Experiment 2-4

Mass of a Gas

Background Information

Hopefully you noted in Experiments 2-1, 2-2, and 2-3 that mass did not change significantly if you did not add or remove any substance.  If that is what you note when you studied your histograms, then welcome to the crowd of millions who have made that interpretation.  But what if you don't agree?  For science, not agreeing is a bigger deal than agreeing with the majority!  Because it is a bigger deal, you want to make sure.  Discoveries ALWAYS occur when someone notices something different from what everyone expects.  But the risk in making a discovery is that the difference MIGHT be due to a mistake or a poor interpretation.  So when a scientist (or anyone) notices something different, the first task is to review how this interpretation was reached to make sure nothing went wrong.  Recall that no measurement is perfect.  Every piece of equipment (including our own senses) is limited in accuracy, precision, and sensitivity.  If experimental results are outside the expected range of uncertainty, it would be wise to repeat the experiment to make sure the difference is not a fluke.

Countless experiments have been performed to see if mass ever changes.  Mass seems to be exactly conserved if nothing is allowed to enter or leave the system.  This is known as the law of conservation of mass.  It holds for all cases in which no substances is added (such as water condensation on the outside of a cold container) or removed (such as a spill).  It has been checked to one part in 109 (a thousand million in Europe or a billion in the U.S.A.).

Properties that are conserved are immensely useful.  It works just like an algebra equation where we know we have an equality, but one term is missing.  We can solve the equation to find what must be the value of the missing term.  Likewise we can use a conserved property to find missing information.  For example, when we burn a candle, it gets lighter.  Because we know mass is conserved, we know the only explanation must be that some material was removed.  We can use what we know about conservation of mass to determine the amount of mass lost from the burning candle.

Experiment

In this experiment we wish to (1) confirm again that mass is conserved if nothing is allowed to enter or leave the system being measured, and (2) use the conservation of mass to calculate the mass of a material otherwise difficult to measure.  We shall make a gaseous material and try to determine the mass of that gas.

Whenever a gas is generated, pressure may build and potentially cause an explosion.  In all such cases precautions should be made to prevent injury in the case of accidental explosion.  If pressure might be built up, the container could be wrapped with tape and contained in a secondary plastic bag to stop projected fragments if the container fails.

Use a small empty container with a small neck which can be closed with a balloon.  Do NOT add too much!  This is a situation where adding excessive materials could both cause injury, and likely will also cause leakage wasting any measurements.

1. Measure about 10 mL (or less) of vinegar (acetic acid, HC2H3O2, in water) into the bottle.  (Remember, returning excess to the original container could accidentally contaminate it.  Always discard excess.)  Caution: Vinegar is a weak acid which can gradually damage eyes and other materials.  WEAR safety GOGGLES and rinse spills with water.  (For Emergency: see details below).
2. Measure about 2.5 g (about the mass of a penny), or less of baking soda (sodium bicarbonate, NaHCO3) into a balloon.  Caution: Baking soda is a weak base which can damage eyes and other materials.  Wear safety goggles.
3. Place the balloon over the mouth of the bottle in such a way that the baking soda remains in the balloon and none falls into the bottle.
4. Weigh the combination of balloon, powder, bottle, and vinegar as accurately as possible.
5. Without removing the balloon from the bottle, lift the balloon allowing the powder to fall into the vinegar causing a chemical reaction.
6. When the reaction appears to be finished, weigh the apparatus and contents again to determine is mass changed.
7. Then, loosen the neck of the balloon, listening for any sounds and observing any volume change.  Is there evidence of gas escaping?
8. Weight the combination a third time.  Any loss in mass could be due to the mass of gas!

Since no measurement is perfect, do the same experiment several times then find the average change in mass.  To reach a conclusion, use the results that seem consistent and where no errors are apparent.

Make histograms of your results to visualize any pattern in your measurements.  It would be appropriate to make histograms for mass change before loosening the neck of the balloon as well as afterwards.

Record in your science journal the mass measurements, first in table format, then by constructing a histogram.  Write a Formal Report if you need to earn credit.

Consider whether observations and measurements are consistent with your expectations.  Experiments which provide unexpected results are often more valuable than others!  If anything was different, try to figure out an explanation.  For example, if the mass changed when it wasn't expected, consider whether other observed changes could be a factor.  If the balloon changed volume, perhaps displaced outside air might effect the measurement?  How could such buoyancy effect your measurements?  (Reading more about buoyancy might be helpful.)

Standard EMERGENCY procedure: For a hazardous chemical in an eye, flush with tepid (comfortable) water for 15 minutes; FOLLOW immediately with medical attention.  The nervous system attempts to warn of a harmful substance in an eye by stinging.  However if the nerve is also damaged the sting may stop or never occur.  If there is any doubt, flush with water for 15 minutes then secure medical attention.  It often is necessary to hold eye lids open because the flow of water tickles, causing the eye lids to automatically close, blocking needed rinsing.

created 1/9/2003
revised 12/9/2003
by D Trapp