Concentration Molality Molarity Mole

The (able below contains solubility and density data lor the salts Na SQ4 and MgS()4. Express their solubilities in terms of molar concentrations, molalities and mole fractions. Calculate the contractions in volume that occur when the solutions are made from the solid salts and the solvent. Comment on the results in terms of the ell ect of ionic charges. The concentrations have been cho-.cn to be comparable. 

SI base units include the meter (m), kilogram (kg), second (s), ampere (A), kelvin (K), and mole (mol). Derived quantities such as force (newton, N), pressure (pascal. Pa), and energy (joule, J) can be expressed in terms of base units. In calculations, units should be carried along with the numbers. Prefixes such as kilo- and milli- are used to denote multiples of units. Common expressions of concentration are molarity (moles of solute per liter of solution), molality (moles of solute per kilogram of solvent), formal concentration... 

Molality, molarity, mole fraction —> Different concentration units used for quantitative treatment of... 

As it is the mean activity coefficient, and not the activity coefficients of the individual ions, that is measurable, in the remainder of this section our interest is in formulas for y . Also, since we will be concerned mostly with low electrolyte concentrations in aqueous solutions, in the application of these fprmulas the distinction between molality (moles per kilogram of solvent) and concentration in molarity (moles per liter of solution) will sometimes be ignored. 

The activity was introduced by Gilbert Newton Lewis and plays an important role in the thermodynamic description of electrolyte solutions. As can be seen, the chemical potential consists of two contribntions the standard state term and a logarithmic term. The standard state term does not depend on concentration, but the logarithmic term does. Both terms depend on the concentration scale (molality, molarity, mole fraction, etc.) because p, , x and in Equation 1.2 all depend on the concentration scale. However, the chemical potential, p does not depend on the concentration scale. The chemical potential is the most important property of a component in solntion and is a function of temperature, pressure, and composition. It would be more accurate to rewrite Equation 1.2 using additional symbols as follows ... 

Solution Concentration—Any description of the composition of a solution must indicate the quantities of solute and solvent (or solution) present. Solution concentrations expressed as mass percent, volume percent, and mass/volume percent all have practical importance, as do the units, parts per million (ppm), parts per billion (ppb), and parts per trillion (ppt). However, the more fundamental concentration units are mole fraction, molarity, and molality. Molarity (moles of solute per liter of solution) is temperature dependent, but mole fraction and molality (moles of solute per kilogram of solvent) are not. 

Experiments on sufficiently dilute solutions of non-electrolytes yield Henry s laM>, that the vapour pressure of a volatile solute, i.e. its partial pressure in a gas mixture in equilibrium with the solution, is directly proportional to its concentration, expressed in any units (molar concentrations, molality, mole fraction, weight fraction, etc.) because in sufficiently dilute solution these are all proportional to each other. 

The label on a bottle of concentrated hydrochloric acid. The label gives the mass percent of HCI in the solution (known as the assay] and the density (or specific gravity) of the solution. The molality, molarity, and mole fraction of HCI in the solution can be calculated from this information. 

The symbol used is dependent upon the method of expressing the concentration of the solution. The recommendations of the IUPAC Commision on Symbols, Terminology and Units (1969) are as follows concentration in moles per litre (molarity), activity coefficient represented by y, concentration in mols per kilogram (molality), activity coefficient represented by y, concentration expressed as mole fraction, activity coefficient represented by f... 

The molality, m = moles per kilogram of solvent, as a temperature-independent concentration unit instead of the molarity, C = moles per litre of solution this means that the amount of solvent and hence its number of moles is fixed. 

In this chapter, you learned about solutions. A solution is a homogeneous mixture composed of a solvent and one or more solutes. Solutions may be unsaturated, saturated, or supersaturated. Solution concentration units include percentage, molarity, molality, and mole fraction. The solubility of solids in liquids normally increases with increasing temperature, but the reverse is true of gases dissolving in liquids. The solubility of gases in liquids increases with increasing pressure. 

A concentration scale for solutes in aqueous solutions, equal to moles of solute/55.51 mol water. It is frequently used in studies of solvent isotope effects. As pointed out by Schowen and Schowen the choice of standard states can change the sign for the free energy of transfer of a species from one solvent to another, even from HOH and DOD. The commonly used concentration scales are molarity, mole fraction, aquamolality, and molality. Free energies tend to be nearly the same on all but the molality scale, on which they are about 63 cal mol more positive at 298 K than on the first three scales. The interested reader should consult Table I of Schowen and Schowen ... 

Molality ami mole fraction, as expressions of concentration of a solute, possess the advantage over molar concentration of being independent of temperature. 

Salt Molar concentration/mol dm 3 Molality, m Mole fraction, x... 

Molality (in) is concentration expressed as moles of substance per kilogram of solvent (not total solution). Molality is independent of temperature. Molarity changes with temperature because the volume of a solution usually increases when it is heated. 

In daily life, it s often sufficient to describe a solution as either dilute or concentrated. In scientific work, though, it s usually necessary to know the exact concentration of a solution—that is, to know the exact amount of solute dissolved in a given amount of solvent. There are many ways of expressing concentration, each of which has its own advantages and disadvantages. We ll look briefly at four of the most common methods molarity, mole fraction, mass percent, and molality. 

The concentration of a solution can be expressed in many ways, including molarity (moles of solute per liter of solution), mole fraction (moles of solute per mole of solution), mass percent (mass of solute per mass of solution times 100%), and molality (moles of solute per kilogram of solvent). When equilibrium is reached and no further solute dissolves in a given amount of solvent, a solution is said to be saturated. The concentration at this point represents the... 

Molal (m) A measure of concentration given in moles of solute per kilogram of solvent (compare with molar). 

