In: Chemistry
1) Indicate how each of the following errors would affect(increase, decrease, or no change) your calculated value for the molar volume of O2 at STP (assuming that you complete the experiment otherwise correctly). In each case, explain your answer.
a. you forgot to subtract the vapor pressure of water in determining the pressure of O2
b. while heating your sample in the test tube your flame was improperly adjusted so that the black soot formed on the outside of the test tube and did not remove before being weighed
Ideal gas equation is given as
PV = nRT
Where P = pressure = 1 bar at STP
V = volume
n = moles of gas
R = 0.0831 Lbar/mol .K
T = Temperature = 273.15K at STP
V/n = RT/P
Where V/n = molar volume of O2 gas
a. The pressure of gas observed is the sum of actual pressure of O2 gas and vapour pressure of water at the temperature. Thus actual pressure of O2 is lower than observed. We know molar volume of O2 is inversely proportional to pressure of gas at any given temperature. So if you forget to substract the vapour pressure, then pressure of gas observed is higher than actual. Thus molar volume of gas will decrease than actual value.
b. If the black soot is not removed, then it's mass is also calculated along with the mass of gas. Thus moles of O2 gas is also higher than actual. At any given temperature and pressure, volume also remains constant. Thus increasing moles leads to decrease of molar volume (V/n).