Question

CHAPTER 7. HEAT TRANSFER WITH CHANGE OF PHASE 7.9 Problems 7.9.1 Freezing of a Solution Consider freezing of grape juice. Assume that the initial weight percentage of water in grape juice is 85% and the effective molecular weight of solute is 183.61 . I) Predict the percent of initial water frozen in grape juice when the temperature has been reduced to -6°C. 2) -15°C. 3) Would you juice of, let's say, 50% water? Why? Repeat the calculations in 1 when the temperature has been reduced to r answer change if we started from a more concentrated grape 7.9.2 Storing at Subzero Temperature Without Freezing A large biological tissue has to be kept at a temperature as low as possible, but without freezing (ice formation). The water in the tissue has dissolved in it several materials, with an effective molecular weight of 50. The concentration of all dissolved materials combined is 10% of the weight of water. What is the lowest temperature that this tissue can be stored at without ice formation? The latent heat of fusi

Answer 1

Calculate the grams of water per gram of juice from the freezing point depression at -6°C AH1 6013.4 J/mol 1 8.314 J/mol K 273 267 0.15 x 183.61 18 · .x -0.059 0.15 · x 183.61 18 0.9420.15_* 183.61 18 7.695×10-4 +0.05 2x= 0.055x 7.695x 10 x-3x10 x 0.2565 Calculate the fraction of water frozen at -6°C 0.85 0.2565 0.85 - 0.698 -69.8% Calculate the grams of water per gram of juice fom the freczing point depression at -15°C ΔΗ。 .x 6013.4 Jmol 1 8.314 J/mol.K 273 258 18 0.15 . 183.61 18 -0.154 18 .x 183.61 18 0.857 0.15x 183.61 18 7x10 +0.0476x 0.0555x 7.888x 10 x 0.088 Calculate the fraction of water frozen at -15°C 0.85-0.088 0.85 F-15C 0.895 89.5% Yes. The percentage of water frozen will be lower if we started with a more concen trated solution