For my chemistry lab, I reacted Mg powder with HCl to create H2 gas. I measured the volume of H2 gas created by salt water displacement by reading off an inverted test tube. I measured 14.0 mL and calculated the percent yield (used PV=nRT to find theoretical volume and divided it by the actual- 0.014L volume) which was 62.2%.
The next question that makes no sense is: "If you did not get 100% yield, could this be explained by the temperature of the gas? If so, given the yield you observed do you expect the gas temperature might have been above or below the temperature recorded for the salt bath water (the thermometer states 290 K)." Hint was to use ideal gas law and see volume of H2 gas in relation to gas temperature.
Based on my interpretation, the percent yield has nothing to do with the temperature of the gas produced. And by calculating the number of moles of H2 gas produced (using PV=nRT I got 5.9*10^-4 mol) and plugging into the PV=nRT equation, everything cancels out and I get 290K, which makes the gas temperature the same as the salt water temperature. I don't think this reaction involves equilibrium either.
Generally confused on how I should answer this. Should I say that the temperature of the gas is higher than the salt bath, and since temperature is directly proportional to volume, higher temperature (when compared to 'initial' temperature of salt water bath) would yield more volume of H2 gas, which was what we observed.
What am I doing/thinking wrong?
You are on the right track in using the ideal gas law equation (PV = nRT) to analyze the relationship between gas temperature and volume. However, it seems that there may be some confusion in your interpretation.
Firstly, let's clarify the concept of percent yield. Percent yield is a measure of how efficient a chemical reaction is in producing the desired product. It represents the ratio of the actual yield (the amount of product obtained experimentally) to the theoretical yield (the maximum amount of product that could be obtained, calculated based on stoichiometry) expressed as a percentage. In your case, you calculated a percent yield of 62.2%.
Now, regarding the question about temperature and its potential effect on the yield. The ideal gas law equation (PV = nRT) relates pressure, volume, temperature, and the number of moles of gas. If we assume that the pressure and volume remain constant, then we can simplify the equation to show that temperature and the number of moles are directly proportional.
In your calculation, you determined the number of moles of H2 gas using the ideal gas law and found it to be 5.9 * 10^-4 mol. From this, you hypothesized that plugging this value back into the ideal gas law equation should yield the same temperature as the salt water bath (290 K). This is correct.
Now, let's address the question about the potential explanation for the less-than-100% yield related to temperature. Given your observed yield of 62.2%, you need to consider whether a higher or lower temperature than the salt bath temperature could have influenced the volume of H2 gas.
According to the ideal gas law (PV = nRT), if the temperature increases while other variables (pressure, volume, number of moles) remain constant, the volume of the gas will also increase. Similarly, if the temperature decreases, the volume will decrease.
Since the observed volume of H2 gas was less than the theoretical volume (considering 100% yield), it suggests that factors other than temperature may have affected the yield. Therefore, it is unlikely that the temperature of the gas can be used as an explanation for the lower yield.
To summarize, the percent yield of a chemical reaction does not depend solely on the temperature of the gas. Other factors such as reactant purity, side reactions, and experimental errors can contribute to a less-than-100% yield. In your case, it appears that the temperature is not a significant factor affecting the observed yield.