AP CHEM: Molar Mass of a Volatile Liquid Calculations
Note: For the sake of easily referring to a step, I have numbered them in order as they appear on your data sheet.
1) mass empty flask and cover
2) mass flask, cover and vapor
3) temperature of vapor (°C)
measurement
measurement
measurement
4) temperature of vapor (K)
K = °C + 273.15
5) pressure of vapor (mmHg)
measurement
6) pressure of vapor (atm)
conversion
7) mass flask, cover and H2O
measurement
8) mass H2O in flask
#7 − #1
9) temperature of H2O in flask
measurement
10) density of H2O
The density of water changes with temperature. You have a table of water
densities at various temperatures on your info sheet (side 1, right edge).
More than likely your temperature of the water is not on the list. To find the
density you will have to interpolate the data on the list.

#3 + 273.15
(

)
Example: Suppose the temperature of the water in the flask is 23.6°C and I
need the density at that temperature.
 Look on the chart to see if that temperature is there. If not, find the
two closest temperatures on the chart that 23.6°C falls between and
note the density for those temperatures.
20°C → 0.99823 g/mL
25°C → 0.99707 g/mL

Set up a proportion to find the density at 23.6°C as follows.
20°C →
0.99823 g/mL
a
b
c
23.6°C → X
25°C →
d
0.99707 g/mL
−
−
−
−
−4.94115 + 5X = −0.004176
5X = 4.986974
X = 0.99739

Density at 23.6°C is 0.99739 g/mL.

11) volume of flask

PV = nRT
12) mol of vapor in flask
13) molar mass of vapor
14) mean value of molar mass of vapor
15) density of vapor in flask
16) mean value of density of vapor



average of both trials for #13

average of both trials for #15
−
(
−
)