Equations (8.125) and (8.126) are valid for the case in which either mole fractions or molalities are used to express the concentrations. When molarities are used, we must include the temperature derivative of the molarity and of the density or molar volume of the solvent when necessary. Thus, for a solute... 

The number of moles of solute per kilogram of solvent is defined as the molal concentration or molality. Molar Concentration... 

Concentrations expressed as molality or mole fractions are temperature-independent and are most useful when a physical measurement is related to theory over a range of temperature, e.g., in freezing point depression or boiling point elevation measurements (Chapter 11). Since the density of water is close to 1 g/cm3, molal and molar concentrations are nearly equal numerically for dilute aqueous solutions (<0.1 M). 

In the case of solutions (liquid or solid mixtures), besides the molar fraction, we frequently use for expressing the solution composition the molar concentration (or molarity) ct, the number of moles for unit volume of the solution, and the molality mt, the number of moles for unit mass of the solvent (main component substance of the solution) ... 

Do not use italic type for the chemical concentration unit M (molar, moles per cubic decimeter, moles per liter) or the unit N (normal). Use italic type for the unit m (molal, moles per kilogram). Use a space between the number and these abbreviations, that is, on each side of these abbreviations. 

Molality Where molarity is the term used to express the concentration of a solution based on the number of moles per litre, molality is the term used to express the concentration of a solution based on number of moles of a solute (dissolved substance) per mass of solvent. The unit of molality is moles per kilogram (or molal). 

This relation holds true only if we specify concentrations in units of molality or mole fraction, both of which are unaffected by pressure changes. If, however, we use units of molarity (i.e., moles/liter) which are affected by pressure changes (owing to the compressibility of the solution), then we must make the appropriate correction. This can be done by means of the identity F/ P)c = dF/dP)xt + ( F/ Xt)p( Xx/ P)c > where Xt is mole fraction and 0% is molarity of the tth component. Now, dXt/dP)Ct = where = — (d In F/dP) , the isothermal compressibility of... 

Activity is given in the same units as concentration—molarity, mole fraction, and so forth. In (2-9) the concentration C usually is expressed either in moles per liter of solution (molarity M) or in moles per kilogram of solvent (molality m). In dilute aqueous solutions the molarity and molality are nearly equal in nonaqueous solutions molarity is usually larger than molality, since the density of the solvent is usually less than unity. Analytical chemists ordinarily find it more convenient to express concentration in molarity, even though it varies slightly with temperature. The analytical concentration is represented by the symbol C, to indicate the moles of solute added per liter of solution. Analytical concentration should be distinguished from the equilibrium concentration, which is indicated by enclosure in square brackets. 

As was extensively discussed in the preceding section, alternative formulations are found by introducing molarity, q, or molality, nti, as the concentration variable. According to Eq. (2.5.10a) we relate the concentration to the mole fraction to write... 

Concentrations will be expressed as mole fraction of a component or species /, x = nj/E nj as molality, mole per mass of solvent, mol kg or molarity, mole per volume of solution. The concentration scale will depend on the properties of the solutes (i.e., ionic, polar, nonpolar, etc.). Pressure, p, and the gas phase partial pressure of species i, / , will be expressed in bars (approximately equal to atmospheres). 

Isotonic saline contains 0.9% w/v of sodium chloride (mol. wt. = 58.5). Express the concentration of this solution as (a) molarity (b) molality (c) mole fraction and (d) milliequivalents of Na" per litre. Assume that the density of isotonic saline is 1 g cm... 

Measures of concentration include mass and volume percentages, molarity, molality, and mole fraction. 

Express the concentration of a solute in solution in units of mass percentage, molarity, molality, and mole fraction (Section ff.f. Problems 3-8). 

Table 1.2. 1 (a) Concentration units encountered in oceanography Equivalents, eq, is equal to moles x absolute value of the charge of the species. Units indicated as seawater units are those preferred in oceanography. Molality, molarity, normality and volume ratio all have a long history of use in classical chemistry because of their convenience for laboratory preparations. ... 

Because the concentration of a solution is so variable, we need two ways to indicate how much solute is in a particular solution. There are several ways to measure the concentration of a solution, including molarity, molality, and mole fraction. The type of measurements you use will often depend upon the situation or on the calculations that you want to be able to carry out. 

There are several ways to measure the concentration of a solution, five of which you should know for the MCAT molarity (M), molality ), mole fraction (%), mass percentage and parts per million (ppm). Molarity is the moles of the compound divided by the volume of the solution. Molarity generally has units of mol/L. Molality is moles of solute divided by kilograms of solvent. Molality generally has units of mol/kg and is usually used in formulae for colligative properties. The mole fraction is the moles of a compound divided by the total moles of all species in solution. Since it is a ratio, mole fraction has no units. Mass percentage is 100 times the ratio of the mass of the solute to the total mass of the solution. Parts per million is 106 times the ratio of the mass of the solute to the total mass of the solution. 

The standard state of a substance is a reference state that allows us to obtain relative values of such thermodynamic quantities as free energy, activity, enthalpy, and entropy. All substances are assigned unit activity in their standard state. For gases, the standard state has the properties of an ideal gas, but at one atmosphere pressure. It is thus said to be a hypothetical state. For pure liquids and solvents, the standard states are real states and are the pure substances at a specified temperature and pressure. For solutes In dilute solution, the standard state is a hypothetical state that has the properties of an infinitely dilute solute, but at unit concentration (molarity, molality, or mole fraction). The standard state of a solid is a real state and is the pure solid in its most stable crystalline form. 

The units of molality are mole per kilogram, but molality is usually symbolized with a small m. Molality is not as widely used as molarity by most chemists, although it is the preferred concentration unit for two of the colligative properties. 

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