NaCl dissociation

The energy diagram describing NaCl dissociation is quite different to that observed for R—X and [R—X] dissociation. This is because for NaCl the two relevant configuration curves cross (Herzberg, 1950 Levine and Bernstein, 1974), in contrast to the situation for R—X and (R—X) where the two curves are separated. The well-known NaCl curve crossing is illustrated in Fig. 7. At infinite separation Na Cl is more stable than... 

As NaCl dissociates completely into Na and Cl, and the ions exert osmotic pressures independently, the total osmolar concentration is... 

We shall see later that the differences are not evidence against the correctness of the theory, but follow logically from the theory of the electrostatic bond. The reason why the molecules in molten NaCl dissociate into ions, but those in CC14 do not, is immediately apparent... 

To illustrate, let us consider the freezing-point depression that occurs when we put 1 mole of NaCl into 1 kg of water. NaCl dissociates according to the equation... 

Next, calculate the mole fraction of water in the solution. Assuming that NaCl dissociates completely into two particles when dissolved in water, the solution contains 0.626 mol of dissolved particles 0.313 mol Na+ ions and 0.313 mol Cl- ions. Thus, the mole fraction of water is... 

Answer You will use the boiling-point elevation formula to solve this. Because sodium chloride is an electrolyte, it will be necessary to use the van t Hoff factor in this problem. Each NaCl dissociates into one Na+ ion and one Cl ion. Therefore, the van t Hoff factor will be 2. 

MCAT 1 MORGAN if. Chemistry Remember that solution concentrations are always given in terms of the form of the solute before dissolution. For instance, when 1 mole of NaCl is added to 1 liter of water, it is approximately a 1 molar solution and NOT a 2 molar solution even though each NaCl dissociates into two ions. Normality measures the number of equivalents per liter of solution. The definition of an equivalent will depend upon the type of reaction taking place in the solution. The only time normality is likely to appear on the MCAT is with an acid-base reaction. In an acid-base reaction an equivalent is defined as the mass of add or base that can furnish or accept one mole of protons. For instance, a 1 molar solution of H2S04 would be called a 2 normal solution because it can donate 2 protons for each h2so4. 

A is correct. Boiling point elevation is a colligative property. The more particles the higher the boiling point. NaCl dissociates so that the normality is twice the molarity. Thus, the least number of particles will be in 0.5 M glucose solution. 

The coUigative properties of electrolytes require a slightly different approach than the one used for the coUigative properties of nonelectrolytes. The reason is that electrolytes dissociate into ions in solution, and so one unit of an electrolyte compound separates into two or more particles when it dissolves. (Remember, it is the number of solute particles that determines the coUigative properties of a solution.) For example, each unit of NaCl dissociates into two ions—Na and Cl. Thus the coUigative properties of a 0.1 m solution of NaCl should be twice as great as those of a 0.1 m solution con-... 

Blood plasma has an osmolarity equivalent to a 0.30 M glucose solution or a 0.15 M NaCl solution. This latter is true because in solution NaCl dissociates into Na+ and Cl and thus contributes twice the number of solute particles as a molecule that does not ionize. If red blood cells, which have an osmolarity equal to blood plasma, are placed in a 0.30 M glucose solution, no net osmosis will occur because the osmolarity and water concentration inside the red blood cell are equal to those of the 0.30 M glucose solution. The solutions inside and outside the red blood cell are said to be isotonic iso means "same," and tonic means "strength") solutions. Because the osmolarity is the same inside and outside, the red blood cell will remain the same size (Figure 18.20b). 

In the zero approximation the electrolyte solution is treated as a mixture of ideal noninteracting ions. The fact that this approximation gives good results when applied to a dilute solution of, for example, NaCl is strong evidence for the structural hypothesis that NaCl dissociates into Na and Cl" ions in solution. 

The composition of NaCl dissociation products in atmospheric-pressure thermal plasma is shown in Fig. 7-42. The initial concentration of NaCl is 17.1 mol/kg. Sodium formation takes place at temperatures exceeding 3000 K. The energy cost of sodium production from chloride is shown in Fig. 7-43. The minimal energy cost is 9.4eV/atom in the case of absolute quenching, and 8.8 eV/atom in the case of ideal quenching, which can be achieved at a specific energy input of 8.2 eV/mol. 

SOLUTION Yes. Every liter of solution contains 9 g NaCl since the M, of NaCl is 58.44, the concentration of this solution is 154 mmol L". Since NaCl dissociates into two solute particles in water, the solution has an osmolarity of 308 mOsmol L" . This is very close to being isotonic with plasma. 

Since NaCl dissociates to form two particles (ions), the number of moles of NaCl is half of the above result. 

Remember that NaCl dissociates into two ions in solution therefore, we multiply the moles of NaCl by two. 

The rate of NaCl dissociation around 1 atm and 2300-4800 K may be taken as representative of the absolute values found for the rate... 

Besides the cations being attracted to the very negative external electrode during electric impulse, there is also a local electrochemistry elfect at each electrode, which causes chemical changes in these PMA based EAPs. In an aqueous environment, NaCl dissociates into sodium and chloride ions ... 

Electrolyte solutions are those in which the solute (e.g., NaCl) dissociates into charged particles called ions (e.g., Na" " and Cr). Naturally, such solutions are much better conductors of electricity than is the solvent alone (normally water), and the conductivity of the solution will depend on the extent to which the dissociation takes place. That is, if only some of the NaCl dissociates into Na+ and Cl , the solution will be less conducting than if all of it does. Measurement of conductivity is therefore a means of determining the degree of dissociation of solutes. Significant improvements in the speed and accuracy of conductance measurements has been achieved by using a flow-through cell (Zimmerman etal. 1995). 

For colligative properties of strong electrolyte solutions, the solute formula tells us the number of particles. For instance, the boiling point elevation iAT ) of 0.050 m NaCl should be 2 X of 0.050 m glucose (C(5Hj20(5), because NaCl dissociates into two particles per formula unit. Thus, we use a multiplying factor called the van t Hoff factor (i), named after the Dutch chemist Jacobus van t Hoff (1852-1911) ... 

Much of what is known about ion dissociation comes from tw o types of experiment measurements of colligative properties and ion conductivities. Chapter 16 shows that the freezing point depression AT/ should increase with the concentration of a solute species m according to AT/ = Kfin, where Kf is a constant that is independent of the solute (see page 288). But what value of m should you use for a solute, like NaCl, that can dissociate Each mole of NaCl dissociates into two moles of ions, Na and Cl . Should m be the concentration of NaCl or twice the concentration of NaCl If you let m be the concentration of the neutral species then the observed value i in the colligative expression ATf = imKf gives a measure of the degree of dissociation. For example, if you observe that i = 2 for NaCl, it implies full dissociation. 

Solution Osmotic pressure depends on the colligative concentration, i.e. the number of particles per unit volume. Since NaCl dissociates into Na" " and Cl ions, the colligative molality of the above solution is 0.30 M. Using the van t Hoff s equation (8.2.14), we can calculate the osmotic pressure 7t ... 

Osmotic pressure in salt water is easy to calculate. For a salt concentration of 10 g L , we obtain Posm = 8.5 bar at a temperature of 25°C. The molar mass of NaCl being 58 g (35 g + 23 g), 10 g L of salt correspond to 345 mol-m of ions. Now, it is observed that a factor 2 appears, since NaCl dissociates into Na and Cl" ions (i.e. there are 172.5 mol-m" of Na ions and 172.5 mol-m of Cl ions). The main finding is that the osmotic pressure is very high because, for a reasonable concentration of particles in g L , the concentration of particles in mol is large... 

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