When making ice cream, the temperature of the ingredients is decreased to below the freezing pt. pf pure water
by using a surrounding ice bath that contains a large amount of table salt.
a) assuming that it dissolves completely and forms an ideal solution, what mass of NaCl is needed to lower the freezing pt of 5.5 kg of water to -5.5 Celsius
b) making the same assumptions , calculate the mass of CaCl2 needed.
Answer:
Freezing-point depression is the difference between the freezing points of a pure solvent and a solution mixed with a solute. It is directly proportional to the molal concentration of the solution, or more precisely, to the solute activity, according to the equation:
ΔTf = i · Kf · activity
activity is in units of mol/kg, and is equal to an activity coefficient times the molality
ΔTf, the freezing point depression is defined as T - Tf, where T is the freezing point of the solution and Tf is the freezing point of the pure solvent.
Kf, the cryoscopic constant, is a colligative property, given by RTf2/ΔHf, where R is the gas constant, and Tf is the normal freezing point of the solvent and ΔHf is the heat of fusion per kilogram of the solvent
Kf for water is 1.858 K·kg/mol (or more commonly used, 1.858 C/m) which means that per mole of solute dissolved in a kilogram of water the freezing point depression is 1.858 kelvins.
i is the i factor or the van 't Hoff factor (see van 't Hoff), accounts for the number of individual ions formed by a compound in solution.
Examples:
i = 1 for sugar in water.
i = 2 for NaCl in water.
i = 3 for CaCl2 in water.
i = 2 for HCl in water. (complete dissociation)
i = 1 for HCl in benzene. (no dissociation)
Freezing-point depression can be used to measure the degree of dissociation, the activity, or the molar mass of the solute, although this particular process, called cryoscopy (Greek "freeze-viewing"), is not as common as it once was. It was still taught as a useful analytic procedure in Cohen's Practical Organic Chemistry of 1910,[1] in which the molar mass of napthalene is determined in a so-called Beckmann freezing apparatus.
Freezing-point depressions occur whenever a solute is added to a pure solution, such as water. This is due to solute molecules disrupting the ability of the solute to form crystals during the freezing process. Because of this, the liquid range of solvent is increased resulting in a freezing point depression.
Do your homework yourself!
More Questions and Answers:
Calculate the pH if the pOH is 5.3?
Is there a site i can go on to learn what the letters stand for on the periodic table?
Do electrons & protons really exist?
A person traveling west gains an hour when he croses a time zone. (look inside) SCIENCE?
What is the chemical equation for Cu + O2 ; CuSO4 . 5H2O ; Fe + HCl ; Fe+ CuSO4 ; FeCl3+KCNS ; FeCl3+K4Fe(CN)6
Increase acetic acid content of vinegar by evaporation?
Why did the flavour of my chocolate change?
How do i find the formula for this compound?
Studying for organic chemistry??
a) assuming that it dissolves completely and forms an ideal solution, what mass of NaCl is needed to lower the freezing pt of 5.5 kg of water to -5.5 Celsius
b) making the same assumptions , calculate the mass of CaCl2 needed.
Answer:
Freezing-point depression is the difference between the freezing points of a pure solvent and a solution mixed with a solute. It is directly proportional to the molal concentration of the solution, or more precisely, to the solute activity, according to the equation:
ΔTf = i · Kf · activity
activity is in units of mol/kg, and is equal to an activity coefficient times the molality
ΔTf, the freezing point depression is defined as T - Tf, where T is the freezing point of the solution and Tf is the freezing point of the pure solvent.
Kf, the cryoscopic constant, is a colligative property, given by RTf2/ΔHf, where R is the gas constant, and Tf is the normal freezing point of the solvent and ΔHf is the heat of fusion per kilogram of the solvent
Kf for water is 1.858 K·kg/mol (or more commonly used, 1.858 C/m) which means that per mole of solute dissolved in a kilogram of water the freezing point depression is 1.858 kelvins.
i is the i factor or the van 't Hoff factor (see van 't Hoff), accounts for the number of individual ions formed by a compound in solution.
Examples:
i = 1 for sugar in water.
i = 2 for NaCl in water.
i = 3 for CaCl2 in water.
i = 2 for HCl in water. (complete dissociation)
i = 1 for HCl in benzene. (no dissociation)
Freezing-point depression can be used to measure the degree of dissociation, the activity, or the molar mass of the solute, although this particular process, called cryoscopy (Greek "freeze-viewing"), is not as common as it once was. It was still taught as a useful analytic procedure in Cohen's Practical Organic Chemistry of 1910,[1] in which the molar mass of napthalene is determined in a so-called Beckmann freezing apparatus.
Freezing-point depressions occur whenever a solute is added to a pure solution, such as water. This is due to solute molecules disrupting the ability of the solute to form crystals during the freezing process. Because of this, the liquid range of solvent is increased resulting in a freezing point depression.
Do your homework yourself!
The answers post by the user, for information only, FunQA.com does not guarantee the right.
More Questions and Answers: