Molarity and molality

These two similar-sounding terms must not be confused. The molarity of a solution is the number of moles (gram molecular weights) of solute in 1 litre (1 dm ) of solution. The molality is the number of moles of solute in 1 kg of solvent Molality has the unit, mol kg which 

Of the two units, molality is preferable for a precise expression of concentration because it does not depend on the solution temperature as does molarity also, the molality of a component in a solution remains unaltered by the addition of a second solute, whereas the molarity of this component decreases because the total volume of solution increases following the addition of the second solute. 

---

C12-0005. Determine the molarity and molality of quinine in the tonic water described in Section Exercise, assuming that the density of tonic water is 1.00 g/mL. 

C12-0037. A saturated solution of hydrogen peroxide in water contains 30.% by mass H2 O2 and has a density of 1.11 g/mL. Calculate the mole fractions, molarity, and molality of this solution. 

There are many ways of expressing the relative amounts of solute(s) and solvent in a solution. The terms saturated, unsaturated, and supersaturated give a qualitative measure, as do the terms dilute and concentrated. The term dilute refers to a solution that has a relatively small amount of solute in comparison to the amount of solvent. Concentrated, on the other hand, refers to a solution that has a relatively large amount of solute in comparison to the solvent. However, these terms are very subjective. If you dissolve 0.1 g of sucrose per liter of water, that solution would probably be considered dilute 100 g of sucrose per liter would probably be considered concentrated. But what about 25 g per liter—dilute or concentrated In order to communicate effectively, chemists use quantitative ways of expressing the concentration of solutions. Several concentration units are useful, including percentage, molarity, and molality. 

Like the difference in their names, the practical difference between molarity and molality is subtle. Take a close look at their definitions, expressed next to one another in the following equations ... 

The numerators in molarity and molality calculations are identical, but their denominators differ greatly. Molarity deals with liters of solution, while molality deals with kilograms of solvent. A solution is a mixture of solvent and solute a solvent is the medium into which the solute is mixed. 

Problem A careful laboratory technician prepares an ethanol/water solution of precisely known concentration by measuring the number of moles of pure ethanol (net), mass of pure water (Mwa, in kilograms) and volume of the resulting solution (VSoin)- From these, he calculates the precise molarity and molality of the solution,... 

Example Converting Weight Percent into Molarity and Molality... 

Find the molarity and molality of 37.0 wt% HC1. The density of a substance is the mass per unit volume. The table inside the back cover of this book tells us that the density of the reagent is 1.19 g/mL. 

Because the density of water is approximately 1 g/ml, the molarities and molalities of water solutions will have about the same value. This will not be true for most other solvents. 

To prepare a 1.000 m solution of KBr in water, for example, you would dissolve 1.000 mol of KBr (119.0 g) in 1.000 kg (1000 mL) of water. You can t say for sure what the final volume of the solution will be, although it will probably be a bit larger than 1000 mL. Although the names sound similar, note the differences between molarity and molality. Molarity is the number of moles of solute per volume (liter) of solution, whereas molality is the number of moles of solute per mass (kilogram) of solvent. 

Solubilities are generally reported in units such as mg/L or the equivalent g/m3, but other unit systems are used, especially molarity and molality. Solutions exceeding 100 mg/L are relatively easy to handle and analyze. In the range 1 to 100 mg/L, more care is required and accuracy is reduced. In the range 0.001 to 1 mg/L, extreme care is required and reported data are often suspect. Below 0.001 mg/L, solutions are so dilute that it is very difficult to obtain reliable data. Mackay et al. (1997) and Yalkowsky and Banerjee (1992) review methods of measuring solubility and other environmentally relevant properties. 

See Section 2.1 ofEisenberg (1976) for a full discussion of the relation between molal and molar units. Table 21.1 lists partial molar volumes of different salts. The difference between molar and molal units is about 1% for a 0.3 MKC1 solution, and rises to only 3% for 1 MKC1. These percentages are smaller than the error in most measurements of Kohs. 

Upon substitution of Eq. (21.27) into the linkage Eq. (21.26), and including the above approximation that (9himMgCi2/dln< MgCi2) 1, as well as the additional approximation that molarity and molality units are interchangeable, the linkage relation for MgCl2 becomes... 

An aqueous solution labeled as 35.0% HCIO4 had a density of 1.251 g/mL. Calculate molarity and molality of the solution. 

Chloroacetic acid, amonoprotic acid, has a Ka of 1.40 x 10 3. Calculate the freezing point of a 0.10 M solution of this acid. Assume that the numerical value of the molar and molal concentrations are the same. 

But we can also answer this question by converting molarity to molality. So, what is the molal concentration of a 0.28 molar solution of glucose To convert between molality and molarity, we need to know the density of the solution. The density of a D5W solution is 1.0157 g/mL. We also need to be very careful about the definitions of molarity and molality, and keep in mind whether we are dealing with liters of solutions or kilograms of solvent. 

Several terms used in vernacular science are not appropriate for scientific communication. The CIPM does not use such terms as parts per million, parts per billion, or parts per trillion or their abbreviations as expressions of quantities. The word weight is a force with the SI unit of newton, not a synonym for mass with the SI unit of kilogram. Terms for an object and quantities describing the object require a clear different action. Normality, molarity, and molal are obsolete terms no longer used. 

This value is used to calculate molarity and molality ... 

Whereas molarity and molality are the concentration units of choice for chemists, percent solutions are more suitable for consumer products. The three types of percent solutions commonly used are defined as follows. 

The three typical ways to describe solutions in chemistry are the mole fraction, molarity, and molality. The term normality is occasionally used, but the College Board specifically states that it is not on the test, so we ll skip it. The mole fraction is the same mole fraction we discussed earlier. The mole fraction of substance A is expressed in Equation 10.1 ... 

Molarity and molality are very similar words, so be very careful to know the difference. 

For dilute aqueous solutions (solutions where water is the solvent), the molarity and molality are very similar. This is because 1.00 L of water has a mass of 1.00 kilograms (at 4°C, anyway). 

E) Molarity and molality are different measures, although there are some conditions where they are very similar. Molality depends on the mass of the solvent and molarity depends on the volume of the solvent. Since volume is affected by temperature, there may be some fluctuations in the volume of a solution as it... 

The correct answer is (E). You should be very clear on the differences between molarity and molality. Molality is the measure of the number of moles of solute dissolved per kilogram of solvent. The molarity is a measure of the moles of solute dissolved in a given volume of solution. If you know that a given solution has a molarity of 1.00, then you know there is 1.00 moles of solvent per liter of solution. Knowing the density will allow you to determine the mass of a solution. If the mass of the solute is subtracted from the mass of the solution, the result will be the mass of the solvent, which is required for the determination of molality. 

Concentration is expressed on the molar scale in terms of mol L of solvent or on the molal scale in terms of mol kg of solvent. The molal scale gives concentrations that are independent of temperature and pressure. In this chapter, the molar scale will be used on the basis that molarity and molality are almost identical for the low ionic strengths commonly associated with freshwaters. 

Here cj and m are the molarity and molality of i in its pure state as specified in Eqs. (3.5.2) of Chapter 3. For ct - c and mA - m the preceding relations reduce to an identity. The problem of dimensionality now does not arise further, the reference potentials now relate once more to species i in its pure state. It is thus unfortunate that the canonical forms (2.5.10) or (2.5.11) rather than the relations (2.5.12) or (2.5.13) have historically been adopted as a starting point. 

---