Experiment 6 Pre-Lab Questions Name: Section: Answer all calculation questions t

Question

Experiment 6 Pre-Lab Questions Name: Section: 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 . Volume of flask: 273 mL Mass of condensed vapor: 0.65 g The student calculates the molecular weight of the gas to be 2.6 x 10 g/mol. What mistake did he make? a. b. What is the molecular weight of the gas? Show your calculations. Why it is not necessary to measure the volume of the unknown liquid that is placed in the flask? 4. 39

Explanation / Answer

1. We know 1 atm= 760 mm Hg,

which means pressure exerted by atmosphere is equal to pressure exerted by 760 mm Hg.

then, 720.3 mm Hg= (1/760) *720.3 atm

=> 0.95 atm

2. We know that 0°Celsius = 273 Kelvin

and x °C = x+273 K

then 97 °C= 97+273 = 370 Kelvin

3. We can apply Ideal Gas Law in this case

PV= nRT

where P (in Atm) = 741.6/760 = 0.98 atm

V = 273 mL = 273/1000= 0.273 L

T= 99+273= 372 K

n= number of moles of gas

and gas constant R= 0.0821L atm/ K mol

on solving, we get n= PV/RT= 0.0088 moles

now, given mass= 0.65 g

so, molecular weight= 0.65/0.0088= 73.8 g/mol

There are two mistakes in the given solution. One is that units are incorrect. Even the number of moles calculated is incorrect.

4. It is not necessary to to know the volume of liquid because we need the volume of gas for calculation using ideal gas law. We know that in gaseous state, it will occupy the volume of the flask, hence volume of gas equals volume of the flask.

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