Question

Would the following hypothetical errors cause a higher or lower percent yield? Explain your answer in each case. A lab finding the amount of H2 gas produced along with the percent yield. A buret with a flask containing HCL and aluminum was used to cause the reaction.

a. A student forgot to subtract the vapor pressure of water from the atmospheric pressure while doing the calculations. High Low

b. A student held the leveling bulb way below the buret’s water level while reading the final buret volume. High Low

Answer 1

the reaction is:

2Al + 6HCl   2Al³⁺ + 3H_2(g) + 6Cl^-

in case a), the student will overestimate the amount of hydrogen in the sample because the total pressure is:

presure in the buret = atmospheric pressure + vapor pressure of water

Then, the pressure in the buret is higher, so the measured volume is lower and the yield will be lower.

in case b), if the leveling bulb is below the buret's water level this causes the pressure in the buret to decrease and the measured volume to be higher. Therefore, the calculated yield will be higher.