1 ) In a physics lab, a student accidentally drops a 27.0-g brass washer into an

Question

1 ) In a physics lab, a student accidentally drops a 27.0-g brass washer into an open Dewar of liquid nitrogen at 77.2 K. How much liquid nitrogen boils away as the washer cools from 297 K to 77.2 K? The latent heat of vaporization for nitrogen is 199.1 kJ/kg.

2 ) What mass of water at 28.0° C added to a Styrofoam cup containing two 60.0-g ice cubes from a freezer at -20.0° C will result in a final temperature of 5.0° C for the drink?

3 ) Ice at 0.0° C is mixed with 4.00 102 mL of water at 21.0° C. How much ice must melt to lower the water temperature to 0.0° C?

Explanation / Answer

1 ,

heat lost by brass = heat gained by N2 ;

0.377 kJ / kg = 0.377 J / g ; <--------heat capacity of brass l

27 * 0.377 * ( 297 - 77.2 ) = ( 199.1 J / g ) * m ;

m = 11.237 g of N2  

2 .

heat gained by ice = heat lost by water .

specific heat of ice = 2.108 J / g ;

heat additon from 20 to 0 C + Latent heat + heat addition from 0 to 20 =    m * 4.18 * ( 28 -5 )

2.108 * ( 2 * 60 ) * ( 20 - 0 ) + 334.4 * ( 2 * 60 ) + ( 2 * 60 ) ( 4.18 ) ( 5 ) =   m * 4.18 * ( 28 -5 )

or m = 496 .10 g <---------mass of water that should be added .

3 .

let the mass be m ;

334.4 * m = M C T ;

334.4 * m = ( V ) * 4.18 * ( 21 - 0 ) ;

= 1 g / cm6 ;

V = 4 e 2 mL = 4 e 2 cm^3 ;

334.4 * m = ( 1 * 4 e 2   ) * 4.18 * ( 21 - 0 ) ;

or m = 105 g <-----------ans

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