Question

You have a glass of water at 83 degrees C. You want to boil it by putting it in a vacuum. What is the smallest percentage of air molecules you would have to remove from the vacuum chamber to allow it to boil? Hint: to boil, the vapor pressure of water must be equal to the external pressure. (You will have to refer to a chart to figure out the vapor pressure of water at 83 degrees). (Remember, atmospheric pressure is 760 torr.) (Remember, pressure and number of moles of gas are directly proportional.)

  

25%

   

50%

   

75%

   

90%

2.

Three lizards went to warm themselves in the sun. After an hour, the first lizard bragged, "I increased my temperature by 15 Kelvin." (Note- Kelvin is not a "degree" scale) The second lizard said, "That's nothing. I increased my temperature by 20 Celsius degrees." The third lizard said, "I have you all beat. I increased my temperature by 22 Fahrenheit degrees."

Which lizard actually experienced the biggest increase in temperature?

  

The first: 15K

   

The second: 20 C

   

The third: 22 F

   

It was a tie between the second and third.

3.

Select all of the following that are true

  

It is possible to determine the average kinetic energy of sample of gas molecules by directly measuring only the temperature of that sample.

   

As the temperature rises, the root mean squared velocity of the gas sample increases.

   

The molecules in a sample of massive gas will have a higher root mean squared velocity than the molecules in a sample of a less massive gas.

   

The hot air above a candle will be more dense than colder air surrounding it.

Answer 1

(1) The vapor pressure of water at 83 degrees is 400.6 torr.

Also, the atmospheric pressure is 760 torr.

Let, the number of moles of air at 760 torr be n_i, and that at 400.6 torr be n_f.

Since, pressure and number of moles of gas are directly proportional,

n_f / n_i = 400.6 / 760 = 0.5.

So, to start boiling, the number of air molecules must be 0.5 times that of the initial.

Hence, air molecules to be removed : ( 1 - 0.5 ) x 100 % = 50 %.

Hence, correct option is : 50 %.

(2) 1^o C change = 1 K change.

So, 20^o C change = 20 K change > 15 K change.

Also, 100^o C change = 180^o F change.

So, 20^o C change = ( 20 x 180 / 100 )^o F change = 36^o F change > 22^o F change.

So, the greatest change in temperature is 20^o C.

Hence, correct option is : 20^o C.