Solution amount-mass-number relationships

Figure 3.9 Summary of amount-mass-number relationships in solution. The amount (mol) of a substance in solution is related to the volume (L) of solution through the molarity (M mol/L). As always, convert the given quantity to amount (mol) first.

We remember from Chapter 5 that the coefficients in equations such as Equation 7.3 allow the relative number of moles of pure reactants and products involved in the reaction to be determined. These relationships coupled with the mole definition in terms of masses then yield factors that can be used to solve stoichiometric problems involving the reactants and products. Similar calculations can be done for reactions that take place between the solutes of solutions if the amount of solute contained in a specific quantity of the reacting solutions is known. Such relationships are known as solution concentrations. Solution concentrations may be expressed in a variety of units, but only two, molarity and percentage, will be discussed at this time. 

The concentration of a very small quantity of a substance in the human body, or in the environment, can be expressed in parts per million (ppm) and parts per billion (ppb). Both parts per million and parts per billion are usually mass/mass relationships. They describe the amount of solute that is present in a solution. Notice that parts per million does not refer to the number of particles, but to the mass of the solute compared with the mass of the solution. 

Molarity can be thought of as a conversion factor used to convert between volume of solution and amount (mol) of solute, from which we then find the mass or the number of entities of solute. Figure 3.10 (on the next page) shows this new stoichiometric relationship, and Sample Problem 3.13 applies it. 

Figure 3.10 Suminary of mass-mole-number-volume relationships in solution. The amount (in moles) of a compound in solution is related to the volume of solution in liters through the rmlarity 0M) in moles per liter. The other relationships shown are identical to those in Figure 3.4, except that here they refer to the quantities in solution. As in previous cases, to find the quantity of substance expressed in one form or another, convert the given information to moles first...

---