A mixture of dry ice (i.e., solid CO2) and acetone (liquid) is commonly used in

Question

A mixture of dry ice (i.e., solid CO2) and acetone (liquid) is commonly used in laboratories for cooling applications. You mix dry ice (1.00 kg at -78.2C) and acetone (1.00 kg at 25.0C) in a thermally well insulated container. The container is open such that CO2 may leave the flask. NOTE: CO2 at standard pressure sublimes (i.e. turns directly to the CO2 gas) at negative 78.2C.

a. What is the final state of the mixture (phase and temperature of each substance)?

b. What changes are to be expected if the amount of dry ice added to the system is doubled

Explanation / Answer

a. heat lost by dry ice = heat gained by acetone

heat = mCpdT

Cp for dry ice is approximately 0.7 J/oC

Cp for acetone is 2.16 J/oC

Tf = final temperature

1000 x 0.7 x (Tf + 78.2) = 1000 x 2.16 x (25 - Tf)

700Tf + 54740 = 54000 - 2160Tf

740 = -2860Tf

Tf = -0.26 oC

So the final phase of acetone would be solid and dry ice would be gas-solid

Temperature of the mixture would be -0.26 oC

b. If the amount of dry ice is doubled, the resulting temperature would be reduced even further and the phases would remain almost the same as with a. conditions.

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