Barrons SAT Subject Test Chemistry, 13th Edition

component in a solution. It is simply the number of moles of that component

divided by the total moles of all the components. The mole fraction of

component i is written as Xi. For a solution consisting of nA moles of

component A, nB moles of component B, nC moles of component C, and so

on, then the mole fraction of component A is given by:

As an example, if a mixture is obtained by dissolving 10 moles of NaCl in 90
moles of water, the mole fraction of NaCl in that mixture is 10 (moles of NaCl)
divided by (10 + 90) or 100 moles, giving an answer of 0.1, the mole fraction of
NaCl.

DILUTION

In dilution problems, the expression of molarity gives the quantity of solute per
volume of solution. The amount of solute dissolved in a given volume of solution
is equal to the product of the concentration times the volume. Hence, 0.5 liter of
2 M solution contains

Notice that volume units must be identical.

TIP

Use this equation for dilution problems:

M 1 V 1 = M 2 V 2

1 = solution before

2 = solution after

If you dilute a solution with water, the amount or number of moles of solute
present remains the same, but the concentration changes. You can use the
expression:

This expression is useful in solving problems involving dilution.

Example