Question

I placed 1 gram of NaBH₄ in a balloon and placed it over a glass round bottle filled with water and acetic acid. While secured, I emptied the balloon into the bottle and made sure to wash the inside of the balloon. The reaction taking place is NaBH₄+2(H₂O)⟶NaBO₂+4(H₂).

The yield in this case should be 213 grams of H₂. According to Wikipedia, hydrogen has a density of 0.08988 g/L. That should leave me with 2.369 L of H₂. But my little experiment showed a balloon with the about 13 cm in diameter, which a volume of sphere that wide would be 1.15 liters of H₂. This would mean a yield of .103 grams or 48.5%. But that just doesn't make sense. My NaBH₄ has not degraded by half.

If you were to double the given density of H₂ to 0.17976 g/L, the results are much more in line with what I'd expect. The yield of H₂ would be .206 grams or 97%, which would make sense since not everything is ideal and my NaBH₄ is a bit old. Obviously there will be minor errors in measurement, and its not a perfect sphere and I didn't control for temperature, but that cannot account for a 50% reduction in yield. Maybe the balloon is compressing the gas? But I don't think so.

So the only other thing I can think of is that the presented density of 0.08988 g/L for hydrogen is technically only for monoatomic Hydrogen, or H, which doesn't make sense and no one ever uses. The density of H₂ at STP should be 0.17976 g/L then to match my observations. There must be something I'm missing here entirely that I'm not understanding.

Answer 1

The confusion stems from miscalculating the expected yield of hydrogen gas due to not considering experimental conditions like temperature, pressure, and water vapor content. The density of diatomic hydrogen (H₂) at STP is 0.08988 g/L, not 0.17976 g/L as the student assumed.

Step-by-step explanation:

The student's question appears to be a discrepancy in expected vs. measured yield of hydrogen gas (H₂) during a chemical reaction involving NaBH₄ and acetic acid. The density value for hydrogen gas provided by the student, 0.08988 g/L, is for hydrogen gas at standard temperature and pressure (STP) conditions.

If the experiment was not performed under STP conditions, then the volume and consequently the yield of H₂ measured might not align with the expected yield calculated using the STP density. Another cause of discrepancy might be the presence of water vapor in the gas collected or inaccuracies in the diameter measurement of the non-spherical balloon.