Molality, of a solution

There are several different scales 011 which the activity of a solute may be defined.1 In thermodynamic expressions for a solute in a non-ideal solution the activity on the molality scale plays the same part that is played by the molality of a solute in an ideal solution. Since the activity is expressed in the same units as the molality, the ratio of the activity to the molality—the activity coefficient—is a pure number whose value is independent of these units it is also indopendont of the particular b.q.s. that has been adopted. Thus the numerical values of all activities and molalities would change in the same ratio, if at any time a new choice were made for the b.q.s. 

The molality of a solution is readily calculated if the masses of solute and solvent are known (Example 10.3). 

Two measures of concentration that are useful for the study of colligative properties, because they indicate the relative numbers of solute and solvent molecules, are mole fraction and molality. We first met the mole fraction, x, in Section 4.8, where we saw that it is the ratio of the number of moles of a species to the total number of moles of all the species present in a mixture. The molality of a solute is the amount of solute species (in moles) in a solution divided by the mass of the solvent (in kilograms) ... 

EXAMPLE 8.5 Sample exercise Calculating the molality of a solute... 

The molality of a solute in a solution is the amount (in moles) of solute divided by the mass (in kilograms) of solvent used to prepare the solution. 

EXAMPLE 16.3. Calculate the molality of a solution prepared by adding 0.200 mol of solute to 100g of water. 

Calculate the molality of a solution with mole fraction 0.100 of C2H,OH in water. Ans. Assume 1.00 mol total, which contains ... 

A—To calculate the molality of a solution, both the moles of solute and the kilograms of solvent are needed. A liter of solution would contain a known number of moles of solute. To convert this liter to mass, a mass to volume relationship (density) is needed. 

Although molalities are simple experimental quantities (recall that the molality of a solute is given by the amount of substance dissolved in 1 kg of solvent) and have the additional advantage of being temperature-independent, most second law thermochemical data reported in the literature rely on equilibrium concentrations. This often stems from the fact that many analytical methods use laws that relate the measured physical parameters with concentrations, rather than molalities, as for example the Lambert-Beer law (see following discussion). As explained in section 2.9, the equilibrium constant defined in terms of concentrations (Kc) is related to Km by equation 14.3, which assumes that the solutes are present in very small amounts, so their concentrations (q) are proportional to their molalities nr, = q/p (p is the density of the solution). 

In the two previous examples, the sucrose and santonic acid existed in solution as molecules, instead of dissociating to ions. The latter case requires the total molality of all ionic species. Calculate the total ionic molality of a solution of 50.0 grams of aluminum bromide (AlBr3) in 700 grams of water. Because the gram formula weight of AlBt3 is... 

What is the molality of a solution made by dissolving 36 g of sodium chloride (NaCl) in 6 L of water (Remember that the density of water is 1.0 kg/L.)... 

Boiling point elevations are directly proportional to the molality of a solution, but chemists have found that some solvents are more susceptible to this change than others. The formula for the change in the boiling point of a solution, therefore, contains a proportionality constant, abbreviated K, which is a property determined experimentally and must be read from a table such as Table 13-2. The formula for the boiling point elevation is... 

The molality of a solute is the number of moles of solute in a solution divided by the mass of the solvent in kilograms ... 

To calculate the molality of a solution prepared by dissolving 10.5 g of sodium chloride in 250 g of water, we convert the mass of sodium chloride to moles of NaCl (by dividing the mass by the molar mass) and divide it by the mass of water in kilograms ... 

Calculate the mole fraction and molality of a solute, given the masses or mass percentages of solute and solvent, Examples G.6 and G.7. 

What is the molality of a solution made by dissolving 1.45 g of table sugar (sucrose, C12H22O11) in 30.0 mL of water The molar mass of sucrose is 342.3 g/mol. 

PROBLEM 11.6 What is the molality of a solution prepared by dissolving 0.385 g of cholesterol, C27H460, in 40.0 g of chloroform, CHC13 What is the mole fraction of cholesterol in the solution ... 

When the concentration of a multicomponent system is expressed in terms of the molalities of the solutes, the expression for the chemical potential of the individual solutes and for the solvent are somewhat different. For dilute solutions the molality of a solute is approximately proportional to its mole fraction. (The molality, m, is the number of moles of solute per kilogram of solvent. When two or more substances, pure or mixed, may be considered as solvents, a choice of solvent must be clearly stated.) In conformity with Equation (8.68), we then express the chemical potential of a solute in a solution at a given temperature and pressure as... 

The molality of a solution is the number of moles of solute per kilogram of solvent in the solution. The molality (m) cannot be computed from the molar concentration (M) unless the density of the solution is known (see Problem 12.88). 

Molar concentration of a solution = M Normality of a solution = N Molality of a solution = m Mole fraction of a component = X... 

The molality of a solution of ethyl alcohol, C2H5OH, in water is 1.54 mol/kg. How many grams of alcohol... 

Benzoic acid, C6H5COOH, is soluble in benzene, CgHg. What is the molality of a solution in which 3.55 g benzoic acid are dissolved in 75 mL of benzene (Density benzene is 0.866 g/mL at the temperature of the experiment.)... 

Calculate the molality of a solution that contains 106 g naphthalene (C10H8) dissolved in 3.15 mol carbon tetrachloride (CC14). 

Calculate the molality of a solution of calcium chloride (CaCl2) made by dissolving 222 grams in 1 kilogram of water. 

Molality is a bit different. It is calculated as the moles of solute per kilogram of solvent. Two main differences here first, you are measuring units of mass instead of units of volume, and second, you are using only the amount of solvent in the denominator. That s where the confusion usually comes from with molarity. With molarity, you are dividing the moles by the amount of solution, whereas in molality, you are dividing the moles by the amount of solvent. To calculate the molality of a solution where substance A is dissolved in some solvent, you would use Equation 10.3 ... 

Sample What is the molality of a solution that has 27.0 g of potassium chromate (K2Cr04, molar mass 194.2) dissolved in 110.0 g of water ... 

Calculate the molality of a solution of methyl alcohol, CH3OH, in water in which the alcohol has a mole fraction of 0.133. 

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