved from the 0.1 g of calcite is around 24 cc (T=21 degree and P is 1010 mb). But, the experimental value that I obtained is around 30 cc.
I have no idea why there is such a big mismatch.

note:
I filled a measuring cylinder with water and inverted it above a bigger container filled with water. I collected the gas obtained from the reaction in the measuring cylinder and and noted the reading.
Is this method correct? Please help me.

 

[Borek]

Calcite doesn't have to be a pure CaCO₃, it may contain substantial amounts of magnesium carbonate. Around 2-3% positive error is guaranteed to occur due to the fact you are ignoring presence of water vapor. Then, some of the carbon dioxide gets dissolved in water, producing negative error.

Thats without countless experimental errors

 

[duke656]

Thank you Borek.
I also conducted the similar experiments using commercial calcium carbonate. Every time the amount of CO2 gas is around 25% more than the theoretical value. Are there any other causes of positive error? Thanks in advance.

 

[Borek]

Nothing obvious that I can think of. So far we neglected the pressure (it is important to keep the level of the liquid below collected gas identical with the level of the liquid outside of the cylinder), but in typical lab setup it should not give error larger than another single digit % (most likely positive error).

I would first check calibration of the glass and balance (checking the thermometer and barometer won't hurt either, although even completely ignoring them and just assuming RTP would not give 25% error). Then I would try to scale the experiment up (say, 0.2 g of carbonate), to see if the difference stays constant.

 

[Bystander]

What's the volume of acid you added?

 

[duke656]

I have done the similar experiments for 0.1, 0.2, 0.3, 0.4 and 0.5 g of CaCO3. The amount of gas collected is also nearly 30, 60, 90 120 and 150 cc respectively.

Bystander, I used 30 ml of 2.0 mol/L HCl. It is more than enough to completely neutralize the used amount of CaCO3. Is there any chances the volume of acid gives the positive error.
Will you help me to recalculate the volume of gas given by 0.1 g of CaCO3 at a temperature of 21 degree and at a pressure of about 1000 mb. I am sorry for being stupid.

 

[Bystander]

Now is the time for all good men to describe the apparatus, paying particular attention to the reaction vessel and method used to initiate the reaction. Are you throwing everything in a bottle and shoving in a stopper? Injecting the acid with a syringe? Breaking a sealed bulb of acid (or calcium carbonate) after sealing the reactor? Also, when you say "commercial" calcium carbonate, what does "commercial" mean? CP? Reagent grade? Or just something from the local nursery sold as soil sweetener?

 

[duke656]

Bystander, let me describe my apparatus.
I used a 500 mL conical flask with a air tight rubber stopper (cork). The stopper has two holes in which stopper funnel and tube for gas collection is inserted.
The collection tube is a brass tube that directly goes into the water filled measuring cylinder (inverted above the water container).
The stopper funnel has a flow regulation mechanism. I poured acid into the flask and closed it to stop the flow and I am sure there is no air leakage through the stopper.
The lab we use is temperature controlled.
The chemicals used are also guaranteed reagent and 98% pure.
The interesting thing is the graph that I obtained is almost linear. Say 0.1 g calcite gives 30 cc and 0.2 g gives 60 cc till 0.5 gives 150 cc. I am in a big problem. Please share your idea.

 

[Borek]

Will you help me to recalculate the volume of gas given by 0.1 g of CaCO3 at a temperature of 21 degree and at a pressure of about 1000 mb.

0.100 g yields 24.2 mL of gas (21 °C/1010 mbar), I checked that your calculations are in the right ballpark already yesterday.

 

[Bystander]

30 cc per 0.1 g? Two things to look at: use just water, no carbonate, and see what happens; second, did you observe any fizzing at all?

 

[cjl]

Where are you located? If you are at an elevation of ~1.5km above sea level, the pressure difference would explain the results...

(if the pressure reported of 1010mbar is directly measured at the lab though, this doesn't explain it)

 

[Chestermiller]

Is it possible that air dissolved in the water is being stripped out by the CO2, or is this too little to be important?

Chet

 

[duke656]

Thank you all. This problem is solved by reducing the volume of acid at the same time increasing it`s concentration. I am very thankful to you all.