Partial molar volumes of sodium chloride solutions PDF

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Objective: In this experiment the partial molar volumes of sodium chloride solutions will be calculated as a function of concentration from densities measured with a pycnometer. Introduction: Thermodynamic variables fall into two types: extensive and intensive properties. Intensive properties do not depend on the quantity of matter. Examples include density, state of matter, and temperature. Extensive properties do depend on sample size. Examples include volume, mass, and size. A partial molar property is a thermodynamic quantity which indicates how an extensive property of a solution or mixture varies with changes in the molar composition of the mixture at constant temperature and pressure These other partial molar quanties can be defined as:

(eq. 1) Where Q is any extensive property (V, E, H, S, A, G) of the thermodynamic functions ni is the number of moles of component i, and p and t are pressure and temperature respectively. Similarly, this concept can be applied to chemical potential also known as the partial molar gibbs energy. dG μi=( ) (eq.2) dni p,t,n’ In much the same fashion as the partial molar volume is defined, the partial molar Gibbs function is defined for compound i in a mixture. For a mixture, G=naμa+nbμb. For a binary solution it can also be written as: ´ 2 −X 1 dQ = ´ 1 X2 dQ

(eq.3)

´ ❑ is any extensive variable Q. where the Xi are mole fractions, Xi =ni/∑ni and Q Considering a system with pure solid NaCl at equilibrium with saturated aqueous solution. If the partial molar volume of solute in aqueous solution is greater than the molar volume of solid solute, an increase in pressure will increase the chemical potential of solute in solution relative to that in the solid phase; solute will then leave the solution phase until a lower, equilibrium solubility is attained. If the partial molar volume in the solution is less than that in the solid, the solubility will increase with pressure. Method: 1 Kg (55.51 moles) of water and the total volume of the solution is given by: V  n1V1  n2V2  55.51V1  mV2

(eq.4)

where the subscripts 1 and 2 refer to solvent and solute, respectively. Let V1 be the molar volume of pure water (18.069cm3 mol 1 at 25 C ) then we define the apparent molar volume  of the solute by the equation:

¿

1 1 0 0 V 1 ) = (V −55.51 ~ V1) (V −n1 ~ m n2

Where m is the moles and for volume: V=

(eq.5)

~0 is the molar volume of water. This can be rearranged to solve V1

1000+m M 2 3 cm d

(eq.6)

Where d is density of the solvent and M2 is the solute molar mass in gram. When and √ m are plotted, the slope at the required concentration is taken and both volumes can be obtained. PHYSICAL DATA: COMPOUND

MOLECULA R WEIGHT (g/mol)

MELTING POINT (℃)

BOILING POINT (℃)

DENSIT Y (g/cm3)

SOLUBILIT Y IN WATER

HAZARDS

Acetone, colorless liquid

58.080

-94.7

56.05

0.7845

Miscible

EYE IRRITANT. Can irritate the nose and throat

Sodium Chloride, White Powder NaCl

58.44

805

1465

2.17

360 g/L

Ingestion of this compound may cause nausea, vomiting, diarrhea, etc.

Procedures : 1) Make up 200 ml of approximately 3.2m (3.0M) NaCl in water by weighing the salt accurately and use a volumetric flask. 2) Solutions of 1/2, 1/4, 1/8, and 1/16 of the initial molarity are to be prepared by successive volumetric dilution; for each dilution pipette 100 ml of solution into a 200-ml volumetric flask and make up to the mark with distilled water. 3) The pycnometer is rinsed with distilled water and thoroughly dried before each use. The pycnometer should be weighed empty and dry (We ). 4) Fill the pycnometer with distilled water and hang it in the thermostat bath (30.0C) with the main body below the surface. Allow at least 15 min for equilibration.

5) Remove the pycnometer from the bath and quickly but thoroughly dry the outside surface with a towel or filter paper. Weigh the pycnometer(Wo). 6) By the similar procedures, record the weight (W) of pycnometer when filled with each NaCl solutions as prepared in step (2)....