Rapid Calculations for Preparing Solutions Diego Lozano Calero Servicio de Bioquimica, Hospital La Paz. Paseo de la Casteilana, 281,28041 Madrid, Spain Students have problems when they want to prepare a diluted solute concentration from a more concentrated solution that cannot be weighed on a balance. The trouble comes from the fact that the density and mass percentage composition concepts are not clear for many of them. Therefore, i t would be interesting to give them other strategies that help them, as is shown here. If we want to prepare a solution with a solute concentration X (morn) and a volume V from a more concentrated solution with P% by mass concentration and density p, the calculations are as follows: the grams of solute, g, , for preparing the X(moyL) solution are, X = gofsalute 8, /liters of solution =-/V molar mass M.....
v=g"
substituting eq 2 in eq 3,
reorganizing eq 4,
P
v . p m
g , =.. . F
This equation connects directly the grams of solute, g,, with the required volume of the concentrated solution. If g, is substituted in eq 5 by its value given by eq 1, a new relation is obtained:
then g, = V X M ,
(1)
the grams of the concentrated solution, g,., containingg,,
the volume V of the concentrated solution containingg,.,
424
Journal of Chemical Education
This equation gives directl; the volume of the coneentrated solution needed to prepare the X (mole&) solution. The uniqueprecaution is to put the data in the appropriate units (with V Mm, and p i n L, glmol, g/mL, respectively; the V unit is mL ). These equations can be pn~grilrnrnedin a porket calculator smce they are ven/ useful in the daily work in the lsbo-
