Experiment 8 Pre-Lab Questions Answer all calculation questions to the correct n

Question

Experiment 8 Pre-Lab Questions Answer all calculation questions to the correct number of significant figures. 1. What pressure in atmospheres corresponds to 720.3 mm Hg? 2. What temperature in K corresponds to 97 °C? 3. A student performs the experiment and records the following data: Barometric pressure: 741.6 torr Temperature of water bath: 99 °C Volume of flask: 273 mL . Mass of condensed vapor: 0.65 g a. The student calculates the molar mass of the gas to be 2.6 x 10-. What mistake did he make? b. What is the molar mass of the gas? Show your calculations. 4. Why it is not necessary to measure the volume of the unknown liquid that is placed in the flask? 5. Refer to the equation that relates'to the mass of the vapor (w). If either w or T is somehow too large or if either P or V is too small, then M/will be too large. Give an example of a procedural error that would result in obtaining a molar mass value larger than the true value. 59

Explanation / Answer

We have,

1

1atm = 760 mm Hg

720.3 mm Hg = 1 atm /760 mm Hg x 720.3 mm Hg = 0.948 atm

2.

K = oC + 273.15

for 97 oC

K = 97 + 273.15 = 370.15K

3.

Molar mass cannot be less than 1g/mol as no element can be lighter than hydrogen atom and hence there is some calculation mistake

We have to use ideal gas equation to find no. of moles

PV = nRT

P = pressure  

V = volume

n = no. of moles

R = gas constant

T = temperature in K

1 torr = 1mmHg

741.6 torr = 741.6 /760 x 1 atm = 0.9758 atm

99 oC = 99 + 273.15 = 362.15K

volume = 273 mL = 0.273 L

n = PV/RT = 0.9758 atm x 0.273L / 0.08206atm.L/mol.K x 362.15K = 0.008964 mol

molar mass = mass / moles = 0.65g/ 0.008964 moles = 72.51g/mol

4.

to know the actual volume of gas ,we should know the volume of liquid in the chamber as gas gives the total volume over in the vessel

total gas volume = volume of chamber - volume of liquid present

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