Which of: 1) number of atoms
2) number of electrons
3) mass
4) numbers of molecules
is/are conserved in a chemical reaction?

1, 2, 3

Explanation:

Using a simple chemical reaction as an example;

2H2  +  O2  --> 2H2O

1) number of atoms

In the reactant side (left side of the reaction), there are 4 Hydrogen atoms which is equal with the number of atoms in the product (right side of the reaction). In the reactant side, there are 2 oxygen atoms, same thing with the product side.  This means it is conserved.

2) Number of electrons

The number of electrons are also balanced. Although there is sharing of electrons, This means it is conserved.

3) In a chemical reaction, the law of conservation of matter is obeyed. Since the number of atoms is conserved, the mass is also conserved.

4) numbers of molecules

Although this is true for some reactions, it is not always the case as seen in the reaction above. This means it is not conserved.

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