Read the following concerning exp 12. It should answer most questions I usually get concerning this exp. You are using both the Ideal Gas Law (IGL) and the van der Waal's (VDW) equation. When using either eqn. you are using the Universal Gas Constant, R. The value of R to use in these eqns is 0.08206 L*atm/(mol*K), when needed to 4 sig. fig. (often used as 0.0821 to 3 s.f.). Note this is in liters for volume and atmospheres for pressure. You need to make sure your P and V in the equations have units of atmospheres and liters so they agree with the units for R. The temperature should ALWAYS be in Kelvin. This is true for ALL equations you will encounter that involve just T (temp.). If your eqn. involves a change in T you can use Celsius (as you did for exp 6). Since I've mentioned temperatures I will first discuss sig. fig. about temps This will generally apply for most of your labs. For your temperatures you will generally be able to estimate them to at least one decimal place (such as 25.3 C). That means in Celsius you would have 3 s.f. When you record temps in your notebook and report forms in Celsius you should record them to the proper number of decimal places, which will depend on the thermometer being used. Both the digital thermometers (used in exp 6 and this exp) and the alcohol thermometers (used in exp 14) allow you to report temps in Celsius to 1 decimal place giving 3 s.f. for T in Celsius. What happens if you convert this to Kelvin, as you MUST in most cases? Since you have 1 decimal place for T in Celsius you should add 273.15 to convert to Kelvin. When you add 273.15 you know this number to 2 decimal places and the T in Celsius to 1 decimal place. Thus, T in Kelvin should be reported to 1 decimal place. This means the T in Kelvin will wind up with an extra s.f. (4 s.f. if the T in Celsius has 1 decimal place). For instance, if 25.3 C is converted to K by adding 273.15 you will have 298.45 K and should report this to the first decimal place. It will wind up with 1 decimal place (298.4 or 298.5 depending on how your lecturer wants you to handle dropping an exact "5") and 4 s.f. For exp 12 you should have recorded the barometric pressure to 1 decimal place, meaning it should have 4 s.f. The s.f. for the masses in parts A and B depend on whether you used the analytical or top-loading balances. For part D you use the sheet with the elemental analysis the TA gave you to determine the empirical formula. Remember, if you get non-integer values for the subscripts in the EF you need to multiply them by an integer until you get whole numbers. Start by multiplying each subscript by 2. Then try 3 if using 2 doesn't give subscripts close to whole numbers. When the numbers are w/in 0.1 of whole numbers you're done with finding which integer to use. Then you need to find the molecular formula (MF) using the experimental MW from part C and your empirical formula wt. (EFW). Remember, the MF is always an integer multiple of the EF and thus the MW is the same multiple of the EFW. You find this multiple by dividing the MW by the EFW. This should be close to an integer. However, it's been determined the systematic error in this exp. generally causes the multiplying factor to be a little higher than it should be. This is why the lab manual (p. 74) tells you to round down. Even if it's something like 1.6 you should round down to 1. If it's 1.995 you probably should go ahead and round up. If the multiplying factor is less than 1 round up to 1. Also, if you round down and have an odd number of H atoms in the formula there's a problem. For the molecules we are using containing C and H or C, H and O there must be an even number of H atoms in the molecular formula. The empirical formula can have an odd number of H atoms but the molecular formula must have an even number of H atoms. Thus, when you round down, if you get an odd number of H atoms in the resulting molecular formula you should go back and round up. Once you have the MF you need to calculate the actual MW. Use the AWs from the inside cover of your book (to at least 4 decimal places). This will give a more exact MW to 4 decimal places (likely about 6 s.f. for most unknowns). For part E use 0.08206 as the excepted value of R. Determine the % error in R for each of the 3 trials using the mass and temp for each trial, along with the average for the volume of the flask, the barometric pressure and your exact MW (from part D). Record all three calculated R values on the report sheet. Report the value of R and % error for all three trials. For "b" in the VDW eqn. use the density of the liquid given on the sheet from the TA. This density has units of g/mL. However, the unit for "b" is L/mol. You will need to convert the density on the sheet to g/L. Use your exact MW obtained from part D. For "a" in the VDW eqn. use the trial that gave the best value for R in part E. Use the mass and temp. from this trial, with the exact MW, the true value for "R" (not your calculated value), the barometric pressure (part A), the average volume of the flask (part B) and the value of "b" calc. for part F.2. This is not the best way to determine "a". As a matter of fact it can give down right awful values for "a", sometimes even negative. You should think about whether the values you get for "a" and "b" are reasonable. What should "a" and "b" be for an ideal gas? What sample calculations do you need to do? Since the report form states "all relevant items" that pretty much means show a sample calc. for anything requiring a calculated value. You only need to do these sample calc. for one trial in parts A, B, C and E. For part D you should show sample calc. for each step in the determination of the EF and even for obtaining the MW from your actual MF. For part F show a sample calc. for the VDW constants, a and b. I think that's about it.