Aqueous solutions boiling-point elevation

What is the evaporation rate and yield of the sodium acetate hydrate CH3C00Na.3H20 from a continuous evaporative crystalliser operating at 1 kN/m2 when it is fed with 1 kg/s of a 50 per cent by mass aqueous solution of sodium acetate hydrate at 350 K The boiling point elevation of the solution is 10 degK and the heat of crystallisation is 150 kJ/kg. The mean heat capacity of the solution is 3.5 kJ/kg K and, at 1 kN/m2, water boils at 280 K at which temperature the latent heat of vaporisation is 2.482 MJ/kg. Over the range 270-305 K, the solubility of sodium acetate hydrate in water s at T(K) is given approximately by ... 

At 13.5 kN/m2, water boils at 325 K and, in the absence of data on the boiling point elevation, this will be taken as the temperature of evaporation, assuming an aqueous solution. The total enthalpy of steam at 325 K is 2594 kJ/kg. 

What is the yield of sodium acetate crystals (CH3C00Na.3H20) obtainable from a vacuum crystalliser operating at 1.33 kN/m2 when it is supplied with 0.56 kg/s of a 40 per cent aqueous solution of the salt at 353 K The boiling point elevation of the solution is 11.5 deg K. 

ACTIVITY COEFFICIENT. A fractional number which when multiplied by the molar concentration of a substance in solution yields the chemical activity. This term provides an approximation of how much interaction exists between molecules at higher concentrations. Activity coefficients and activities are most commonly obtained from measurements of vapor-pressure lowering, freezing-point depression, boiling-point elevation, solubility, and electromotive force. In certain cases, activity coefficients can be estimated theoretically. As commonly used, activity is a relative quantity having unit value in some chosen standard state. Thus, the standard state of unit activity for water, dty, in aqueous solutions of potassium chloride is pure liquid water at one atmosphere pressure and the given temperature. The standard slate for the activity of a solute like potassium chloride is often so defined as to make the ratio of the activity to the concentration of solute approach unity as Ihe concentration decreases to zero. 

What is the molality of an aqueous glucose solution if the boiling point of the solution at 1 atm pressure is 101.27°C The molal boiling-point-elevation constant for water is given in Table 11.4. 

Concentrations expressed as molality or mole fractions are temperature-independent and are most useful when a physical measurement is related to theory over a range of temperature, e.g., in freezing point depression or boiling point elevation measurements (Chapter 11). Since the density of water is close to 1 g/cm3, molal and molar concentrations are nearly equal numerically for dilute aqueous solutions (<0.1 M). 

Aqueous solutions vaporize with nearly the same coefficient as pure water if attention is given to boiling-point elevation and if tbe solution does not become saturated and care is taken to avoid dry wall conditions. 

Molality is used in certain physical chemical calculations (e.g., calculations of boiling-point elevation and freezing-point depression). For dilute aqueous solutions, m and M will be quite close. In order to interconvert m and M, we need to know % w/w. 

The molal boiling point elevation constant, S), is the difference in boiling points between aim nonvolatile, nonelectrolyte solution and a pure solvent. It is expressed in units of °C/w and varies for different solvents. Values of A j, for several common solvents are found in Table 15-4. Note that water s A j, value is 0.512°C/w. This means that a m aqueous solution containing a nonvolatile, nonelectrolyte solute boils at 100.512°C, a temperature 0.512°C higher than pure water s boiling point of 100.0°C. 

A lab technician determines the boiling point elevation of an aqueous solution of a nonvolatile, nonelectrolyte to be 1.12°C. What is the solution s molality ... 

The expressions for boiling-point elevation and freezing-point depression apply accurately to dilute solutions only. A saturated aqueous solution of Nal (sodium iodide) in water has a boiling point of 144°C. The mole fraction of Nal in the solution is 0.390. Compute the molality of this solution. Compare the boiling-point elevation predicted by the expression in this chapter with the elevation actually observed. 

The boiling point elevation (BPE) of some 40 saturated aqueous solutions of inorganic salts have been reported by Meranda and Furter (1977) who proposed the correlating relationship... 

F ure 12.10 Phase diagreim illustrating the boiling-point elevation and teezing-point depression of aqueous solutions. The dashed cunres pertain to the solution, and the solid curves to the pure solvent. 4s you can see, the boiling point of the solution is higher than that of water, and the freezing point of the solution is lower them that of water. 

Table 12.2 lists values of A), for several common solvents. Using the boiling-point elevation constant for water and Equation (12.6), you can see that if the molality of an aqueous solution is 1.00 m, the boiling point will be 100.52°C. 

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