A wire made of gallium metal is placed in a solution of copper(II) nitrate solution. If 10g of gallium were required, how many moles of copper (II) nitrate solution were needed?

First, you need to write and balance the equation of gallium replacing copper in a solution. Here's a skeleton equation. You need to balance it (hint: the easiest way is to use half-reactions -- at least IMHO).

Ga + Cu^{2+} \rightarrow Ga^{3+} + Cu

Ga \rightarrow Ga^{3+} + 3e^-

Cu^{2+} + 2e^- \rightarrow Cu

Once you get the balanced equation, you can calculate the number of moles of gallium that will react with copper (as a ratio). You start with 10 grams of gallium. You compute the number of moles of gallium in 10 grams using the inverse of the atomic weight (atomic mass) of gallium (in g/mole):

\cancel {g\,of\,gallium} \,\times\, \frac {moles\,of\,gallium}{\cancel {g\,of\,gallium}} = moles\,of\,gallium

From the ratio of copper to gallium, you can then compute the number of moles of copper that are involved.

From the atomic weight of copper, you could then calculate the mass of copper involved, but that is not requested in the problem.

Gallium - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Gallium)