Alight wave travels at a speed of 3.0 × 108 meters/second. if the wavelength is 7.0 × 10-7 meters, what is the frequency of the wave?

c = f*L 

If you cant remember this just consider the units of each (denoted [c] is units of c): 

[c] = meters/second, [f] = 1/seconds or Hertz, [L] = meters 

So [c] = [f*L], an easy way to remember it. 

So to get the frequency, just solve for f: 

f = c/L = (3.00 x 10^8)/(5.93 x 10^-7) m/s = 5.05902192 × 10^14 m/s ~ 5.06 x 10^14 m/s 

first, consult an activity series chart to determine if al is more reactive than fe. it is, so it will replace the fe. now, one can continue with the problem.

next, make sure you have a correctly balanced equation.

3feo + 2al ==> 3fe + al2o3

now, determine which reactant (feo or al) is in limiting supply.

moles feo present = 125 g x 1 mole/71.8 g = 1.74 moles

moles al present = 25.0 g x 1 mole/26.98 g = 0.927 moles

since from the balanced equation, it take 3 mol feo for every 2 mol al, al will be limiting.

you can determine this simply by dividing moles of each by their coefficient in the balanced equation. for example: 1.74/3 = 0.58 and 0.927/2 = 0.464. al is less, so it is limiting.

finally, using the stoichiometry in the balanced eq. determine mass of fe that can be produced from 0.927 moles of al.

0.927 moles al x 3 moles fe/2 moles al x 55.8 g/mole = 77.6 g